Muscles in invertebrates

The most obvious characteristic of animals may be movement. Movement requires a mechanism. The mechanisms allowing multicellular animals of all kinds to change position or posture are muscle cells.

The Mechanism of Movement

A muscle cell, also called a muscle fiber, is specialized for contraction. Muscle is composed of many muscle fibers, and each muscle cell contains contractile molecules permitting the cell to shorten and thereby change its shape. Each muscle fiber moves by drawing its ends together or by contracting.

Several types of contractile molecules are common in the muscles of invertebrates and vertebrates. Actin and myosin are the most prominent contractile molecules. Each actin molecule is rounded and forms myofibrils by assembling repetitive monomers to form a helix, a springlike shape. Two of these helices are coiled around each other like two intertwined strings of pearls.

Myosin is a more complex myofibril. Each myosin molecule is composed of a head and a tail. The tails of two myosin molecules are wound around each other helically, while the heads project from the same end but in different directions. In the presence of calcium ions, each head can swivel. If an actin molecule is exposed, the myosin head can attach to it, forming a crossbridge. Since they lie parallel to each other in the cell, when the myosin returns to its original position, it pulls the actin myofibril. The ends of the contractile unit move closer to each other, shortening the muscle cell.

Myosin tails lie parallel to each other, but at the opposite ends of the myosin myofibrils, the heads project in opposite directions. The heads lie close to the two end walls that attach the actin myofibrils. There is a small central area where only the oppositely facing tails of the myosin myofibrils are found.

A contractile unit is called a sarcomere. A sarcomere is a repeated unit within a muscle cell consisting of overlapping thick myofibrils (myosin) and thin myofibrils (actin). Since there are areas in which there is no actin and areas in which there is no myosin, some light stripes are seen crossing the myofibrils in a microscopic preparation of certain muscle fibers. Actin myofibrils connect to thick walls that mark the ends of a sarcomere. The myosin myofibrils are connected across the center of the sarcomere. During relaxation, actin is absent from the center, producing a light band. The regular pattern of light and dark areas along the length of the muscle fiber is characteristic of striated or skeletal vertebrate muscle. This vertebrate muscle type is the most studied in physiology and, therefore, is the best known.

The Specifics of Invertebrate Muscles

Invertebrate muscles have similar components, but the arrangement is less regular, the alignment of sarcomeres often being oblique. Actin and myosin myofibril arrangement is not readily delineated, but similar structures exist. A sarcoplasmic reticulum, similar to the endoplasmic reticulum of ordinary cells, is characteristic of vertebrate and invertebrate muscle fibers. It is arranged in a series of flattened sacs over a sarcomere’s myofibrils. The sarcoplasmic reticulum can quickly absorb calcium ions, keeping their concentration around the sarcomere low. Without calcium ions, the myosin heads cannot be cocked.

Transverse, or T, tubules extend along the thick end wall of each sarcomere. They conduct the nerve impulse throughout the sarcomere to initiate contraction rapidly and simultaneously. When the impulse reaches the sarcoplasmic reticulum, stored calcium ions are released. This release begins contraction. Contraction ceases only if adenosine triphosphate (ATP) is present. Because they no longer make ATP, dead animals’ contracted muscles do not relax; the muscles become stiff (exhibit rigor mortis).

Other molecules are also typically found in muscle cells. Tropomyosin is a strand of molecules that lies in a groove of the helically wound actin myofibrils. It blocks the attachment of myosin heads to actin molecules. Troponin is a three-part molecule that rests along the actin myofibril. One subunit attaches calcium ions. This changes troponin’s conformation so that another subunit attracts the tropomyosin molecule, unblocking the actin myofibril so that the cocked head of myosin molecules can attach to the actin.

Paramyosin is a structural protein molecule similar to myosin found in thick filaments of invertebrates’ muscles. It forms a paracrystalline core that acts as a “filler” for myosin, keeping the myosin myofibrils aligned to interact with the actin. It may be the contractile protein of some invertebrate muscle types. It is prominent in the “catch” muscles of mollusks, allowing them to exert minimal energy during long periods of muscle tension.

Muscle Response Rates

Muscles may vary in their response rates even though they look the same. “Fast” muscle fibers contract rapidly in response to a nerve impulse. “Slow” muscle fibers contract several times more slowly. Although both types use the same contractile machinery, the slow muscle cells have less sarcoplasmic reticulum. This means that the calcium ions necessary to initiate contraction are not released as quickly everywhere along the contractile fibrils and are not pumped away from them as quickly as they are in the fast muscle cells.

Most invertebrate muscles are synchronous. This means that each contraction is initiated by a nerve impulse. The rate of contraction of synchronous muscles is determined by the rate of passage of nerve impulses. Even in flight, these muscles usually contract only about thirty-five times per second. Synchronous flight muscles are connected directly to the wings and are found in insects, such as grasshoppers, moths, butterflies, and dragonflies.

True flies and bees, as well as beetles and true bugs, have a specialized kind of flight muscle called an asynchronous muscle. In asynchronous muscles, every contraction is not initiated by a nerve impulse. The contraction rates, up to one thousand times per second, are so rapid that nerve impulses could not be received and acted on quickly enough to produce that contraction rate. In these muscles, contraction is initiated by a nerve impulse received and transmitted by the T tubules, but there is no sarcoplasmic reticulum. Even fibrils removed from the cell and placed in a solution containing calcium ions and ATP contract and relax in an oscillatory fashion. The rates seem to be regulated by the myofibrils rather than by the calcium concentration. The T tubules seem to signal the contraction to begin and later turn off the cycling behavior of the myofibrils.

Asynchronous muscles are not directly attached to the wings of the insect. Elevator muscles are attached to the roof of the thorax. Their contraction pulls the roof of the thorax down and elevates the wings. Contraction of the wing depressor muscles pulls down the wings, but this stretches the wing elevators, stimulating them to contract and allowing the thorax roof to “pop up.” Raising the thorax shortens the wing depressor muscles and terminates the active state of the depressors. They relax until stretched again by the elevation of the wings. The elevator and depressor muscle contractions follow the same sequence of stretch ® contraction ® shortening ® relaxation ® stretch, but are out of phase with each other. The frequency of wing beats depends upon the mechanical properties of the thorax and wings and not upon the frequency of nerve impulses.

Unlike vertebrate animals, there is a correlation between sarcomere length and speed of contraction. Rapidly contracting arthropod muscle fibers have short sarcomeres and relatively low ratios of thin to thick fibrils. Slowly contracting muscle fibers have long sarcomeres and high ratios of thin to thick fibrils. Intermediate types of fibers can exist within the same muscle.

Variations of Invertebrate Muscle Types

Because invertebrates are so varied, there is no generalization to which an exception cannot be found. For example, lobsters and crayfish have two separate muscle fiber types in their tail musculature. The tails are important in swimming and, particularly, in escape maneuvers. They must flex and extend rapidly to evade predators or aggressors. The bulk of the tail muscles consist of short sarcomere, rapidly contracting flexors and extensors. Thin sheets of long sarcomere—slowly contracting flexors and extensors—lie near the carapace. They are used for postural adjustments and for slow movements.

Flexors and extensors are good examples of another principle of muscle action. Muscles contract and they shorten. This shortening moves a body part. Relaxation allows the muscle fibers to lengthen. The force needed to lengthen the relaxed muscle comes from the contraction of its antagonist, a muscle that produces movement in the opposite direction from that of the first muscle. For example, flexing the tail of a lobster or crayfish is performed by the tail flexors. Relaxation does not return the tail to its extended position. Contraction of the extensor muscles moves the tail away from the body and extends the tail again. Relaxation of the extensors may allow the tail to be less rigid in its extension, but the tail will not flex until the flexor muscles are contracted. Muscles work in antagonistic groups to produce opposite movements (flexion and extension) and to produce postural changes that allow a lobster or crayfish to maintain its position when the current changes direction or speed.

Based on muscle striation patterns, invertebrate muscle is generally composed of three main muscle types, though exceptions and outliers exist for each group. In all invertebrate groups except arthropods, smooth muscles—involuntary muscles triggered by neural, hormonal, or environmental stimulation—exist. However, invertebrate smooth muscle has more myofilaments that are thicker than those found in vertebrate smooth muscle. Transversely striated muscle similar to that of vertebrates occurs in arthropods. Obliquely striated muscles comprise thick and thin fibers observed in brachiopods, annelids, mollusks, and chaetognaths. Other invertebrates, like the parasite Schistosoma mansoni, have so-called hybrid muscles that combine traits seen in classical muscle structures.

Studying Invertebrate Muscles

Numerous methods are used to study the muscles of the invertebrates. Initially, the contraction patterns of whole muscles were studied by electrically stimulating muscles and their nerves. As in all biology, however, work is being done on the molecular level.

Glycerinated muscle is a preparation used to study the molecular elements necessary for contraction. Soaking muscle cells in glycerin removes the cell membrane and sarcoplasmic reticulum but leaves the contractile fibrils intact. These can be used to determine the effect of presence or absence of ATP, calcium, magnesium, or other factors that might influence the working of the fibrils. They are used much the same as a whole muscle preparation. The ends of the fibrils are attached to one point that does not move and to another that does move when the muscle fibrils contract and relax. The movement can be recorded on paper or on an oscilloscope screen. The tension generated, or the length of shortening, can be calculated.

Microscopic analysis of the structural organization of invertebrate fibers also adds to the store of knowledge. Most invertebrate muscles do not have the striated appearance of vertebrate skeletal muscles. The myofibrils vary in their arrangements and are often difficult to discern. Microscopic analysis shows that rapidly contracting fibers have low ratios of thin to thick fibrils, and slowly contracting fibers have larger ratios of thin to thick fibrils.

Muscles do not function without the coordination of the nervous system. Its contribution is different from that in vertebrates. Invertebrate muscle fibers do not exhibit the all-or-none response characteristic of vertebrates. Invertebrate muscle fibers receive innervation from several nerve fibers. A single nerve may serve many muscle fibers. Any muscle fiber is served by more than one nerve. The contraction strength of invertebrate muscle fibers depends upon the number and types of nerves sending impulses to that muscle cell at any time.

The study of the biochemical composition of myofibrils is also important. Actin and myosin are similar in all species. Vertebrate myosin occurs in two forms. One form has a higher intrinsic ATPase activity and responds quickly; the other has a slower intrinsic ATPase activity and twitches slowly. Invertebrate muscle fibrils have no such differences between myosin molecules. Paramyosin is a molecule peculiar to invertebrates. It is thought to be part of a supporting structure for the myosin tails. Some invertebrate muscles have been found not to have troponin and to have different forms of tropomyosin.

Electrophysiological studies have shown that invertebrate muscle fibers receive excitatory and inhibitory nerve fibers. Excitatory nerve fibers secrete acetylcholine and cause contraction. Inhibitory nerve fibers secrete serotonin and dopamine. The relaxation produced by these fibers is thought to be mediated by cyclic adenosine monophosphate (cAMP), a derivative of ATP. The relaxation may be a result of the breaking of crossbridges stabilized by paramyosin.

Many invertebrates in several groups, such as Mollusca, Arthropoda, and Chordata, can regenerate portions of their body that are cut off, including their muscles, using epimorphosis. After the wound heals, blastemas form (a group of undifferentiated cells), and as the blastema creates patterns and the cells are differentiated, the missing muscle forms. The intricacies of this process are not entirely understood, but acetylcholinesterase and myogenic regulatory factors are suspected to be critical in muscle regeneration.

Simple and Complex Arrangements

Invertebrate neuromuscular systems are organized along the same general principles as in vertebrates: They coordinate body movements. All muscle cells shorten after receiving a stimulus, but the shortening will produce motion only if there is a skeleton, a structure to which the muscle fiber is anchored and another part to which force can be applied.

Sedentary sea anemones are relatively simple coelenterates that escape from threats by withdrawing their soft tentacles and contracting toward the substrate protected by thick body walls. Their body wall is a cylinder composed of two layers of cells separated by mesoglea, a jellylike material that allows diffusion of nutrients, wastes, and gases. Two groups of muscles are embedded in the body wall. In the outer body wall, longitudinal muscles parallel the long axis of the body. Their contraction shortens the body wall and draws it downward toward its base; its attachment is to the substrate. The circular muscles ring the inside layer of the body wall. Their contraction lengthens the body by squeezing the contents inward and narrowing the cylinder like a Chinese finger puzzle.

More complex invertebrates have more complex arrangements of muscles and skeletal parts. Their movements become more complex also. The best-known examples come from the larger and more numerous invertebrate phyla, the annelids, mollusks, and arthropods.

Annelids and Mollusks

Annelids and many other invertebrates have a hydrostatic skeleton system composed of circular and long muscles, like that of the sea anemone. Their bodies are divided into discrete segments, each with a fluid-filled cavity. Circular muscles of a segment contract, pressing against the fluid in the cavity extending it. This contraction of the circular muscles stretches the longitudinal muscles and sets the tiny bristles along the side of each segment into the substrate. The contraction of the longitudinal muscles squeezes the fluid into a shorter segment which stretches the circular muscles and widens the segment. The body is pulled forward by the bristles that had been set in the substrate.

One of the few examples of the use of hydrostatic pressure in place of muscles occurs in some spiders. The hind legs of jumping spiders have flexor muscles that bring the legs toward the body. They have no extensor muscles. Rapidly increased body fluid pressure straightens the legs, causing the spider to jump forward.

Mollusks include bivalves—sedentary clams, mussels, and oysters; gastropods—the slowly moving snails and slugs; and cephalopods—some of the quickest, most intelligent, and largest invertebrates, octopus and squid. The muscular systems of these animals are as varied as their lifestyles.

The bivalves settle as adults in one place. They are unable to move about. Whenever a predator threatens them or environmental conditions change (for example, if the tide goes out), they must close their shells to shield their delicate body tissues. “Catch” muscles protect them by closing tightly without using much energy. These obliquely striated adductor muscles can maintain their contracted state for a long period. A short train of nerve impulses initiates a contraction that may last hours or days with no further nerve impulses. The catch muscle does not stretch readily, so prolonged pressure by a predator, such as a starfish, does not lengthen the muscle. The catch muscle remains contracted until a relaxation mechanism is activated by neural impulses in separate neurons. These catch muscles contain paramyosin surrounded by myosin molecules.

Arthropods

The arthropods are characterized by an exoskeleton, the joints of which allow movement. The skeleton is outside the body. It is composed of the cuticle, a hardened covering of chitin, with a thin, pliable hinge in the joints. Muscles cross the gap covered by the hinge. In arthropods, muscle tension is controlled largely or entirely by gradation of contraction within a motor unit. The degree of depolarization of these muscle fibers depends upon the frequency of impulses transmitted in motor neurons. Each fiber is a part of several motor units. Additionally, inhibitory factors to arthropod muscles can prevent depolarization and, hence, contraction of the muscle (unlike vertebrate muscles). The neurons fire in short bursts to produce rapid movements.

Most rapidly contracting, short-sarcomere fibers are innervated by a phasic axon capable of producing rapid movements. The slowest, long-sarcomere muscle fibers are supplied only by the tonic axon that is active most of the time. These fibers provide for slow movements and postural adjustment. Muscle fibers with intermediate contraction times are innervated by both phasic and tonic axons. A muscle can, therefore, contract over a range of speeds and durations. An extreme example of this is the crustacean claw-opener muscle and the stretcher (extensor) of the proximal leg segment, which are innervated by a single excitatory neuron. Separate inhibitory neurons supply each muscle so they can be controlled independently.

In lobsters, the differentiation of the claws into a large crusher and a small cutter is correlated with muscular and neural activity. In their younger states, juvenile lobster claws have the same shape. Either claw can develop into the crusher. In the cutter claw, all the muscle fibers transform to fast fibers. In the crusher claw, all the muscle fibers become slow fibers. The change in shape and muscle type depends on the presence of a manipulable environment and on the animal having unequal neuromuscular feedback to the central nervous system. The presence of a crusher on one side prevents the development of a crusher on the opposite side. Few generalizations describe invertebrates. Their muscles exhibit the same variety as the organisms that make up this diverse assemblage.

Principal Terms

Actin: One of the two major types of contractile proteins; it forms the thin myofibrils of the sarcomere

Fast Muscle: Muscle cells that respond quickly to nervous impulses; in invertebrates, these muscle fibers have short sarcomeres and a low ratio of thin to thick myofibrils

Myosin: One of the two major contractile proteins making up the thick myofibrils

Paramyosin: A structural protein associated with myosin myofibrils and thought to support them

Slow Muscle: Muscle cells that respond slowly to nervous impulses; in invertebrates, these muscle fibers have long sarcomeres and a high ratio of thin to thick myofibrils

Tropomyosin: A double-stranded protein that lies in the grooves of actin myofibrils, blocking actin from attachment to myosin

Troponin: A globular protein composed of three subunits; one subunit binds calcium ions, and another draws tropomyosin away from actin, which allows myosin to form crossbridges constituting the third subunit

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