Physiology of Finfish and Shellfish (2+1)

3.2.13. Crustacean muscle

Crustacean muscles have form some characteristics that to varying degrees are present in the muscles of a number of other animals. We shall discuss two points: (1) the gross arrangement of the muscle fibers into parallel fibered or pinnate muscles, and (2) the multiple innervations of a single muscle fiber and its responses to the different nerves.

Nearly everybody, who has handled live crabs or lobsters has made contact with the impressive force these animals can exert with their claws. This is not because the strength of the muscle is extraordinarily high, but rather because of the anatomical arrangement of the fibers. In the claw of the crab, the muscle is pinnate; this means that the fibers, instead of being parallel to the direction of the pull, are arranged at an angle, which greatly increases their mechanical advantage (Figure 11.18).

In the enclosed space of the claw, a pinnate muscle of a given volume can have more and shorter fibers than a parallel-fibered muscle. The increased force, attributable to this mechanical arrangement, is gained at the expense of the distance over which the attachment can be moved. In the crab claw, the closing muscle can exert, because of the pinnate arrangement, about twice as much force as it could if it were parallel fibered (Alexander 1968).

Another advantage of the pinnate muscle in the crab claw is related to the confined space in which it works. As a parallel-fibered muscle contracts, its cross section increases, and in the rigid enclosure of the claw this would cause difficulties. Because of the arrangement of the pinnate muscle, however, the cavity of the crab claw can be almost completely filled with muscle, for as the fibers shorten, their angle is changed so, that there is space for the thickening. This is impossible with a parallel-fibered muscle in a limited space.

A pinnate muscle can perform only the same amount of work as a parallel-fibered muscle of equal volume because the increased force of contraction is offset by a corresponding decrease in shortening distance. It should be noted that pinnate muscles are not unique to crustaceans; they also occur quite commonly in vertebrates.

The most interesting aspect of crustacean muscle is the multiple innervations. Individual muscle fibers may be innervated by two or more nerve fibers. In addition to multiple nerves that stimulate the muscle to contract, many arthropod muscles also have inhibitory nerves whose stimulation causes the muscle to relax, if it is already in a state of contraction.

Another crustacean characteristic is that a whole muscle is often innervated by only a few or a single axon. This makes the whole muscle act much as a single unit and gradation of response results from variations in the nerve impulses in combination with the balance between excitatory and inhibitory impulses. One muscle may, therefore, depending on the impulses it receives, act as a fast or as a slow muscle.

For example, the jumping muscle of the hind-leg of a locust is innervated by three axons: one fast excitatory, one slow excitatory, and one inhibitory. The response to the fast axon is a rapid twitch; the response to the fast axon is a rapid twitch; the response to an impulses produce increasing intensity in the response. Some of those muscle fibers that are innervated by the slow axon are also innervated by an inhibitor axon. Impulses in this axon reduce the response to the slow axon and induce muscle relaxation (Usherwood 1967).

The difference in the innervation of vertebrate and crustacean muscle is shown in Figure 11.19. As stated above, the most striking characteristic of crustacean muscle is that it is supplied by two or more different nerve fibers. Often, there is a fast excitatory fiber, a slow excitatory fiber, and finally an inhibitory nerve fiber. In many muscles, the majority of fibers is innervated by the fast nerve. In combination with the inhibitory fibers, this permits a wide range in the gradation of the contractions.

The other characteristic of crustacean muscle that differs from vertebrate muscle is that a whole muscle is usually innervated from a very few neurons in the central nervous system and is reached by a correspondingly small number of axons that branch and connect the fibers of the entire muscle.