Electrical and Mechanical Responses of the Skeletal Muscle of a Primitive Insect

Abstract

INNERVATION of arthropod excitatory muscle is of two types, distinguished by mechanical effect; these are “fast” (twitch) and “slow” (tonic), and only in the fast type is there a significant mechanical response to a single nerve impulse. The classes Insecta and Crustacea are the only arthropods the neuromuscular mechanisms of which have been studied in any depth, and considerable differences have been found between them, both in the anatomy and in the form of electrical events which accompany the fast and slow mechanical responses. Nearly all insect muscle fibres are fast-innervated and the electrical response is large, with a non-propagated spike component (active membrane response). Some fibres are also slow-innervated, and in these the electrical response is much smaller, but summates and facilitates greatly to repetitive stimulation, producing large depolarizations^1–5. In crustaceans there are relatively fewer fast-innervated fibres, but all are slow-innervated. The two types of response are often difficult to distinguish electrically, and the fast response has no spike component; summation is incomplete although facilitation can be significant, and large depolarizations to either fast or slow stimulation are very rare^6,7. Investigations of some of the more primitive insects could possibly show some responses which do not have the large spike activity of the higher insects, and some interesting neuromuscular patterns have been discovered in dragonfly larvae (species of Aeschna) which are thought to be primitive for several other reasons.