Common inhibitory motoneurones in insects.

Motor nerve fibres to insect muscles are usually classified as either fast, slow or inhibitory, depending on the mechanical and electrical responses they elicit in the muscle (Hoyle, 1953a; Usherwood, 1962; Usherwood & Grundfest, 1964), although some motor axons cannot be satisfactorily placed within any of these categories (Hoyle, 1959, 19666). Histologically, single insect muscles have been seen to be innervated by up to eight different axons (Dresden & Nijenhuis, 1958). Electrophysiological experiments, however, have not yet revealed that any simple muscle or branch of a compound muscle receives more than five axons. All insect muscles so far described receive at least one fast axon and they may or may not be innervated by slow excitatory axons and/or an inhibitory axon. Unlike crustacean muscle, no insect muscle has been found having dual inhibitory innervation. Single fibres of some crustacean muscles receive up to six different axons (Kennedy & Takeda, 1965), whereas only one insect muscle has been described in which single fibres are more than triply innervated; this being the basalar fibrUlar muscle of the beetle, Oryctes rhinoceros, where a small number of fibres receive four excitatory axons (Ikeda & Boettiger, 1965). The usual innervation pattern of single insect muscle fibres is either single or dual excitatory. The number of peripheral inhibitory nerve fibres so far found in insects is few compared to those for Crustacea, and there is some doubt in insects as to whether they have any real functional role (Hoyle, 19666). Usherwood & Grundfest (1965) have described an inhibitory axon innervating the extensor tibiae muscle of the locust and grasshopper. Although stimulation of this axon always produced mechanical inhibition of the slow excitatory response, its normal function was not clear. Recently, however, Runion & Usherwood (1968), recording from the motor nerve to the extensor tibiae muscle in freely moving locusts, have shown that for regular walking the inhibitory axon fires just before the discharge of excitatory axons to the flexor tibiae muscle. This suggests that the function of this particular inhibitory axon may be to facilitate relaxation in the extensor tibiae muscle, thus allowing a more rapid and stronger flexion of the tibia. A nerve fibre that can produce mechanical inhibition in the anterior coxal adductor muscle of the locust and grasshopper has been described by Hoyle (19666). Hoyle found, however, that activity in this axon produced mechanical inhibition of the excitatory response only in about one third of his preparations. In another third it