The Innervation of Muscle

This chapter discusses the details concerning the innervation of the various invertebrate skeletal muscles and the responses produced by their efferent axons. There are two basic ways in which muscle fibers are activated by impulses from the central nervous system. One method involves the production of a propagated action potential that arises at one point on the muscle fiber surface; this action potential passing rapidly to all regions of the fiber to activate the contractile system. The second method involves the production of large numbers of non-propagated action potentials or graded depolarizations that arise at a number of points along the muscle fiber. As these membrane disturbances occur simultaneously at all points, they synchronize the activation of the contractile machinery of the whole fiber. The former method is found in vertebrate skeletal muscles, while the latter is found in most invertebrates and particularly in the arthropods. The vertebrates and invertebrates, thus, differ in that the former have focal innervation with the exception of the intrafusal muscle fibers of the muscle spindle proprioceptor and the tonic fibers of amphibia, birds, and mammals, while the latter have multiterminal innervation.