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by Robert McNeill Alexander, Norman R. Alpert, Robin Huw Crompton, Nancy A. Curtin, Robert E. Davies, John Gergely, M.D., John M. Newsom-Davis, Warren F. Walker, Jr., David M. Warshaw, Bernard Wood
Muscle, contractile tissue found in animals, the function of which is to produce motion.
Movement, the intricate cooperation of muscle and nerve fibres, is the means by which an organism interacts with its environment. The innervation of muscle cells, or fibres, permits an animal to carry out the normal activities of life. An organism must move to find food or, if it is sedentary, must have the means to bring food to itself. An animal must be able to move nutrients and fluids through its body, and it must be able to react to external or internal stimuli. Muscle cells fuel their actions by converting chemical energy in the form of adenosine triphosphate (ATP), which is derived from the metabolism of food, into mechanical energy.
Muscle is contractile tissue grouped into coordinated systems for greater efficiency. In humans the muscle systems are classified by gross appearance and location of cells. The three types of muscles are striated (or skeletal), cardiac, and smooth (or nonstriated). Striated muscle is almost exclusively attached to the skeleton and constitutes the bulk of the body’s muscle tissue. The multinucleated fibres are under the control of the somatic nervous system and elicit movement by forces exerted on the skeleton similar to levers and pulleys. The rhythmic contraction of cardiac muscle is regulated by the sinoatrial node, the heart’s pacemaker. Although cardiac muscle is specialized striated muscle consisting of elongated cells with many centrally located nuclei, it is not under voluntary control. Smooth muscle lines the viscera, blood vessels, and dermis, and, like cardiac muscle, its movements are operated by the autonomic nervous system and thus are not under voluntary control. The nucleus of each short tapering cell is located centrally.
Unicellular organisms, simple animals, and the motile cells of complex animals do not have vast muscle systems. Rather, movement in these organisms is elicited by hairlike extensions of the cell membrane called cilia and flagella or by cytoplasmic extensions called pseudopodia.
This article consists of a comparative study of the muscle systems of various animals, including an explanation of the process of muscle contraction. For an account of the human muscle system as it relates to upright posture, see muscle system, human.
General features of muscle and movement
Muscle powers the movements of multicellular animals and maintains posture. Its gross appearance is familiar as meat or as the flesh of fish. Muscle is the most plentiful tissue in many animals; for example, it makes up 50 to 60 percent of the body mass in many fishes and 40 to 50 percent in antelopes. Some muscles are under conscious control and are called voluntary muscles. Other muscles, called involuntary muscles, are not consciously controlled by the organism. For example, in vertebrates, muscles in the walls of the heart contract rhythmically, pumping blood around the body; muscles in the walls of the intestines move food along by peristalsis; and muscles in the walls of small blood vessels constrict or relax, controlling the flow of blood to different parts of the body. (The effects of muscle changes in the blood vessels are apparent in blushing and paling due to increased or decreased blood flow, respectively, to the skin.)
Muscles are not the only means of movement in animals. Many protists (unicellular organisms) move instead by using cilia or flagella (actively beating processes of the cell surface that propel the organism through water). Some unicellular organisms are capable of amoeboid movement, in which the cell contents flow into extensions, called pseudopodia, from the cell body. Some of the ciliated protozoans move by means of rods called myonemes, which are capable of shortening rapidly.
Nonmuscular methods of movement are important for multicellular animals as well. Many microscopic animals swim by means of beating cilia. Some small mollusks and flatworms crawl using cilia on the underside of the body. Some invertebrates that feed by filtering particles from water use cilia to create the necessary water currents. In higher animals, white blood cells use amoeboid movements, and cilia from cells lining the respiratory tract remove foreign particles from the delicate membranes.
Muscles consist of long slender cells (fibres), each of which is a bundle of finer fibrils Figure 1). Within each fibril are relatively thick filaments of the protein myosin and thin ones of actin and other proteins. When a muscle fibre lengthens or shortens, the filaments remain essentially constant in length but slide past each other as shown in Figure 2: The arrangement of the myofilaments in a striated muscle. The muscle is extended in the …Figure 2. Tension in active muscles is produced by cross bridges (i.e., projections from the thick filaments that attach to the thin ones and exert forces on them). As the active muscle lengthens or shortens and the filaments slide past each other, the cross bridges repeatedly detach and reattach in new positions. Their action is similar to pulling in a rope hand over hand. Some muscle fibres are several centimetres long, but most other cells are only a fraction of a millimetre long. Because these long fibres cannot be served adequately by a single nucleus, numerous nuclei are distributed along their length.
The work done by muscle requires chemical energy derived from the metabolism of food. When muscles shorten while exerting tension and performing mechanical work, some of the chemical energy is converted to work and some is lost as heat. When muscles lengthen while exerting tension (such as in slowly lowering a weight), the chemical energy that is used, along with the mechanical energy absorbed by the action, is converted to heat. Generation of heat is an important function of muscle in warm-blooded animals. Shivering is muscle activity that generates heat and warms the body. Similarly, some insects vibrate their wings for a while before flight, heating the muscles to the temperature at which they work best.
Diversity of muscle
Muscle fibres differ from species to species of animal and between parts of the same animal. Apart from the distinction between voluntary and involuntary muscles, muscles differ in structure and activity.
Muscles differ in the arrangement of their myofilaments. The principal types of muscles are striated muscle, in which the filaments are organized in transverse bands as in Figure 2; obliquely striated muscle, in which the filaments are staggered, making the bands oblique (Figure 3: The arrangement of the myofilaments in obliquely striated muscle.Figure 3); and smooth muscle, in which the filaments are arranged irregularly. In vertebrates, all voluntary muscles are striated, and all involuntary muscles are smooth, except for cardiac muscle, which is involuntary but striated. Obliquely striated muscle is found only in some invertebrate groups (the nematodes, annelids, and mollusks) and has the protein paramyosin in the thick filaments as well as myosin.
Muscles differ in the stimuli required to activate them. In vertebrates, voluntary muscles require action potentials (electrical signals) in their nerves to initiate every contraction. Some involuntary muscles are spontaneously active, and the action potentials in their nerves only modify the natural rhythm of contraction. The leg muscles of all insects, and the wing muscles of many, require action potentials to initiate every contraction; however, the wing muscles of other insects consist of fibrillar muscle, which requires only occasional action potentials to maintain its rapid rhythmic contractions. The wings of these insects are attached to the body in such a way as to have a resonant frequency of vibration (like a guitar string that vibrates when plucked at its resonant frequency). When fibrillar muscles are active, they contract so as to maintain the vibrations of the resonant system.
Muscles differ in the ability to exert stress. Muscles that exert large stresses have long, thick filaments that carry larger numbers of cross bridges. The result is more cross bridges than are found in other muscles. This means that more force can be transmitted from each thick filament to the adjacent thin filaments, and larger stresses can be exerted. Less stress can be exerted when the fibres are shortening than when they are maintaining constant length, and more can be exerted when they are being forcibly stretched.
Muscles differ in the manner in which their forces are controlled. Most of the fibres in the voluntary muscles of mammals can only be switched on or off, and different degrees of force are obtained by activating different numbers of fibres. In many other muscles, however, the force exerted by each fibre can be varied. In these muscles, force is not controlled by activating different numbers of fibres but by changing the intensity of muscle activation as a whole.
Muscles differ in the ranges of length over which they can operate. Smooth muscles generally work over wider ranges of length than striated ones, but there are a few exceptional striated muscles. One such muscle in the tongue of chameleons can shorten to one-sixth of its fully extended length.
Muscles also differ in their speed of action, including the rates at which they develop force and shorten. If a muscle shortens by one-tenth of its length in one-tenth of a second, its rate of shortening is one length per second. Maximum rates of shortening vary between species and between muscle fibres in a single animal. For example, two muscles in the limbs of mice have maximum shortening speeds (at 37 °C) of 24 and 13 lengths per second.
Finally, muscles differ in their metabolism. The adenosine triphosphate (ATP) that they use as their immediate energy source may be produced either by oxidative reactions, in which food is oxidized to carbon dioxide and water, or by processes that do not require oxygen (anaerobic processes). Vertebrates and crabs use the anaerobic process of glycolysis, converting the carbohydrate glycogen to lactic acid, for short bursts of vigorous activity, such as sprinting. The burst of activity is followed by a recovery period in which oxygen is used to oxidize some of the lactic acid, releasing the energy needed to convert the rest back to glycogen. The advantage of using anaerobic metabolism in this way is that the intensity of activity during the burst is not limited by the rate at which the blood can bring oxygen to the muscles.
In vertebrates, many muscle fibres perform only oxidative metabolism or only glycolysis, though some perform both. Oxidative fibres are commonly red, due to the presence of the pigment myoglobin. Most fishes show an obvious distinction between the main bulk of white swimming muscle and a narrow strip of red muscle along the side of the body. Slow swimming is powered by the red (oxidative) muscle and bursts of fast swimming by the white (glycolytic) muscle. Red and white muscles are also easy to distinguish in the domestic chicken, in which the pale meat of the breast consists mainly of white fibres and the dark meat of the legs consists of red fibres. The breast muscles are the main muscles of the wings, which are used by chickens only for occasional short bursts of flight. Other birds that practice sustained flight (e.g., hummingbirds) mainly have red breast muscles.
Muscles that work skeletons
A clamshell is an example of a simple system in which a rigid skeleton is worked by muscles. The two rigid parts of the shell (Figure 4: Rigid skeletons worked by muscles.Figure 4A) are hinged together. They can be closed to protect the animal within or allowed to open. A block of rubbery protein, the inner hinge ligament, lies just inside the hinge. When the adductor muscle contracts, it closes the shell, but, in so doing, it compresses the inner hinge ligament. When it relaxes, the ligament recoils elastically, reopening the shell. This is an unusual system, in that it is worked by just one muscle. Most other skeletal systems need muscles in antagonistic pairs, in which each muscle is paired with a muscle of the opposite effect.
This antagonism is illustrated by the human ankle (Figure 4B). The tibialis anterior muscle flexes the ankle (raising the toes) and the soleus muscle extends the ankle. These muscles make up an antagonistic pair. In this particular case there is another muscle, the gastrocnemius, which cooperates with the soleus, helping it to extend the ankle. (The gastrocnemius, however, crosses the knee as well as the ankle and affects both joints.)
The ankle is not a simple hinge joint. As well as flexion and extension, it can exhibit inversion (the sole of the foot faces the other leg) or eversion (the opposite movement). These movements are controlled by the tibialis posterior, which inverts the ankle, and the peronaeus muscles, which are antagonistic and evert it.
A hinge such as the clam joint or the human knee performs just one kind of movement, flexion/extension, expressed in technical terms as allowing one degree of freedom of movement. The human ankle performs two kinds of movement, flexion/extension and inversion/eversion, allowing two degrees of freedom. Ball-and-socket joints, such as the human hip, allow three degrees of freedom. Most animal joints have at least two muscles (an antagonistic pair) for each degree of freedom.
Muscles of the human forearm (anterior view, superficial layer). [Credit: Encyclopædia Britannica, Inc.]Seldom are muscle fibres as long as a muscle, but many muscles, such as the biceps in the human arm, are composed of relatively long fibres lying nearly parallel to each other. These parallel muscles are attached to tendons or apodemes (in arthropods, chitinous rods that serve as sites for muscle attachment) only at their extreme ends. Since muscle fibres can contract about one-third of their resting length, this arrangement is suitable to an extensive and quick movement. The deltoid muscle in the human shoulder is said to be pennate; relatively short fibres attach diagonally onto a tendon that penetrates far into the muscle. The ankle muscles shown in Figure 4B are pennate muscles, but most of the hamstring muscles (at the back of the thigh) are parallel. The adductor muscles of the shells of clams are parallel, but most of the leg muscles of arthropods are pennate. A pennate muscle may contain many more and shorter fibres than a parallel muscle of equal mass. Therefore, the pennate muscle can exert a greater force but cannot shorten a great deal; the parallel-fibred muscle can exert only a relatively small force but can shorten significantly. The presence of pennate muscle in a given structure may have the same effect as a longer lever arm. In the slender legs of arthropods, with insufficient space for bulky muscles or long lever arms, many of the muscles are pennate.
Tendons and apodemes have elastic properties. Tendons in the legs of mammals serve as springs, reducing the energy cost of running: energy that is lost as the foot hits the ground and decelerates the body is stored as elastic strain energy in tendons and is subsequently returned in an elastic recoil. An apodeme in the hind legs of locusts, for example, is one of the important elastic elements in the catapult mechanism that powers jumping.
Muscle in soft animals
Slugs, worms, and many other invertebrate animals have no skeleton, and thus movement is not produced by lever action. Even vertebrates have parts of the body that have muscles but no skeletal component (for example, the tongue). Many soft-bodied animals have muscle systems based on the principle illustrated by a simple wormlike animal, as shown in Figure 5: Changes in body form in wormlike soft-bodied animals. (A) The longitudinal muscle …Figure 5. The longitudinal muscle fibres run lengthwise along the body, and the circular fibres encircle it. The body contents are liquids or tissues that can be deformed into different shapes, but they maintain a constant volume. If longitudinal muscles contract and the body shortens, it must widen to accommodate its volume; if the circular muscles contract and the body thins, it must lengthen. Thus, the longitudinal and circular muscles are antagonistic, and shortening of either extends the other. Further, if the length of a circular muscle remains constant while the longitudinal muscle of one side of the body shortens, the body bends, and the longitudinal muscle of the other side is stretched. Thus, the longitudinal muscles of the left and right sides can be antagonistic toward each other. In worms the body fluids render muscles antagonistic through hydrostatic forces. The principle involved is sometimes called the principle of the hydrostatic skeleton.
This principle can apply to individual muscles as well if their fibres run in several directions. For example, a muscle that has some fibres running longitudinally and others running circularly and/or radially will become shorter and fatter when the longitudinal fibres shorten and will become longer and thinner when the circular and radial fibres shorten. There are many examples of muscle structure like this in the mollusks. One such example is the shell muscle of the abalone Haliotis, which connects the domed shell of the animal to its adhesive foot. When the muscle shortens, with the foot attached to a rock, the shell is pulled down over the animal to protect it. When the muscle lengthens (by contraction of circular and radial fibres), the shell is raised from the rock, allowing respiratory water currents to circulate.
Invertebrate muscle systems
The phylum Cnidaria includes the hydras, jellyfishes, and sea anemones. Cnidarians have two main body forms: the cylindrical tentacled polyp, exemplified by the hydra and the sea anemone, and the bell-shaped (or inverted saucer-shaped) medusa. Hydras are some of the simplest multicellular animals to have muscle. They are hollow, cylindrical, freshwater creatures about 10 mm long. One end attaches to a plant or some other support, and the other end is free and has a mouth surrounded by tentacles. The body wall consists of two layers of cells with a middle gelatinous layer called mesoglea. In hydras and other two-layered animals, one kind of cell serves as both muscle and epithelial cells. The compact body of each cell is packed closely with the adjacent cells to form an epithelium, and the base of each cell, where it meets the mesoglea, is drawn out into a long muscle fibre.
In the hydra the musculoepithelial cells that cover the outer surface of the body have longitudinal muscle fibres; those that line the gut cavity (the gastrodermis) have circular muscle fibres. Sea anemones have all of the muscle fibres in the gastrodermis, though some of the fibres are longitudinal and some are circular. When the mouth of the sea anemone is closed, the water in the gut cavity acts as a hydrostatic skeleton, permitting the animal to grow longer and thinner or shorter and fatter or to bend in any direction. These changes result from the interaction of the longitudinal and circular muscles through movements that are not as simple as those in the schematic worm shown in Figure 5. The hydra can reduce its volume by using its muscles to squeeze water out of the gut cavity through the open mouth. It can reinflate using cilia to circulate water into the gut cavity. Its movements are also influenced by the viscoelastic properties of the mesogleal jelly.
The largest and most familiar medusae are the jellyfishes of the class Scyphozoa, some of which grow to a diameter of two metres. Though large, the scyphozoan jellyfishes have only a single layer of cells on the outer surface of the body and a single layer lining the gut cavity; most of the volume of the animal is occupied by the gelatinous mesoglea. The epidermis of the undersurface of the bell includes the musculoepithelial cells responsible for the animal’s weak swimming movements. The muscle fibres contract, reducing the diameter of the bell and forcing out a stream of water. The bell then returns to its original shape by elastic recoil of the mesoglea. These movements are performed in a regular rhythm with a period of a few seconds, propelling the animal through the water. Medusae are among the simplest animals that use muscles to make rhythmic movements. In at least some medusae, the circular muscles, which do most of the work of swimming, are striated. In contrast, most of the other muscles of cnidarians are smooth.
Although all worms have more than two layers of cells and most have long slender bodies, the various groups of worms are different from each other in other respects.
The simplest worms are the flatworms (phylum Platyhelminthes), most of which have flattened shapes like leaves or ribbons. Although musculoepithelial cells have been found in some flatworms, the muscle cells in most are distinct from the epithelial cells. There is a layer of circular muscle fibres immediately under the epidermis, a layer of diagonal fibres, and a still deeper longitudinal layer. There are also dorsoventral muscle fibres running from the upper to the lower epidermis of the flattened body. These sets of muscle fibres act in various combinations to make the body long and thin, short and fat, or bent to one side or the other. These muscles are also used by some of the larger flatworms to pass waves of muscular contraction along the body, enabling the worm to crawl in a snail-like fashion.
Many flatworms have a mouth opening connected to the pharynx, a muscular tube that carries food from the mouth to the intestine. In some flatworms the pharynx is protruded and inserted into invertebrate prey, to digest and suck out the contents. The sucking is done by peristalsis, waves of muscular contraction that move along the tube from the mouth toward the gut. Although the muscle cells of flatworms are generally not musculoepithelial, their nuclei are found in large cell bodies. The muscle fibres of vertebrates and higher invertebrates, on the other hand, have no projecting cell body.
Roundworms (phylum Nematoda) also have large cell bodies on their muscle cells, but these muscle cells are unique in that nerve fibres do not travel to them as they do in the muscles of other animals. Instead, narrow projections of the muscle cell bodies extend to the principal nerves and contact nerve cells there.
Roundworms have obliquely striated, longitudinal muscle but no circular muscle. They are enclosed in a thick cuticle that allows bending but prevents swelling. Therefore, contraction of the longitudinal muscle can only bend the body. Roundworms do not bend from side to side like eels or snakes, but up or down (dorsal or ventral). By preventing swelling, the cuticle ensures that shortening of one muscle group stretches the other; thus, it makes the dorsal and ventral longitudinal muscles antagonistic to one another. Most crawl between soil particles or among the villi of a host’s gut by undulating waves of muscular contraction. Similar movements also enable some roundworms to swim.
The segmented worms (phylum Annelida) include the earthworms and many marine worms. Inside the body, between the body wall and the gut, is a fluid-filled cavity, the coelom, which in some annelids, including earthworms, is divided into successive segments. The body wall has an outer layer of circular muscle and an inner layer of longitudinal muscle.
Earthworms crawl by peristaltic contractions of the body wall. Each segment is alternately elongated (by contraction of its circular muscles) and shortened (by contraction of its longitudinal muscles). The muscles of each segment contract just after those of the segment in front, so that waves of contraction pass backward along the body, enabling the worm to move slowly forward. The same movements also serve for burrowing. While shortened, the segments are pushed against the burrow wall; when they elongate again, the worm moves forward.
The phylum Mollusca includes the gastropods (snails, slugs, and periwinkles), bivalves (clams, oysters, mussels, and scallops), cephalopods (octopods and squids), and other, smaller classes. All mollusks, except the cephalopods, have a highly muscular organ called the foot, through which muscle fibres run in all directions. The foot of a gastropod is a flat structure used for crawling. Waves of muscular contraction travel along its length, moving the animal slowly over the ground. The foot of a bivalve mollusk is a bulbous or tonguelike organ that is used for burrowing in sand or mud. The foot pushes down into the substrate, swells to anchor itself, and then pulls the rest of the animal down behind it.
In addition to the muscles of the foot, gastropod and bivalve mollusks have large muscles attached to their shells. The columellar (shell) muscles of gastropods pull the foot and other parts of the body into the shell. The adductor muscles of bivalves (Figure 4) shorten to close the shell or relax to allow the shell to spring open, enabling the mollusk to extend its foot or to feed. The adductor muscle can shorten rapidly and close the shell quickly. The muscle is also capable of maintaining the tension needed to hold the shell shut against the spring action of the hinge ligament without using much metabolic energy. Economy of energy is particularly important if the shell has to be kept closed for long periods—for example, for several hours while the mollusk is exposed on the beach at low tide. Fast muscles can shorten rapidly because their cross bridges detach and reattach quickly; however, they use much energy while maintaining tension because there is an energy cost every time a cross bridge detaches and reattaches. Muscles that are economical in their energy usage are generally slow. Accordingly, most bivalve mollusks have two parts to their adductor muscles: a translucent part, which is fast, and an opaque part, which is slow but economical.
Squids and other cephalopod mollusks also swim by jet propulsion. They draw water into the mantle cavity (the cavity that houses the gills) and expel it rapidly. Vigorous movements of this kind provide jet propulsion, but gentler ones serve for breathing by circulating water, and thus oxygen, through the mantle and gills. Fast-swimming squid have mantle cavities whose muscular walls make up as much as 35 percent of the mass of the body.
These walls mainly consist of circular muscle fibres that squeeze water out of the mantle cavity when they contract. Other fibres run radially through the thickness of the wall. These fibres make the wall thinner when they contract, stretching the circular muscle and enlarging the cavity again. Cephalopods do not have longitudinal muscle fibres; however, layers of collagen fibres on the outer and inner surfaces of the muscle prevent the animal from lengthening when the muscles contract. Thus, the circular and radial muscle fibres are antagonistic. Enlargement of the cavity, however, is not solely due to the radial muscle fibres; the cavity tends to expand by elastic recoil of the tissues when the circular muscles relax.
Though many mollusks have shells, most molluscan muscle systems depend on the principle of the hydrostatic skeleton. In some cases, body fluids are involved; for example, the feet of clams are extended and inflated by the inflow of blood. In other cases the muscle itself serves as the incompressible element that must thicken as it shortens or become slender as it elongates, to maintain constant volume. Examples include the shell muscle of the abalone and the tentacles of squid, which are shortened by contraction of longitudinal muscle fibres and lengthened by circular and transverse ones.
Arthropoda is the largest phylum of invertebrate animals and comprises crustaceans, insects, arachnids (spiders and scorpions), and other classes. Some arthropods have soft-bodied young stages in which the principle of the hydrostatic skeleton is important. Most adult arthropods are encased in a skeleton with jointed appendages formed from a stiff cuticle that is divided into separate plates to assist in movement. This skeleton, working as a system of levers, is largely responsible for making muscles antagonistic.
The wing muscles of dragonflies (Odonata) and those of some other insects are worked in simple, direct ways by pulling on the wing bases and making them pivot about their joints. More-advanced insects, including flies (Diptera), work their wings indirectly by muscles that attach to other parts of the skeleton. Although the details of the mechanisms are complicated, the basic principle is simple. Each wing-bearing segment of the body is enclosed by two main plates of cuticle, a tergum above and a sternum below. These plates are flexible enough to be distorted by muscle action. Distortions of the tergum are particularly important in the wing mechanism.
The principal wing muscles are the dorsoventral muscles, which run vertically from the sternum to the tergum, and the longitudinal muscles, which run lengthwise along the segment. Contraction of the longitudinal muscles makes the tergum bow upward, and contraction of the dorsoventral muscles pulls it down again. The wings have joints connecting them to the tergum and to the sternum. Upward movement of the tergum (from contraction of the longitudinal muscles) lowers the wings, and downward movement (from contraction of the dorsoventral muscles) raises them.
All arthropod muscles seem to be striated, not obliquely striated or smooth, and the sarcomeres are of varying lengths. In locusts the sarcomeres (the primary structural and functional unit responsible for contraction; see below The myofilament) of wing muscles are 3.9 micrometres (μm) long, but the sarcomeres of leg muscles (which do not have to contract so quickly) are 8.5 μm long. Wing muscles in many other insects have shorter sarcomeres, often about the same length as those in mammalian muscle (about 2.5 μm).
The force exerted by the muscle is controlled by varying the frequency of action potentials in the axons (an extension of the nerve cell that conducts nerve impulses away from the cell body). The higher the frequency, the larger the force, within limits. In contrast, in vertebrates each muscle is served by many motor axons, each of which is connected to only a small group of muscle fibres. In the twitch muscles that predominate in vertebrates, each muscle fibre is either inactive or fully active, and force is varied by recruiting different numbers of muscle fibres. Like those of other animals, most arthropod muscles require an action potential to initiate each contraction.
Fibrillar muscle is found in the sound-producing, or tymbal, muscles of some cicadas and in the wing muscles of several orders of insects, including the Diptera (flies), Coleoptera (beetles), Hymenoptera (wasps), and Hemiptera (bugs). Most fibrillar muscles work at high frequencies, often of several hundred cycles per second, but they are kept working by action potentials arriving at much lower frequencies. They contract at the resonant frequency of the tymbal or of the wing system. Clipping the wings of an insect that has fibrillar wing muscles increases the frequency of the wing beat, because reduction of the vibrating mass increases the resonant frequency.
All insect wing muscles work aerobically and produce high power outputs. Consequently, they need many mitochondria (the site of aerobic energy production in cells), which may occupy 40 percent or more of their volume in both fibrillar and non-fibrillar muscles. Non-fibrillar muscles that work at high frequencies also need large sarcoplasmic reticulums, but fibrillar ones do not.
Although insect muscles seem to always work aerobically, some crustacean muscles can work anaerobically. The leg muscles that the crab Callinectes uses for swimming include two types of fibres. One type resembles the red muscle fibres of vertebrates in that it is deep pink and contains a high proportion of mitochondria. The other resembles vertebrate white fibres because it is white, with far fewer mitochondria, and presumably works anaerobically. Similar differences occur in other crustacean muscles. Crabs use anaerobic metabolism for short bursts of violent activity in the way that vertebrates do.
The phylum Echinodermata comprises the starfishes, sea urchins, and their relatives. Their internal skeletons are made of porous blocks of calcium carbonate, and they have muscles to work their skeleton. Echinoderms also have a hydraulic system, the water-vascular system, with movable projections from the body called tube feet.
The details of the tube feet differ among the different groups of echinoderms. In the arrangement found in sea urchins (Figure 6: Tube foot of the sea urchin.Figure 6), five double rows of tube feet project through the test, so every part of the body surface has tube feet near it. The tube feet are slender tubes, with a sucker on the closed end. Muscles in the sucker enable it to attach to objects, so the tube feet can be used by the animal to anchor, to move, or to manipulate its prey. Connective tissue in the tube feet limits their diameter but allows them to lengthen, to shorten, and to bend. The tube feet have only longitudinal muscles, which extend the length of the cavity of the tube foot. They are extended by water that has been forced into them by muscles in the wall of the ampulla at their bases.
Vertebrate muscle systems
Major types of vertebrate muscles
In terms of its microscopic structure, the musculature of vertebrates is usually divided into three types: striated, cardiac, and smooth muscle. Smooth and cardiac muscle are under the control of the involuntary, or autonomic, nervous system. Striated muscle, on the other hand, is mainly under the control of the voluntary, or central, nervous system. Smooth and cardiac muscle are also similar in their development, being generally associated with the yolk sac. Striated muscle develops directly from the middle of the three embryonic layers, arising largely from the mesodermal somites (see below). In the adult, smooth and cardiac muscle are associated with organs or tubes (viscera), and striated (skeletal) muscle is associated with the bony or cartilaginous skeleton.
The two major divisions of the vertebrate musculature are the visceral musculature and the somatic musculature (the striated muscles of the body wall). Somatic musculature may be divided into appendicular, or limb, muscles and axial muscles. The axial muscles include the muscles of the tail, trunk, and eyeballs as well as a group of muscles called hypobranchial muscles, which separate and migrate from the others during development.
Basic pattern of development
The gastrula is the stage of embryonic development at which the embryo appears as three distinct layers of cells (the germ layers): the exterior ectoderm, the middle mesoderm, and the interior endoderm. The mesoderm differentiates to form most of the tissues, structures, and organs of the body. As the embryo lengthens, the mesoderm lying along the midline differentiates to form the notochord, a hollow cartilaginous nerve tube. In the adult the notochord contributes only to the structure of the vertebrae. The mesoderm lateral to this midline then divides into three parts that ultimately form the somites (which subsequently form the vertebrae, the somatic muscles, and the skin), the structures of the urogenital system, and the coelom and its associated structures.
One problem in discussing the differences in arrangement of muscles between the various vertebrate groups is in deciding which muscles in each species are homologous—that is, which have the same evolutionary and developmental origin. The problem arises because the position and attachment of muscles change during evolution; a muscle lying in the same position and attached to the same bone or cartilage in one vertebrate may have different origins from those of another vertebrate species. Comparison of the development of muscles in the embryo of each species and of their nerve supply would probably give the best clues. No single method may be relied upon in all cases, and many different types of evidence are considered before the homology is decided upon.
Vertebrate muscles are given names derived from Latin according to their attachments. In this system the Latin names of the bony points of attachment are either joined, as in sternocleidomastoid, naming the human muscle that runs from the sternum and clavicle to the mastoid region of the skull, or they may be named for their form or their gross function. There are several standard terms that describe form and function. A muscle may have more than one point of origin; thus, it may be described as having, for example, two “heads,” as in biceps femoris (bi- for two, -ceps for heads, femoris meaning “of the femur”). It may be long, longus, or short, brevis. It may run transversely across a body segment, transversus, or obliquely, obliquus. It may lie close to the surface, superficialis, or deep, profundus. In describing function, flexors are muscles that tend to close the angle made by the two bones to which they are attached; extensors tend to increase the angle. Adductors pull a bone or cartilage closer to the axis of the body, or limb, while abductors pull away from the axis. Rotators turn one bone or cartilage with respect to another or with respect to the midline. Pronators turn the sole of the foot or the palm of the hand to face the ground, while the opposite function is performed by supinators. Constrictors and sphincters diminish the volume of spaces or the area of structures, and dilators increase them. The names of muscles in humans often have been applied to grossly equivalent muscles in animals, a situation that often causes confusion.
The earliest known vertebrates were jawless fishes of the class Agnatha, and their only living representatives are the cyclostomes—the lampreys and the hagfishes. The modern agnathans retain much of the general organization of the ancestral vertebrates, and, therefore, much of their musculature is relevant to an understanding of the evolution of muscles in more-advanced vertebrates.
The cyclostomes are free-swimming animals with prominent axial somatic musculature, which during contraction produces undulating waves that propagate from head to tail to produce thrust. The axial muscles form a single segmented mass running vertically down the side of the fish. These muscle segments, known as myomeres, consist of relatively short fibres that insert into septa of connective tissue, the myocommata, between the adjacent myomeres. There is only a rudimentary axial skeleton and no appendicular skeleton, so there are no limb muscles. The eyes of cyclostomes are degenerate structures, and the six axially derived muscles normally found associated with vertebrate eyes are diminished or absent. The branchiomeric muscles in cyclostomes are represented by a sheet of constrictors that compresses the gill pouches and helps the pumping mechanism draw water through the pharynx to the gills. Other muscles of the branchiomeric series have been modified for specialized feeding functions. The branchiomeric musculature of more primitive jawless fishes would probably have been similar for each of the gill arches.
The sharks and other cartilaginous fishes (the class Chondrichthyes) have modified the structure of the first two arches; the cartilages of the anterior arch form the mandible and upper jaw (palatoquadrate), and modifications also have taken place in the second, hyoid arch. The posterior five gill arches of more primitive sharks, however, are a good model for the condition in the ancestral jawless fishes. Each arch has a visceral skeleton comprising five cartilages named, from dorsal to ventral, the pharyngobranchial, epibranchial, ceratobranchial, hypobranchial, and basibranchial. The cartilages are arranged at angles to each other. Each cartilaginous arch is provided with a set of branchial muscles that receives separate, visceral innervation. Superficially, a thin sheet comprising dorsal and ventral constrictor muscles runs in the flap of skin that covers each gill slit and forms the gill septum. Most fibres attach, dorsally and ventrally, to connective tissues (fascia) that sheath the body. Some of the deeper fibres attach to the gill bar and may run between adjacent bars. These thin, broad muscles squeeze the pharynx closed as part of the pumping action necessary for gill breathing. Dorsal and deep to this layer, a levator muscle runs from the sheathing fascia to the pharyngobranchial, and it can elevate the gill arch. In some sharks, however, the most posterior sets of levator muscles, whose fibres run diagonally down and back, may join adjacent levators, become enlarged, and attach to the pectoral girdle. This mass is known as the trapezius and evolves into the tetrapod muscle of the same name. Adductor muscles are positioned so as to close the angle between the epibranchial and ceratobranchial, and an interarcual muscle performs the same function for the angle between the pharyngobranchial and epibranchial cartilages.
In the jawed fishes, including the sharks, the axial musculature of the trunk and tail (a single block in cyclostomes) differentiates into dorsal and ventral components, which are separated by connective tissue. The dorsal block of muscle is known as the epaxial musculature, and the ventral block, the hypaxial. The epaxial block runs from the back of the skull to the end of the tail, while the hypaxial block is not present any farther forward than the pectoral (shoulder) girdle (because of the presence of the branchial [gill] apparatus). The hypaxial musculature in the tail forms a solid block of muscle, while in the trunk it encloses the body cavity. Ribs develop in the horizontal septum separating the two blocks of muscle and usually lie in the myocommata, the fascial tissue separating each myomere. In fishes, the ribs primarily serve to improve the leverage of the myomeres in producing the undulatory movements of swimming. The ribs are short in sharks but may develop to considerable length in bony fishes. Unlike the cyclostomes, where the myomeres form a series of essentially vertical strips of muscle, the myomeres of all jawed fishes are folded in a complex fashion. This development is related to the development of a more powerful swimming ability in the jawed fishes. The myomeres are folded in a zigzag pattern, projecting strongly forward halfway down the side of the fish, with a smaller, backward projection both dorsal and ventral to this point; the effect is of a W on its side. These projections become sharper and more cone-shaped deep to the surface of the fish and thus come both to be overlapped by the folds of several anterior myomeres and to overlap those of several more posterior myomeres. The folding and overlapping of myomeres has the effect that contraction of a single myomere produces curvature over a considerable part of the body of the fish. The fishes that swim faster thus tend to have a greater degree of folding and overlapping. In the tunny, for example, one myomere may have an overlap with 20 others. The undulations of the body and caudal (tail) fin produced by these axial muscles can produce much greater thrust than is produced by the beating of the appendicular fins. The latter are mostly used in slow “precision” swimming, as when a fish is investigating food, while undulations of the body are used for faster, powerful swimming. The axial musculature of fishes contributes up to half the weight of the fish, while the appendicular muscle contributes less than a fifth of the fish’s mass.
In all higher vertebrates, the most anterior element in the axial musculature is the set of six eye muscles derived from the three pre-otic somites (those anterior to the ear region of the embryo). The rectus muscles move the eyes about the longitudinal axis of the body, that is, superiorly (upward) or inferiorly (downward), or about a vertical axis, in other words, laterally (backward) or medially (forward), according to their position relative to the eyeball. The oblique muscles, superior and inferior, rotate the eyes about a transverse axis.
Jawed fishes have single midline fins and two sets of paired fins. The unpaired dorsal and anal fins of teleosts (advanced bony fishes) have axially derived muscle sheets on either side, which, when contracted, may change their angle and even fold the fins. The paired pectoral fins and the weaker pair of pelvic fins, however, have a mass of musculature both dorsal and ventral to them that is derived from mesenchymal cells. The dorsal muscle mass lifts the fin or pulls it posteriorly; the ventral mass pulls it down or forward. The two major muscle masses are attached at one end to the pectoral or pelvic girdle and on the other to the base of the fin. The amount of downward or upward movement of the fin versus the amount of backward or forward movement can be adjusted, in some fishes, by small slips of muscle derived from the major dorsal and ventral masses, which twist the fin.
The hypobranchial muscles of jawed fishes are straplike muscles running from the pectoral girdle to the structures of the visceral skeleton, the jaws, and the gill bars. Some muscles, such as the coracomandibularis, are specialized as jaw openers, although most of the work of jaw opening is done by gravity.
In bony fishes the gill septum of the hyoid arch is greatly modified to become a single, movable, bony covering for the whole gill chamber—the operculum. The individual gill septa are lost, and there is a great modification of the posterior branchial muscles, with many of the elements found in sharks (e.g., levators and adductors) becoming reduced or absent. The superficial constrictor of the hyoid arch in sharks is remodeled in bony fishes to control the opening and closing of this protective cover.
Electric organs appear to have arisen independently in several fishes. They are modifications of the axial musculature of the tail, as in the electric eel Gymnotus, a teleost, or of the muscles of the pectoral fins, as in the ray Torpedo. In a few cases electric organs lie superficially to the musculature and may be derived from modified glandular tissue, as in the Nile catfish Malapterurus.
Origins of the tetrapod limbs
The invasion of land led to a complete change in emphasis in the propulsive elements of the muscular system. In fish the axial musculature is much more important as a mover of the body than is the appendicular musculature. The evolution of land vertebrates is characterized by an increasing emphasis on the limbs for propulsion and by a corresponding de-emphasis on the axial musculature. The limbs of tetrapods are generally similar in overall pattern. Primitively at least, most major groups have similar characteristic features: the fore and hind feet have five digits; there is one bone in the proximal part of the limb (nearest to the body) and two in the distal part (away from the body); and there are a wrist or ankle joint, an elbow or knee joint, and a shoulder or hip joint. Although most muscles have several roles, the major actions of tetrapod limb muscles are similar: some primarily resist the downward force of the body at hip and shoulder, others press the supporting fore or hind feet down onto the ground at wrist or ankle or pull back on the supporting limbs (at all three joints) to create thrust, and others primarily pull the “swing” limbs forward into a new support position.
The limbs may originally have developed more as supportive struts. Structurally, the tetrapod limb can be derived from the pattern found in the paired fins of Sarcopterygii, a class of lobe-finned fishes. These were once a large radiation but have been largely replaced by the Actinopterygii, the class of ray-finned fishes. Today the lobe-finned fishes are represented by the coelacanth (Latimeria) and the lungfishes (Dipnoi). The lungfishes, denizens of shallow and seasonal waters, habitually use their fins as supports, but propulsion is largely achieved by undulations of the body, as is the case with other fish.
In the living urodeles (newts and salamanders) of the class Amphibia, the axial muscles are most important for propulsion. The limbs of urodeles are quite weak and tend to be carried forward passively with the undulations of the body. As the primary propulsive force is provided by the muscles of the trunk, urodeles retain large axial muscles. The axial muscles are still segmented, separated by myocommata, although the myomeres run vertically and without the elaborate folding seen in jawed fishes. The epaxial muscles, given the name dorsalis trunci in tetrapods, are little changed, although some modification has taken place to promote a facility for dorsoventral bending of the spine that occurs in tetrapods but rarely in fishes.
The anurans (frogs and toads) have rather similar, but considerably reduced, epaxial muscles. There is, however, a trend in tetrapods toward finer control of muscular action with increasing complexity. In reptiles the epaxial muscles, although still retaining a semi-segmental structure, are divided into several structural and functional units. The deepest set of muscles, the transversospinalis group, are short and run obliquely forward, over one to four vertebrae, from the transverse process of one vertebra to the lamina (the flat plate of bone at the base of the vertebral spine) of a more anterior vertebra. The transversospinalis group is particularly responsible for rotatory movements of the spine. Superficial to transversospinalis lies longissimus, with much longer fibres, which is important in extension of the back. More superficial still and lateral to these muscle blocks is iliocostalis, a flat sheetlike muscle that runs from the pelvic girdle upward and laterally to attach to the ribs. It is particularly important in lateral flexion (bending) of the spine. This general pattern is further complicated in snakes, which have secondarily returned to the propulsive use of the axial muscles. In birds the vertebral column of the trunk region undergoes much fusion, and this complexity is reduced, as indeed it is in chelonians (turtles and tortoises). Mammals retain the broad pattern of the reptile epaxial musculature but (with the exception of the innervation of the musculature) have greatly reduced the segmentation that is present in reptiles.
In the tails of urodeles the hypaxial muscles are also largely unchanged. As with all land vertebrates, however, the demands of supporting the viscera when living in an air environment have brought about major modifications of the hypaxial musculature of the trunk. In typical tetrapods a strong series of ribs has developed for the same reason. Although urodeles have secondarily reduced their ribs, they show many of the typically tetrapod features of the hypaxial musculature. The muscles fall into three groups. A group of subvertebral muscles forms ventral to the vertebrae, in the region of their joints with the ribs at the transverse processes. It acts in ventral and lateral flexion (bending) of the spine. A rectus abdominis muscle runs longitudinally along the ventral aspect of the body wall between the pectoral and pelvic girdles, and laterally this muscle is associated with the third group, the lateral hypaxial muscles. The third group consists of three major layers of muscle whose fibres are oriented in differing directions, a feature that gives additional strength to the body wall. Superficially lies the external oblique muscle, with fibres running longitudinally but somewhat ventrally; deep to this lies the internal oblique, with fibres running longitudinally and somewhat dorsally; and deepest lies the transversus muscle, whose fibres run dorsoventrally.
In the higher tetrapods the external and internal obliques tend to become further divided into layers in the abdominal region. The thoracic representatives of these muscles tend to become divided into discontinuous, rather thin muscle layers between the ribs (external and internal intercostals), superficial to the ribs (supercostals), and deep to the ribs (subcostals). While only the rectus abdominis tends to retain visible evidence of segmental origin, in its tendinous intersections (which are present even in humans) the segmental innervation of the hypaxial muscles is retained in all tetrapods.
In tetrapods, unlike fishes, the pectoral girdle does not have a solid bony connection to the axial skeleton but rather is supported by a series of muscles derived from the outer layer of hypaxial trunk muscles. This is no doubt another adaptation to life in an air environment, where the cushioning effect of water has been lost. These muscular slings are not readily demonstrated in the living amphibians, which are either skeletally degenerate, as in urodeles, or highly specialized toward leaping, as in anurans (frogs and toads). In more typical tetrapods, there are two major derivatives of the external oblique attaching the scapula (shoulder blade) to the body: the serratus, made up of numerous fingerlike slips running from the scapula to the neighbouring ribs, and the levator scapulae, which are fused with serratus along its caudal (tail-end) border. Levator scapulae consist of fibres running more anteriorly to ribs or transverse processes of the neck. Mammals and some reptiles have a third such muscle, attaching the pectoral girdle to the region of the spine, called rhomboideus. Mammals also have utilized part of the hypaxial musculature to form a muscular septum between the region of the lungs and heart (the thoracic cavity) and the region of the digestive and reproductive viscera (the abdominal cavity). This is the diaphragm, which is the most important respiratory muscle in the mammalian body.
The six axially derived eye muscles of fishes undergo only small modifications in tetrapods. Eye movements are changed, partly according to changes in the orientation of the orbit, such as the trend toward orbital frontality that is typical in primates. Additional eye muscles may be derived by splitting some of these six muscles. An example of this is the retractor bulbi muscle, which is derived from the lateral rectus muscle. In amphibians and some reptiles it pulls the eyeball deeper into the orbit for protection, and in amphibians it is an aid in swallowing. Another example is the levator palpebrae superioris, derived from the superior rectus, which elevates the upper eyelid to open the eye.
The limb muscles of typical tetrapods are derived from the dorsal and ventral muscle blocks of the paired fins of fishes. In tetrapod development, this pattern of derivation from dorsal and ventral muscle blocks is repeated. As a consequence, the homologies of the muscles of the typical tetrapod limb often can be traced by considering the source of innervation of each muscle from the nerves of the dorsal (or extensor) compartment or the ventral (or flexor) compartment.
In the pectoral limb the dorsal, extensor group of muscles includes several that appear consistently and with similar roles. Beginning with the muscles that act on the humerus (the proximal bone of the limb), all tetrapods have a large sheetlike muscle known as the latissimus dorsi that runs from the side of the trunk to the humerus. The latissimus dorsi muscle retracts the humerus and thus propels the body forward. Acting to rotate, flex, or adduct the humerus, depending on limb posture, is a muscle known as subcoracoscapularis in amphibians, reptiles, and birds and as subscapularis in mammals. It runs from the deep surface of the shoulder girdle to the humerus. In amphibians the dorsalis scapulae arise from the anterior edge of the scapula. The same muscle is known as the deltoideus in reptiles and mammals; in the latter, part of its origin moves from the scapula to the clavicle (collarbone). It is a major abductor of the shoulder in most tetrapods. At the elbow joint, all tetrapods have a muscle called triceps as the major extensor. It arises in several heads from the shoulder girdle and humerus. There are always a variable number of extensor muscles for the wrist and digits (fingers and toes) arising from the region of the elbow joint, on the lateral aspect of the humerus.
On the ventral, flexor aspect of the pectoral limb, the pectoralis is found in all tetrapods. The pectoralis runs from the chest wall to the humerus, on which it acts to pull the humerus downward and backward. This muscle not only is important in providing forward thrust in quadrupedal locomotion but is the chief depressor of the forelimb in birds and bats. The major elevator of the wing in birds, supracoracoideus, is present in all tetrapods. In mammals the supracoracoideus retains its attachment to the humerus, but its previous point of origin (the coracoid plate) disappears, and the muscle now appears as two separate blocks of muscle arising on either side of the spine of the scapula as an abductor muscle (supraspinatus) and a rotator and flexor (infraspinatus). Coracobrachialis and (except in amphibians) biceps arise from the tip of the coracoid and act to flex the elbow. In this they are aided by the brachialis muscle, which arises from the humerus. As on the extensor aspect, there are always a number of flexors of the wrist and digits. These arise on the medial side of the distal humerus.
The muscles of the pelvic limb cannot be readily compared beyond the reptiles and mammals. Even in these cases, changes in limb posture have led to major changes in the arrangement and function of muscles. On the dorsal aspect, a single large muscle in reptiles, puboischiofemoralis, runs from the bones of the pelvis to the femur (the proximal bone of the hind limb). This reptilian muscle appears to be represented by three mammalian hip muscles: psoas, iliacus, and pectineus. Iliofemoralis acts as an abductor of the hip in reptiles and appears to be represented by the gluteal muscles in mammals, but the function of the gluteal muscles is different. More similar in reptiles and mammals is the quadriceps or quadratus femoris, which consists of multiple heads (four in mammals) that arise from the pelvic girdle and femur and insert by a common tendon into the tibia (the larger bone of the distal pectoral limb). It is the sole extensor of the knee joint in both the reptiles and the mammals. The extensors of the ankle and digits in both reptiles and mammals are not dissimilar to those of the pectoral limb and take origin from the lateral and anterior surfaces of the two distal bones of the pelvic limb. On the ventral aspect of the hind limb, small, deep muscles run from the internal and external pelvis to the head of the femur and help in adduction and rotation. Of these, the puboischiofemoralis externus of reptiles appears to be represented by the obturator externus of mammals, and, similarly, the ischiotrochantericus of reptiles appears to be the homologue of the obturator internus of mammals. Again, the major adductor of the hip of reptiles, adductor femoris, appears to be homologous with some of the muscles called the adductors in mammals. There seem to be some homologies between the major flexors of the hip and thigh in reptiles, such as puboischiotibialis, and two deeper muscles, flexor tibialis externus and internus, and some functionally equivalent muscles in mammals: the gracilis, semimembranosus, and semitendinosus. In reptiles the axial muscle of the tail is strong, and the caudofemoralis, a powerful flexor of the thigh that originates in the tail, is consequently large. The tail in mammals, although usually present, is much more gracile, and, as a result, caudofemoralis is represented by only a few small muscles. Another major change is in the flexors of ankle and digits. In reptiles, these insert by long tendons passing below the ankle joint, much as in the forelimb. In mammals, however, the equivalent long flexor, gastrocnemius, inserts on a new bony process, the calcaneal tuberosity, or heel bone, which gives more efficient leverage.
The hypobranchial muscles of tetrapods are both reduced and modified in comparison with those of jawed fishes. In tetrapods these straplike muscles still arise from elements of the pectoral girdle but now pass to the new derivatives of the gill arches of fishes: the hyoid bone and laryngeal cartilages. They act primarily in the gross movements of these structures in swallowing and the production of sound—for example, as depressors of the hyoid (sternohyoid, omohyoid) or of the larynx (sternothyroid). Fibres of the hypobranchial muscles in the region of the hyoid are utilized to form the internal musculature of the tongue.
The branchial musculature is also modified in tetrapods from the condition seen in jawed fishes. The development of a shoulder muscle, the trapezius, from the levator muscles of the gill arches of fishes, as previously discussed, is taken further in tetrapods by the separation of further slips of muscle to form muscles such as sternocleidomastoid, a muscle important for humans in movements of the head and in breathing. In mammals that lose the clavicle, these slips may be further modified to form muscles running from the head to the pectoral limb. Tetrapods, with the exception of mammals, utilize part of the constrictor muscle of the hyoid arch to form the depressor mandibulae, which replaces the hypobranchial muscles as the major jaw-opening muscle. The restructuring of the posterior jaw in mammals leads to the further replacement of this new muscle by the digastric, which is a compound muscle made up of parts of the constrictors of the first and second branchial arches. Thus, it is partly innervated by the mandibular division of the fifth cranial nerve (as is the case with other jaw muscles and the tensor tympani, one of the muscles of the ear) and partly by the seventh cranial nerve, the facial nerve (which also supplies an ear muscle associated with the stapes, an ear bone derived from the hyoid arch). The levator palatoquadrati, which elevates the upper jaw in jawed fishes, is retained as a jaw muscle in birds and in some reptiles, as they share the ability of fishes to move the upper jaw. The adductor mandibulae is much altered in tetrapods, although its overall function is retained. During the course of tetrapod evolution, it becomes a superficial muscle, and in mammals it splits into several functional units arising from the undersurface and side of the skull and attaching to various points on the mandible. These are the lateral pterygoid, which pulls the jaw forward; the medial pterygoid and its partner, the masseter, which close the jaw and move it from side to side; and the temporalis, which closes the jaw and pulls it backward. All are innervated by the first-arch cranial nerve, the fifth nerve. The intermandibularis of jawed fishes is retained as the mylohyoid of tetrapods, which is an elevator of the tongue.
Finally, the constrictor muscle of the hyoid arch, which in bony fishes is used to control the operculum, is remodeled in tetrapods as a sheathing superficial muscle of the neck, the sphincter colli. It derives its innervation from the nerve of the hyoid arch, the seventh, or facial, nerve. This cranial nerve is named from the further adaptation of the sphincter colli muscle in mammals, particularly in higher primates, as the many small muscles of facial expression, which allow people to smile, laugh, and frown.
Primitive contractile systems
Cilia and flagella
Unicellular organisms such as the paramecium, a protozoan that lives in freshwater ponds and streams, propel themselves by the action of cilia. Cilia occur in large numbers and move in a coordinated way. Ciliated cells within the vertebrate body propel fluid and mucus along interior passages, such as the lining of the respiratory tract.
Flagella are structurally similar to cilia, except that they are longer (sometimes up to 50 times longer) than cilia and usually number only one or two per cell. Sperm cells of most higher organisms move using flagella. Many types of unicellular algae and protozoans use flagella in swimming through water.
Both cilia and flagella contain a regular pattern of tubules extending along their lengths; there is an outer ring of nine pairs of tubules surrounding a central pair of tubules. Each tubule is composed of filaments comprising a string of globular subunits. The movement of a cilium or a flagellum requires energy, which is obtained from the breakdown of adenosine triphosphate (ATP), catalyzed by a protein attached to the outer tubules, dynein.
Some types of bacteria have flagella whose motion seems to depend on a cellular particle called the basal body, to which the flagellum is attached. Such flagella derive their energy from a difference in hydrogen ion concentration across the cell membrane.
Amoeboid movement occurs as an extension of the cytoplasm, called a pseudopod (“false foot”), flows outward, deforms the cell boundary, and is followed by the rest of the cell. Many pseudopodia may be formed at the same time, and their actions do not seem to be coordinated.
Although amoeboid motion is characteristic of the amoeba, a unicellular protozoan, it is also found in nonmuscle cells of multicellular organisms. These cells contain myosin and actin, which differ in some aspects of their structure from the corresponding proteins in muscles because of variations in the genes that encode them.
Striated, or striped, muscle constitutes a large fraction of the total body weight in humans. Striated muscle contracts to move limbs and maintain posture. Both ends of most striated muscles articulate the skeleton and thus are often called skeletal muscles. They are attached to the bones by tendons, which have some elasticity provided by the proteins collagen and elastin, the major chemical components of tendons.
Each striated muscle has blood vessels and nerves associated with it. The vessels transport blood to and from the muscle, supplying oxygen and nutrients and removing carbon dioxide and other wastes. The signals that initiate contraction are sent from the central nervous system to the muscle via the motor nerves. Muscles also respond to hormones produced by various endocrine glands; hormones interact with complementary receptors on the surfaces of cells to initiate specific reactions. Each muscle also has important sensory structures called stretch receptors, which monitor the state of the muscle and return the information to the central nervous system. Stretch receptors are sensitive to the velocity of the movement of the muscle and the change in length of the muscle. They complete a feedback system that allows the central nervous system to assess muscular movement and to adjust motor signals in light of the movement.
The muscle fibre
Muscle is composed of many long cylindrical-shaped fibres from 0.02 to 0.08 mm in diameter. In some muscles the fibres run the entire length of the muscle (parallel fibres), up to several tens of centimetres long. In others a tendon extends along each edge, and the fibres run diagonally across the muscle between the tendons (pennate fibres). Considerable variation can be found among the different skeletal muscles, the actual arrangement of the fibres depending on the function of the muscle.
There is a high degree of organization within the fibre, a series of alternately dark and light bands. Each band extends perpendicular to the length of the fibre. Each fibre is surrounded by a complex multilayered structure called the sarcolemma. The outermost layer is a fine network of fibrils, which, at the ends of the muscle, extend into the tendons and form the structural link with them. The next layer of the sarcolemma is a foundation, or basement, membrane. The innermost layer is a plasma membrane similar to the ones that surround most cells. The plasma membrane consists of a lipid bilayer with proteins embedded in it. Some of the proteins are embedded entirely within the lipid layer, others extend to one or the other surface, and still others span the whole width of the two layers. These proteins represent enzymes, receptors, and various channels (such as those involved in the movement of ions between the exterior and interior of the cell). The plasma membrane maintains the electrical potential, which plays a major role in stimulating muscle contraction.
Sarcoplasm is the cytoplasm of a muscle fibre. It is a water solution containing ATP and phosphagens, as well as the enzymes and intermediate and product molecules involved in many metabolic reactions. The most abundant metal in the sarcoplasm is potassium. Sodium and magnesium are present in lower concentrations. Most of the calcium of muscle is bound to proteins or stored in the sarcoplasmic reticulum. Contraction is initiated by the release of calcium ions (Ca2+) upon the depolarization of the membrane, which is induced by nerve impulses.
Each striated muscle cell, or fibre, contains many nuclei. This is the result of the fusion of singly nucleated cells that occurs during the embryological development of striated muscle. After fusion, the cells never again divide.
Mitochondria in the sarcoplasm of the muscle fibre contain the enzymes involved in the Krebs cycle and in oxidative phosphorylation. Granules in the sarcoplasm of muscle cells contain glycogen, the storage form of carbohydrate. The breakdown of glycogen and the metabolism of the individual units of the resulting carbohydrate through glycolysis, the Krebs cycle, and oxidative phosphorylation are important sources of ATP, the immediate source of energy for muscle contraction. Muscles that contain many fibres that operate at a steady, low level of activity are red, due to the presence of cytochromes (molecules involved in oxidative phosphorylation) and myoglobin (an oxygen-carrying molecule in the sarcoplasm). Muscles that work in bursts of activity contain fibres that have fewer mitochondria and fewer molecules of cytochromes or myoglobin, are white, and depend more heavily on reactions that do not require oxygen to make ATP.
Electron micrographs of thin sections of muscle fibres reveal groups of filaments oriented with their axes parallel to the length of the fibre. There are two sizes of filaments, thick and thin. Each array of filaments, called a myofibril, is shaped like a cylindrical column. Along the length of each myofibril alternate sets of thick and thin filaments overlap, or interdigitate, presenting alternate bands of dark regions (with thick filaments and overlapping thin ones) and light regions (with only thin filaments). Within a fibre all the myofibrils are in register, so that the regions of similar density lie next to each other, giving the fibre the characteristic striated appearance it shows in the phase-contrast or polarized light microscope. Each light region is divided in two by a dark band. The unit between two dark bands is known as a sarcomere.
Each myofibril is about one or two micrometres (1 micrometre = 10−6 metre) in diameter and extends the entire length of the muscle fibre. The number of myofibrils per fibre varies. At the end of the fibre, the myofibrils are attached to the plasma membrane by the intervention of specialized proteins.
Forty to 80 nanometres (nm) usually separate adjacent myofibrils in a fibre. This space contains two distinct systems of membranes involved in the activation of muscle contraction (Figure 7: Ultrastructure of a group of myofibrils, showing the sarcoplasmic reticulum and … [Credit: Encyclopædia Britannica, Inc.]Figure 7). One system is a series of channels that open through the sarcolemma to the extra-fibre space. These channels are called the transverse tubules (T tubules) because they run across the fibre. The transverse tubular system is a network of interconnecting rings, each of which surrounds a myofibril. It provides an important communication pathway between the outside of the fibre and the myofibrils, some of which are located deep inside the fibre. The exact spatial relationship of the tubules to the filaments in the myofibril depends on the species of animal.
The other membrane system that surrounds each myofibril is the sarcoplasmic reticulum, a series of closed saclike membranes. Each segment of the sarcoplasmic reticulum forms a cufflike structure surrounding a myofibril. The portion in contact with the transverse tubule forms an enlarged sac called the terminal cisterna.
In most vertebrates each transverse tubule has two cisternae closely associated with it, forming a three-element complex called a triad. The number of triads per sarcomere depends on the species; for example, in frog muscle there is one per triad, and in mammalian muscle there are two. In fishes and crustaceans, only one cisterna is associated with each transverse tubule, thus forming a dyad. The sarcoplasmic reticulum controls the level of calcium ions in the sarcoplasm. The terminal cisternae apparently are the sites from which the calcium ions are released when the muscle is stimulated, and the longitudinal tubules are the sites at which calcium ions are effectively removed from the sarcoplasm. The removal of calcium ions (Ca2+) from the sarcoplasm is accomplished by a protein that catalyzes the breakdown of ATP, making the free energy of hydrolysis available for the energy-requiring process of Ca2+ transport.
As mentioned earlier, the myofibril is a columnlike array of filaments. In a longitudinal section through a group of myofibrils (Figure 7), there is a light band of low density called the I band. In the centre of the I band there is a prominent dense line called the Z line, although in reality, considering the three-dimensional structure of the myofibril, it is more appropriate to speak of Z disks. The area between two Z lines, a sarcomere, can be considered to be the primary structural and functional unit directly responsible for muscle contraction. The myofibril can thus be thought of as a stack of sarcomeres. The A band, which contains thick filaments partly overlapped with thin filaments, appears dark.
At high magnification, small bridgelike structures can be seen on the thick filaments extending toward the thin filaments in the overlap region. They are called cross bridges and are believed to be responsible for the movement and force developed during contraction (for the relation of cross bridges to the molecular architecture of thick filaments, see below). In the middle of the A band, where only thick filaments are present, is a region called the H zone; the H zone looks somewhat lighter than the overlap region of the A band. Also in the A band is a narrow, lightly stained region that contains bare thick filaments without cross bridges and is called the pseudo-H zone. In the centre of the A band is a narrow, darkly stained region called the M band, in which occur fine bridges between the thick filaments. These bridges differ from the cross bridges between the thick and thin filaments and are in fact composed of an entirely different protein.
If cross sections of the myofibril at different levels of the sarcomere are examined by electron microscope, the filaments can be seen end-on, and the three-dimensional nature of the lattice of filaments can be appreciated. The I band contains only thin filaments, with a diameter of 6 to 8 nm. In the A band, in the overlap region, the thin filaments appear with thick ones (diameter of 12 nm) in an extremely regular pattern or lattice. In vertebrates the thick filaments are arranged in a hexagonal lattice, and the thin ones are located at the centre of the equilateral triangles formed by the thick filaments. Sections through the H zone contain only thick filaments arranged in the same hexagonal pattern they form in the overlap region. In the M band the hexagonal array of thick filaments can be seen with M bridges running between them.
Sliding of filaments
The discovery that during contraction the filaments do not shorten but that the two sets—thick and thin—merely move relative to each other is crucial for our current understanding of muscle physiology. During contraction the thin filaments move deeper into the A band, and the overlap of the thick and thin filaments increases. If a longitudinal section of the sarcomere is considered, the thin filaments on the left side of the A band would move to the right into the A band, and the filaments on the right of the A band would move to the left into the A band. Directionality of the motion partly results from the structural polarity of both the thick filaments, since in the two halves of the filament the myosin molecules are oriented in opposite directions, and the actin filaments, in which the actin molecules are oriented with respect to the Z bands.
Proteins of the myofilaments
To understand the finer structural details of the myofilaments and the mechanism by which sliding, and ultimately muscle contraction, occurs, one must understand the molecular components of the filaments and of the structures associated with them. The myofilaments are composed of several different proteins, constituting about 50 percent of the total protein in muscle. The other 50 percent consists of the proteins in the Z line and M band, the enzymes in the sarcoplasm and mitochondria, collagen, and the proteins in membrane structures. Of the myofilament proteins, myosin and actin are known to play a direct part in the contractile event. Troponin and tropomyosin, which are located in the thin filaments together with calcium ions, regulate contraction by controlling the interaction of myosin and actin.
The main constituent of the thick filaments is myosin. Each thick filament is composed of about 250 molecules of myosin. Myosin has two important roles: a structural one, as the building block for the thick filaments, and a functional one, as the catalyst of the breakdown of ATP during contraction and in its interaction with actin as part of the force generator of muscle. The individual myosin molecule contains two major protein chains and four small ones, the entire molecule being about 160 nm in length and asymmetrically shaped. The rodlike tail region, about 120 nm long, consists of two chains of protein, each wound into what is known as an α-helix, together forming a coiled-coil structure. At the other end of the molecule, the two protein chains form two globular headlike regions that have the ability to combine with the protein actin and carry the enzymatic sites for ATP hydrolysis.
Thick filament assembly
In the middle portion of the thick filament, the molecules are assembled in a tail-to-tail fashion. Along the rest of the filament, they are arranged head to tail. The tail parts of the molecules form the core of the filament; the head portions project out from the filament. The cross bridges are actually the globular head regions of myosin molecules extending outward from the filament, and the smooth pseudo-H zone is the region of tail-to-tail aggregation, in which there are only tails and no heads.
The precise three-dimensional arrangement of the cross bridges projecting from the thick filament cannot be seen easily in electron micrographs but can be determined from X-ray diffraction study of living muscle. The three bridges project 120 degrees from the opposite sides of the filament every 14.3 nm along the length of the filament. Each successive set of bridges is located in a position rotated 40 degrees farther around the filament. The pattern of nine bridges (three sets of three bridges) repeats itself every 42.9 nm along the thick filament. Some variation may exist from species to species and muscle to muscle.
Thin filament proteins
The thin filaments contain three different proteins—actin, tropomyosin, and troponin. The latter is actually a complex of three proteins.
Actin, which constitutes about 25 percent of the protein of myofilaments, is the major component of the thin filaments in muscle. An individual molecule of actin is a single protein chain coiled to form a roughly egg-shaped unit. Actin in this form, called globular actin or G-actin, has one calcium or magnesium ion and one molecule of ATP bound to it. Under the proper conditions, G-actin is transformed into the fibrous form, or F-actin, that exists in the thin filament in muscle. When the G-to-F transformation takes place, the ATP bound to G-actin breaks down, releasing inorganic phosphate (Pi) and leaving an adenosine diphosphate (ADP) molecule bound to each actin unit. Actin molecules repeat every 2.75 nm along the thin filament. They give rise to a helical structure that can be viewed as a double or single helix. The apparent half-pitch is about 40 nm long. Actin is believed to be directly involved in the process of contraction because the cross bridges can become attached to it.
Tropomyosin is a rod-shaped molecule about 40 nm long. Two strands of tropomyosin molecules run diametrically opposed along the actin filaments. Tropomyosin has a structure similar to that of the myosin tail, being a coiled unit of two protein chains. Each tropomyosin molecule is in contact with seven actin units.
Troponin is a complex of three different protein subunits. One troponin complex is bound to every tropomyosin molecule. A troponin molecule is located approximately every 40 nm along the filament. Troponin and tropomyosin are both involved in the regulation of the contraction and relaxation of muscles. One of the subunits (TnC) is the receptor for Ca2+ released from the sarcoplasmic reticulum on activation of the muscle. It is thought that calcium binding then causes further structural changes in the interaction of actin, tropomyosin, and another troponin subunit (TnI) that lead to contraction by activating the actin-myosin interaction.
Actin-myosin interaction and its regulation
Mixtures of myosin and actin in test tubes are used to study the relationship between the ATP breakdown reaction and the interaction of myosin and actin. The ATPase reaction can be followed by measuring the change in the amount of phosphate present in the solution. The myosin-actin interaction also changes the physical properties of the mixture. If the concentration of ions in the solution is low, myosin molecules aggregate into filaments. As myosin and actin interact in the presence of ATP, they form a tight compact gel mass; the process is called superprecipitation. Actin-myosin interaction can also be studied in muscle fibres whose membrane is destroyed by glycerol treatment; these fibres still develop tension when ATP is added. A form of ATP that is inactive unless irradiated with a laser beam is useful in the study of the precise time course underlying contraction.
If troponin and tropomyosin are also present, however, the actin and myosin do not interact, and ATP is not broken down. This inhibitory effect corresponds to the state of relaxation in the intact muscle. When calcium ions are added, they combine with troponin, inhibition is released, actin and myosin interact, and ATP is broken down. This corresponds to the state of contraction in intact muscle. The exact mechanism by which troponin, tropomyosin, and calcium ions regulate the myosin-actin interaction is not fully agreed upon. In the thin filament there are one troponin and one tropomyosin molecule for every seven actin units. According to one view, Ca2+ binding to troponin (actually the TnC subunit) induces a change in the position of tropomyosin, moving it away from the site where myosin also binds (steric blocking). Alternatively, the calcium-induced movement of tropomyosin in turn induces changes in the structure of actin, permitting its interaction with myosin (allosteric model). In smooth muscles, Ca2+ activates an enzyme (kinase) that catalyzes the transfer of phosphate from ATP to myosin, and the phosphorylated form is then activated by actin.
A somewhat different scheme of regulation operates in the muscle of mollusks. As in vertebrate muscles, calcium ions act as the initiator of contraction. The difference is that the component that binds calcium ions in the molluscan muscle is myosin rather than a component of the actin-containing thin filaments. The interaction of actin and myosin provides a basis for molecular models of force generation and contraction in living muscle.
The neuromuscular junction
The signal for a muscle to contract originates in the nervous system and is transmitted to the muscle at the neuromuscular junction, a point of contact between the motor nerve and the muscle. In higher organisms each muscle fibre is innervated by a single motor nerve fibre; in other species (e.g., crustaceans) inhibitory fibres are also present. As the nerve approaches the muscle, it loses its myelin coat but remains partially covered by processes of the Schwann cells, which elsewhere surround the nerve and produce myelin. The nerve then branches several times, indenting the surface of the muscle to form the end plate that occupies only a small region of the total surface area of the muscle. The narrow (50 nm) synapse separates the nerve from the muscle and contains the basement membrane (basal lamina). In the subneural region the muscle membrane is deeply folded, forming secondary synaptic clefts into which the basement membrane penetrates.
The neural signal is an electrical impulse that is conducted from the motor nerve cell body in the spinal cord along the nerve axon to its destination, the neuromuscular junction. No electrical continuity exists between the nerve and the muscle; the signal is transmitted by chemical means that require specialized presynaptic and postsynaptic structures.
Storage of acetylcholine in the nerve terminal
The nerve terminal contains many small vesicles (membrane-enclosed structures) about 50 nm in diameter, each of which contains 5,000–10,000 molecules of acetylcholine. Mitochondria are also present, providing a source of energy in the form of ATP. Acetylcholine is formed in the nerve terminal from choline and acetyl-CoA through the catalytic action of the enzyme choline acetyltransferase. Choline is obtained by the active uptake of extracellular choline, a breakdown product of previously released acetylcholine. Concentrations of acetylcholine (and ATP) are several hundredfold lower in the cytoplasm than in the vesicles. Packaging of the transmitter into the vesicles occurs within the nerve terminal and is an energy-requiring process.
Release of acetylcholine from the nerve terminal
The vesicles cluster close to specialized regions of the nerve terminal membrane called active zones. Freeze-fracture electron microscopy reveals an orderly array of small particles (about 10 nm in diameter) within these active zones, which are believed to represent voltage-gated calcium channels. The channels are opened by depolarization (an increase in membrane potential) of the nerve terminal membrane and selectively allow the passage of calcium ions.
The nerve impulse is a wave of depolarization traveling along the axon of the motor nerve such that the resting membrane potential of about −70 millivolt is reversed, becoming briefly positive. At the nerve terminal, the nerve impulse causes voltage-gated calcium channels at the active zones to open until depolarization subsides. This allows calcium ions to enter the nerve terminal along their concentration gradient. The region of raised calcium concentration within the nerve terminal is localized close to the active zones and, by a process that is not yet understood, causes vesicles in this region to fuse with the nerve terminal membrane and to open outward (exocytosis), thereby discharging their contents into the synaptic cleft. A nerve impulse causes the release of about 50–100 vesicles of acetylcholine in humans and somewhat more in some other species.
At high rates of stimulation, sufficient to cause a smooth contraction (tetanus) of the muscle, the quantity of transmitter released per impulse declines for the first few impulses (synaptic depression), which may be due to a reduction in the number of vesicles ready for release.
Following the voltage-dependent influx of calcium into the nerve terminal, it is necessary for calcium to be removed to prevent continuous discharge of neurotransmitter. Mechanisms underlying this process are likely to involve sodium-calcium exchange across the nerve terminal membrane and possibly calcium uptake by mitochondria.
Acetylcholine is released from the nerve terminal by two other processes, independently of the nerve impulse. Neither of these processes leads to muscle contraction. The first occurs spontaneously when individual vesicles randomly fuse with the nerve terminal membrane and discharge their contents, generating a small potential change (about 0.5–1 millivolt), the miniature end plate potential. This potential is below the threshold at which an action potential is triggered in the muscle cell and thus does not lead to muscle contraction. The frequency of such events varies; in humans they occur at each end plate about once every five seconds. The second process of acetylcholine release occurs as a continuous “molecular leakage” of neurotransmitter from the nerve terminal rather than from vesicles. The overall amount released in resting muscle by this means greatly exceeds the spontaneous release of individual vesicles.
The acetylcholine molecules diffuse across the synaptic cleft and react with the acetylcholine receptors. The number of available acetylcholine binding sites greatly exceeds the number of acetylcholine molecules released. Acetylcholine is either rapidly broken down by the enzyme acetylcholinesterase, which is anchored in the basement membrane, or diffuses out of the primary cleft, thus preventing constant stimulation of acetylcholine receptors. Drugs that inactivate acetylcholinesterase and thereby prolong the presence of acetylcholine in the cleft can lead to repetitive firing of the muscle cell in response to a single nerve stimulus.
Acetylcholine receptors are ion channels that span the postsynaptic membrane, and they have extracellular, intramembranous, and cytoplasmic portions. They are located principally over the peaks of the postsynaptic folds, where they are present at high density. They consist of five subunits arranged around the central ion channel.
The supply of junctional acetylcholine receptors is continuously renewed. Receptors are internalized by the muscle cell and degraded in lysosomes (specialized cytoplasmic organelles), while new receptors are synthesized and inserted into the muscle membrane.
In normally innervated muscle, receptors are confined to the neuromuscular junction. In non-innervated fetal muscle and in denervated adult muscle, however, acetylcholine receptors are found elsewhere as well. These receptors have slightly different properties from junctional receptors, notably a much higher rate of turnover.
Acetylcholine-acetylcholine receptor interaction
The resting membrane potential of the muscle cell is held at about −80 millivolt. Binding of acetylcholine to its receptor causes the receptor molecule to alter its configuration so that the ion channel is opened for about one millisecond (0.001 second). This permits the entry of small positive ions, mainly sodium. The resulting local depolarization (the end plate potential) causes voltage-gated sodium channels located around the end plate to open. At a critical point (the firing threshold for the muscle cell) a self-generating action potential is triggered, causing the membrane potential to reverse and become briefly positive. The action potential propagates over the muscle fibre membrane to activate the contractile process.
The amplitude of the end plate potential is normally sufficient to bring the membrane potential of the muscle cell well above the critical firing threshold. The extent to which it does so represents a “safety factor” for neuromuscular transmission. The safety factor will be reduced by any event that, by interfering with presynaptic or postsynaptic function, reduces the size of the end plate potential.
The physical aspects
Vertebrates are able to move about and to exert and bear forces because of the contraction of the striated muscles. These activities usually involve several structures operating in different ways. The skeleton to which the muscles are attached operates as a lever system. When a muscle shortens, it moves the joints that it spans. In addition, in coordinated movement usually several muscles contract in different ways. As some muscles shorten, others develop a force while at a fixed length, and still others may be lengthened by an external force even as they contract.
The force that a muscle develops is a “pulling” force, never a “pushing” force. If the load is small enough, the muscle can shorten and produce a pulling motion (an isotonic condition). If the load is just equal to the maximum force the muscle can develop, the length of the muscle will remain the same (an isometric condition). An even larger load will stretch the muscle.
The size and the rate of the mechanical responses to stimulation, whether by a nerve in the body or by direct electrical shocks of an isolated muscle, depend on the muscle and the temperature. In a frog sartorius muscle (of the leg) at 0 °C (32 °F), the action potential reaches its peak of depolarization about 1.5 milliseconds after the stimulus.
The very early tension changes require much more rapid and sensitive measuring and recording instruments than are necessary for studying other aspects of the contraction process. The latent period, the first seven milliseconds, is the amount of time needed for the electrical signal, which appears as an action potential at the surface membrane, to be translated and to travel to the contractile apparatus within the muscle fibres. The explanation for latency relaxation (a four-millisecond period during which the tension drops slightly), however, is not so clear. It may be related to a change in shape of the sarcoplasmic reticulum, which releases a large amount of calcium ions at about the time latency relaxation occurs. The tension begins to rise after 15 milliseconds.
Twitch and tetanus responses
Skeletal muscles respond to a single electric shock of sufficient magnitude by rapid, intense contractions called phasic contractions. If the ends of a frog sartorius muscle (at 0 °C) are fixed to prevent shortening, the tension increases for about 200 milliseconds and then begins to decrease, at first rather rapidly and then more slowly. More happens during this mechanical response to a single stimulation, called a twitch, than the tension record suggests.
The mechanical response to repeated stimulation depends on the rate of the stimulation. Muscle, like other excitable tissues, has a period following its action potential during which the membrane will not respond to stimulation regardless of the strength. This absolute refractory period in the frog sartorius at 0 °C lasts about 10 milliseconds after stimulation. Therefore, a second pulse within that time span will not elicit any response. If, however, the pulses are 300 milliseconds apart, the muscle will be relaxing when the second pulse is given, and the tension will appear in waves in phase with the stimulation, causing an unfused tetanus. It is possible to stimulate the muscle at a frequency between these extremes so that the tension developed by the muscle remains constant. This latter type of contraction is called a fused tetanus, and the rate of stimulation that produces it is called the fusion frequency. The exact rate depends upon the particular muscle and the temperature.
Usually, the maximum tetanus tension is from 1.2 to 1.8 times greater than the maximum tension during a twitch. Within the muscle, many elastic structures, connected in series with the contractile elements, are stretched during contraction. The attachment of the muscle fibres to the tendons at the end of the muscle and the attachment of the thin filaments to the Z line contribute to this elastic component. In single fibres, however, most of the elasticity of the series of elastic elements is contributed by the actin-myosin cross bridges themselves. Full maximum tension is not apparent at the end of the muscle until the contractile elements have shortened enough to stretch the elastic elements—somewhat like taking up the slack in a rope before a pull on one end can be felt at the other end. In a twitch, the activity of the muscle is so brief that the contractile elements cannot extend the elastic elements completely before relaxation begins; as a result, the tension at the ends of the muscle does not reach the maximum possible level. During a tetanus, on the other hand, the activity of the contractile elements is maintained, and they can eventually shorten enough to extend fully the series of elastic elements. When this has been accomplished, the maximum tension is apparent at the ends of the muscle.
The force developed by a muscle, whether it is contracting or resting, is strongly dependent on the length of the muscle. Resting skeletal muscle does not exert any force at lengths less than the normal length of the resting muscle in the body. When resting skeletal muscle is extended somewhat beyond the normal length of the muscle, however, a passive force begins to assert itself. The exact length at which this passive force occurs depends on the particular muscle. This force is characterized as passive because it is developed in noncontracting or inactive muscles by the elastic elements of the muscle.
When a muscle is to lift a constant load (isotonic conditions) after stimulation starts, the force increases, just as in an isometric contraction, and, when the force is equal to the load, the muscle begins to shorten and lifts the load. When both the activity of the muscle and the force in it begin to decline, the load stretches the muscle back to its initial length. The tension in the muscle is equal to the load during the shortening and the lengthening of the muscle, except during brief periods of acceleration as the muscle begins to move. Only after the muscle has returned to its initial length does the tension begin to diminish. The size of the load also determines the velocity of shortening; this relationship between load and velocity also applies to cardiac and smooth muscles.
When a chemical reaction occurs, energy is absorbed or released. In a contracting muscle, chemical reactions release energy that appears either as mechanical work or as heat. The first law of thermodynamics, or the law of conservation of energy, states that the heat and work produced must equal the energy released by the chemical reactions. The muscles that shorten and do external work liberate more energy as heat and work than do those that contract under isometric conditions and do not shorten or do external work. In light of the first law of thermodynamics, this finding means that the amount of chemical reaction that takes place during contraction depends on the type of contraction performed by the muscle. In other words, the flow of energy is subject to regulation.
The efficiency of the process of muscle contraction depends on the fate of the free energy released in chemical reactions—i.e., whether it is converted primarily into work or is degraded into heat. The second law of thermodynamics sets limits to the amount of energy that can be transformed into mechanical work. Although the production of heat can detract from the efficiency of working muscle, energy that appears as heat is not always wasted. In warm-blooded animals, for example, the heat released by muscles maintains a constant body temperature regardless of the environmental temperature. When an animal shivers in the cold, a large amount of heat is generated in the muscles. The muscles alternately contract and relax, releasing energy chiefly as heat.
Source of energy for muscle work
Muscles use the free energy released by chemical reactions by coupling the chemical reaction to physical changes in the contractile proteins. The exact molecular details of this fundamental coupling process are not yet completely known. Of the reactions that have been identified, the splitting of ATP is the energy-yielding reaction nearest to the contractile event. Water participates in this reaction in which ATP is broken down to ADP and phosphate (Pi); the reaction that occurs in the muscle, during which chemical free energy is converted into work, can be written as follows:
ATP + H2O + contractile elements → ADP + P i + contractile elements + work + heat
This equation emphasizes the obligatory role of the contractile elements and the coupled nature of the reaction that produces work.
In skeletal muscle most ATP is produced in metabolic pathways involving reactions of the sugar glucose or some other carbohydrate derived from glucose. During contraction, for example, glucose is made available for these reactions by the breakdown of glycogen, the storage form of carbohydrate in animal cells. The concentration of Ca2+ is transiently increased on activation of muscle. The ions are also activators of the process of glycogen breakdown. During the recovery period, the glycogen supply is replenished by synthesizing glycogen from glucose supplied to the muscle tissue by the blood. For a more detailed discussion of the metabolic pathways producing ATP, see metabolism.
In a resting vertebrate muscle, the available supply of ATP can sustain maximal muscle work for less than one second. The muscle, therefore, must continuously replenish its ATP store, and this is done in many different ways. One mechanism for the formation of ATP operates so rapidly that for a long time scientists were unable to detect any change in the amount of ATP in the muscle as a result of contraction. This immediate rebuilding of ATP is accomplished by the reactions of compounds called phosphagens. All of these compounds contain phosphorus in a chemical unit called a phosphoryl group, which they transfer to ADP to produce ATP (these compounds are also referred to as high-energy phosphates).
During rapid and intense contraction, phosphagen can be utilized to rebuild ATP rapidly and maintain its level as long as the phosphagen lasts, which in a maximally working human muscle is just a few seconds. After contraction, ATP is utilized to form phosphagen from creatine; ADP is also formed.
The amount of phosphagen is higher in skeletal muscle than it is in cardiac or smooth muscle. This correlates with the type of activity of the muscles. Skeletal muscle operates in bursts of activity, whereas cardiac and smooth muscle contract in a regular pattern. Skeletal muscle needs an immediate supply of a large amount of ATP, which is provided by the phosphagen reaction; cardiac and smooth muscle, which use ATP at a lower rate, rely on slower reactions to fill their energy requirements.
Molecular mechanisms of contraction
The nerve impulse that ultimately results in muscle contraction appears as an action potential at the sarcolemma, the membrane that surrounds the muscle fibre. This electrical signal is communicated to the myofilaments inside the fibre in the following way. When the action potential reaches the opening of the transverse tubules (channels that open through the sarcolemma to the space outside the fibre; see above The myofibril) in the surface of the fibre, it travels down into the fibre along the tubular membranes, which are continuous with the surface membrane, to within a fraction of a micrometre of each functional contractile unit (Figure 7). In frog muscle the transverse tubules surround the myofibril at the level of each Z line, and in mammalian muscles they are located at the edge of the A bands and I bands. At the triads (the three-element complex consisting of one transverse tubule and two cisternae, which are enlarged saclike membranes), the transverse tubule walls are close to the membranes of the terminal cisternae of the sarcoplasmic reticulum.
By some as-yet-unknown mechanism, the change in the electrical properties of the transverse tubules during an action potential causes the rapid release by the terminal cisternae of relatively large amounts of calcium ions into the sarcoplasm. As the concentration of calcium ions increases in the sarcoplasm, they become bound to the troponin in the thin filaments. This releases (or removes) the troponin-tropomyosin-mediated inhibition of the myosin-actin interaction. As the stimulation of the muscle continues, the terminal cisternae continue to release calcium ions. At the same time, however, some of the calcium ions are being removed from the sarcoplasm by another portion of the sarcoplasmic reticulum, the longitudinal tubules. Once the calcium ions are inside the lumen (cavity) of the longitudinal tubules, many of them slowly diffuse back to the terminal cisternae, where they are bound to a protein, calsequestrin, as a storage site. The removal of calcium ions from the sarcoplasm by the sarcoplasmic reticulum is energy-requiring. The breakdown of ATP is the chemical reaction that supplies the energy, and two calcium ions are apparently removed from the sarcoplasm for each ATP molecule that is split.
The heart is the pump that keeps blood circulating throughout the body and thereby transports nutrients, breakdown products, antibodies, hormones, and gases to and from the tissues. The heart consists mostly of muscle; the myocardial cells (collectively termed the myocardium) are arranged in ways that set it apart from other types of muscle. The outstanding characteristics of the action of the heart are its contractility, which is the basis for its pumping action, and the rhythmicity of the contraction.
Heart muscle differs from its counterpart, skeletal muscle, in that it exhibits rhythmic contractions. The amount of blood pumped by the heart per minute (the cardiac output) varies to meet the metabolic needs of the peripheral tissues (muscle, kidney, brain, skin, liver, heart, and gastrointestinal tract). The cardiac output is determined by the contractile force developed by the muscle cells of the heart (myocytes), as well as by the frequency at which they are activated (rhythmicity). The factors affecting the frequency and force of heart muscle contraction are critical in determining the normal pumping performance of the heart and its response to changes in demand.
Structure and organization
The heart is a network of highly branched cardiac cells 110 μm in length and 15 μm in width, which are connected end to end by intercalated disks. The cells are organized into layers of myocardial tissue that are wrapped around the chambers of the heart. The contraction of the individual heart cells produces force and shortening in these bands of muscle, with a resultant decrease in the heart chamber size and the consequent ejection of the blood into the pulmonary and systemic vessels. Important components of each heart cell involved in excitation and metabolic recovery processes are the plasma membrane and transverse tubules in registration with the Z lines, the longitudinal sarcoplasmic reticulum and terminal cisternae, and the mitochondria. The thick (myosin) and thin (actin, troponin, and tropomyosin) protein filaments are arranged into contractile units (that is, the sarcomere extending from Z line to Z line) that have a characteristic cross-striated pattern similar to that seen in skeletal muscle.
The frequency of contraction
The rate at which the heart contracts and the synchronization of atrial and ventricular contraction required for the efficient pumping of blood depend on the electrical properties of the myocardial cells and on the conduction of electrical information from one region of the heart to another. The action potential (activation of the muscle) is divided into five phases (0–4) and is graphed in Figure 9: The time course of each of the phases of the ventricular action potential (0–4) and …Figure 9. Each of the phases of the action potential is caused by time-dependent changes in the permeability of the plasma membrane to potassium ions (K+), sodium ions (Na+), and calcium ions (Ca2+). The resting potential of the myocytes of the ventricle (phase 4) begins with the outside of the cell being positive—i.e., having a greater concentration of positive ions. Atrial and ventricular myocytes are normally quiescent (nonrhythmic); however, when the resting membrane potential is depolarized to a critical potential (Ecrit), a self-generating action potential follows, leading to muscle contraction. Phase 0, the upstroke, is associated with a sudden increase in membrane permeability to Na+. Phases 1, 2, and 3 result from changes in membrane permeability and conductance to Na+, K+, and Ca2+.
The electrical activity of heart muscle cells differs substantially from that of skeletal muscle cells in that phases 1, 2, and 3 are considerably prolonged (200 milliseconds versus 5 milliseconds, respectively). Another significant difference in excitability is that heart muscle cannot be tetanized (i.e., induced to spasm) by the application of repetitive stimuli (see above Striated muscle), thus ensuring the completion of the contraction/relaxation cycle and the effective pumping of blood.
Because atrial and ventricular cells are normally quiescent, exhibiting action potentials only after the muscle is depolarized to the critical membrane potential (Ecrit), the source of the rhythmic contractions of the heart must be sought elsewhere. In contrast to atrial and ventricular myocytes, the myocytes of the sinoatrial (SA) node, the atrioventricular (AV) node, the bundle branches, and the Purkinje fibre system are made up of specialized cardiac muscle cells that exhibit a spontaneous upward drift in the resting potential toward Ecrit, resulting in the generation of the action potential with all of its phases. The normal rhythmicity of cells from each of these regions depends on the rate at which spontaneous depolarization occurs and the resting membrane potential from which it starts. The region with the fastest intrinsic rate, the SA node, sets the pace for the whole heart. The pacemaker activity is propagated to the rest of the heart by means of the low electrical resistance pathways through the muscle cells (e.g., intercalated disks) and the presence of specialized conducting tissue (e.g., bundle branches and the Purkinje system). The time course of activation and the shape of the action potentials in different parts of the heart are responsible for the synchronous activation and contraction of the muscles of the atrium followed by those of the ventricle.
The normal rhythm of the heart (i.e., the heart rate) can be altered by neural activity. The heart is innervated by sympathetic and parasympathetic nerves, which have a profound effect on the resting potential and the rate of diastolic depolarization in the SA nodal region. The activity of the sympathetic nervous system may be increased by the activation of the sympathetic nerves innervating the heart or by the secretion of epinephrine and norepinephrine from the adrenal gland. This decreases the resting potential of the myocytes of the SA node while increasing the rate of diastolic depolarization. The result is an increase in the heart rate. Conversely, stimulating the parasympathetic nervous system (vagal nerves to the heart) increases the resting potential and decreases the rate of diastolic depolarization; under these circumstances the heart rate slows. The sympathetic nervous system is activated under conditions of fright or vigorous activity (the so-called fight-or-flight reaction), where the increase in force and rate of heart contraction are easily felt; the parasympathetic system exerts its influence during periods of rest.
Immediately following depolarization of the plasma membrane and the ensuing action potential, the heart muscle develops force and then relaxes. The surface action potential is transmitted to the interior of the muscle by means of the transverse tubular system. Calcium ions enter the muscle cell during the plateau phase of the action potential (phase 2), triggering the release of calcium from the terminal cisternae of the sarcoplasmic reticulum. Calcium diffuses to the myofilaments and combines with the troponin-tropomyosin system (associated with the thin actin filaments), producing a conformational change that allows actin and myosin to interact. This interaction in the presence of ATP results in cross-bridge cycling and ATP hydrolysis. The force developed in the whole muscle is the sum of all the forces developed by each of the millions of cycling cross bridges of the muscle. The free calcium ions in the cytosol are removed by an energy-dependent calcium uptake system involving calcium ion pumps located in the longitudinal sarcoplasmic reticulum. These calcium pumps lower the concentration of free calcium in the cytosol, resulting in the dissociation (release) of calcium from the troponin-tropomyosin system. The troponin-tropomyosin system is then transformed back to its original state, preventing myosin and actin from interacting and thus causing relaxation of the muscle. At the same time, calcium is extruded from the cell into the surrounding medium.
Force and velocity of contraction
There are a number of factors that change the force developed by heart muscle cells. In a manner similar to that seen in skeletal muscle, there is a relationship between the muscle length and the isometric force developed. As the muscle length is increased, the active force developed reaches a maximum and then decreases. This maximum point is the length at which the heart normally functions. As with skeletal muscle, changes in length alter the active force by varying the degree of overlap of the thick myosin and thin actin filaments. The force developed by heart muscle also depends on the frequency at which the muscle is stimulated. As the stimulus frequency is increased, the force is increased until the maximum is reached, at which point it begins to decrease. An increase in the level of circulating epinephrine and norepinephrine from the sympathetic nervous system also increases the force of contraction. All these factors can combine to allow the heart to develop more force when required. At any given length the velocity of contraction is a function of the load lifted, with the velocity decreasing as the load is increased.
Response of the heart to stress
Demands on the heart vary from moment to moment and from day to day. In moving from rest to exercise, the cardiac output may be increased tenfold. Other increases in demand are seen when the heart must pump blood against a high pressure such as that seen in hypertensive heart disease. Each of these stresses requires special adjustments. Short-term increases in demand on the heart (e.g., exercise) are met by increases in the force and frequency of contraction. These changes are mediated by increases in sympathetic nervous system activity, an increase in the frequency of contraction, and changes in muscle length. The response to long-term stress (hypertension and thyrotoxicosis) results in an increase in the mass of the heart (hypertrophy), providing more heart muscle to pump the blood, which helps meet the increase in demand. In addition, subtle intracellular changes affect the performance of the muscle cells.
In the pressure-overload type of hypertrophy (hypertensive heart disease), the pumping system of the sarcoplasmic reticulum responsible for calcium removal is slowed while the contractile protein myosin shifts toward slower cross-bridge cycling. The outcome is a slower, more economical heart that can meet the demand for pumping against an increase in pressure. At the molecular level the slowing of calcium uptake is caused by a reduction in the number of calcium pumps in the sarcoplasmic reticulum. The change in the maximum velocity of shortening and economy of force development occur because each myosin cross-bridge head cycles more slowly and remains in the attached force-producing state for a longer period of time.
In the thyrotoxic type of hypertrophy, calcium is removed more quickly while there is a shift in myosin. At the molecular level there are more sarcoplasmic reticular calcium pumps, while the myosin cross-bridge head cycles more rapidly and remains attached in the force-producing state for a shorter period of time. The result is a heart that contracts much faster but less economically than normal and can meet the peripheral need for large volumes of blood at normal pressures.
Because vertebrate smooth muscle is located in the walls of many hollow organs, the normal functioning of the cardiovascular, respiratory, gastrointestinal, and reproductive systems depends on the constrictive capabilities of smooth muscle cells. Smooth muscle is distinguished from the striated muscles of the skeleton and heart by its structure and its functional capabilities.
As the name implies, smooth muscle presents a uniform appearance that lacks the obvious striping characteristic of striated muscle. Vascular smooth muscle shortens 50 times slower than fast skeletal muscle but generates comparable force using 300 times less chemical energy in the process. These differences in the mechanical properties of smooth versus striated muscle relate to differences in the basic mechanism responsible for muscle shortening and force production. As in striated muscle, smooth muscle contraction results from the cyclic interaction of the contractile protein myosin (i.e., the myosin cross bridge) with the contractile protein actin. The arrangement of these contractile proteins and the nature of their cyclic interaction account for the unique contractile capabilities of smooth muscle.
Structure and organization
Smooth muscle contains spindle-shaped cells 50 to 250 μm in length by 5 to 10 μm in diameter. These cells possess a single, central nucleus. Surrounding the nucleus and throughout most of the cytoplasm are the thick (myosin) and thin (actin) filaments. Tiny projections that originate from the myosin filament are believed to be cross bridges. The ratio of actin to myosin filaments (approximately 12 to 1) is twice that observed in striated muscle and thus may provide a greater opportunity for a cross bridge to attach and generate force in smooth muscle. An increased probability for attachment may in part account for the ability of smooth muscle to generate, with far less myosin, comparable or greater force than striated muscle.
Smooth muscle differs from striated muscle in its lack of any apparent organization of the actin and myosin contractile filaments into the discrete contractile units called sarcomeres. Research has shown that a sarcomere-like structure may nonetheless exist in smooth muscle. Such a sarcomere-like unit would be composed of the actin filaments that are anchored to dense amorphous bodies in the cytoplasm as well as dense plaques on the cell membrane. These dense areas are composed of the protein α-actinin, found in the Z lines of striated muscle, to which actin filaments are known to be attached. Thus, force generated by myosin cross bridges attached to actin is transmitted through actin filaments to dense bodies and then through neighbouring contractile units, which ultimately terminate on the cell membrane.
Relaxed smooth muscle cells possess a smooth cell membrane appearance, but upon contraction, large membrane blebs (or eruptions) form as a result of inwardly directed contractile forces that are applied at discrete points on the muscle membrane. These points are presumably the dense plaques on the cell membrane to which the actin filaments attach. As an isolated cell shortens, it does so in a corkscrewlike manner. It has been hypothesized that, in order for a single cell to shorten in such a unique fashion, the contractile proteins in smooth muscle are helically oriented within the muscle cell. This helical arrangement agrees with earlier speculation that the contractile apparatus in smooth muscle may be arranged at slight angles relative to the long axis of the cell. Such an arrangement of contractile proteins could contribute to the slower shortening velocity and enhanced force-generating ability of smooth muscle.
The contractile proteins interact to generate a force that must be transmitted to the tissue in which the individual smooth muscle cells are embedded. Smooth muscle cells do not have the tendons present in striated muscles that allow for transfer of muscular force to operate the skeleton. Smooth muscles, however, are generally embedded in a dense connective tissue matrix that connects the smooth muscle cells within the tissue into a larger functional unit.
Other organelles of the cell interior are related to energy production and calcium storage. Mitochondria are located most frequently near the cell nucleus and at the periphery of the cell. As in striated muscles, these mitochondria are linked to ATP production. The sarcoplasmic reticulum is involved in the storage of intracellular calcium. As in striated muscle, this intracellular membrane system plays an important role in determining whether or not contraction occurs by regulating the concentration of intracellular calcium.
Initiation of contraction
Smooth muscle cells contract in response to neuronal or hormonal stimulation, either of which results in an increase in intracellular calcium as calcium enters through membrane channels or is released from intracellular storage sites. The elevated level of calcium in the cell cytoplasm results in force generation. The rise in the level of intracellular calcium, however, initiates contraction through a mechanism that differs substantially from that in striated muscle. In striated muscle, myosin cross bridges are prevented from attaching to actin by the presence of the troponin-tropomyosin system molecules on the actin filament (see above Striated muscle). In smooth muscle, although tropomyosin is present, troponin is not, which means that an entirely different regulatory scheme operates in smooth muscle. Regulation of the contractile system in smooth muscle is linked to the myosin filament; regulation in striated muscle is linked to the actin filament.
In order for the smooth muscle myosin cross bridge to interact cyclically with actin, a small protein on the myosin molecule called the light chain must be phosphorylated (receive a phosphate group). This phosphorylation is the result of a series of interdependent biochemical reactions that are initiated by the rise in intracellular calcium. For the cell to relax, the concentration of intracellular calcium falls, thus inactivating these biochemical processes associated with light chain phosphorylation. The phosphate molecule that was added in the previous steps, however, still must be removed from the light chain so that attachment of the cross bridge to actin is prevented. Phosphatases are enzymes in the muscle cell that cleave the phosphate group from the myosin light chain.
Cross-bridge cycle and ATP breakdown
Smooth muscle contraction requires the release of chemical energy stored in ATP molecules. The release of this chemical energy by the myosin cross bridge and the resultant mechanical work is commonly referred to as the cross-bridge cycle, which in smooth muscle is believed to be a multistep process similar to that in striated muscle. Therefore, the mechanical properties of smooth muscle, as of striated muscles, are intimately linked to this multistate cross-bridge cycle. For instance, there is a correlation between the rate at which the cross bridges cycle and the maximum shortening velocity of the muscle. Since the actomyosin ATPase cross-bridge cycle in smooth muscle is considerably slower than that in striated muscle, the slower shortening velocity in smooth muscle must be partly due to the reduced turnover rate of the cross bridge. The slower cycling rate could also account for the high economy of ATP utilization that characterizes smooth muscle force production, since fewer cycles are required and less energy is consumed in the generation of force.
The relationship between smooth muscle’s ability to shorten and to generate force is characterized by the force-velocity relationship. The form of this relationship is qualitatively similar to that in striated muscle; however, the smooth muscle force-velocity relationship differs from that of striated muscle in having a slower maximum shortening velocity and a greater force per cross-sectional area of muscle. As mentioned above, the slower shortening velocity may relate to the slower cycling rate of the cross bridge as well as the orientation of the contractile proteins within the muscle cell.
The force-generating capabilities of smooth muscle are greater than those of striated muscle despite the fact that there is considerably less myosin in smooth muscle. Possible explanations for this relate to the arrangement of the contractile apparatus within the cell, which gives rise to more cross bridges effectively operating in conjunction with one another. Also, enhanced force production could be related to the greater amount of time that the cross bridge spends in the attached, high force-producing state (i.e., duty cycle). Evidence for such an increase in the duty cycle does exist in smooth muscle.
When fully contracted, the amount of force that smooth muscle can generate depends on the muscle length. Therefore, as in striated muscle, an optimal length for force production exists, with force being reduced at both lesser and greater lengths (for an explanation of the underlying mechanism, see above Striated muscle). The similarity in shape for the force-velocity and length-tension relationships between smooth and striated muscle suggests that in smooth muscle both a cross-bridge mechanism and a sliding of contractile filaments must occur.
Smooth muscle cells often must generate constant force for prolonged periods of time. In order to do this without depleting the muscle’s energy supply, smooth muscle appears to have adapted by altering the cross-bridge cycling rate during the time course of a single contraction. If shortening velocity is a reasonable indicator of cross-bridge cycling, then the observed reduction in shortening velocity with time of contraction suggests that the cross-bridge cycling rate must be slowing with contraction. At present it is not clear whether the cycling rate of the entire cross-bridge population slows with duration of contraction or whether a subset of slowly cycling cross bridges (i.e., latch bridges) appears with a contraction time that hinders the action of the normally cycling bridges. Regardless of the mechanism, the modulation of cross-bridge cycling rate represents a highly economical means of generating force in a muscle that often exists in a tonic state of contraction.
Smooth muscle in disease states
Smooth muscle cells lining the artery walls have been implicated in cardiovascular diseases such as atherosclerosis and hypertension (high blood pressure). In hypertension an increase in the size of the individual cells (hypertrophy) and in their number (i.e., hyperplasia) has been hypothesized. The increased quantity of smooth muscle in the artery wall could increase the constrictive ability of the artery or the artery wall thickness, either of which could constrict the lumen of the artery, thus reducing blood flow through the vessel. To compensate for this resistance to blood flow, the cardiovascular system responds by elevating blood pressure to ensure that the various tissues of the body are adequately supplied with blood. A consequence of the need to raise blood pressure, however, is a greater workload for the heart, and thus individuals who are diagnosed with high blood pressure are at a higher risk of heart attack or stroke. Medications that relax smooth muscle in an effort to lower blood pressure have been successful in reducing the risk of cardiovascular complications.