Invertebrate nerve-muscle systems

Abstract

Abstract Recent studies of invertebrate motor nerves and muscles are summarized, with particular attention given to insects and crustaceans. Skeletal muscles, generally, are innervated by both excitatory and inhibitory motoneurons. Excitatory motoneurons may fire in short bursts and elicit rapid, twitch-like contractions (phasic motoneurons) or they may fire in long bursts and produce slow, graded contractions (tonic motoneurons). Inhibitory motoneurons form axo-axonal synapses with excitatory motoneurons (presynaptic inhibition) and neuromuscular synapses (postsynaptic inhibition). Only a few neurons innervate each muscle, but each forms thousands of synapses. Skeletal muscle fibers may contract rapidly (few msec) or slowly (few sec). The former (phasic fibers) have short sarcomeres (2–4 μm); the latter (tonic fibers) have long sarcomeres (6–14 μm). Inhibitory motoneurons release γ-aminobutyric acid (GABA) which increases chloride ion conductance. Excitatory motoneurons in arthropods may release glutamic acid, while those of annelids and molluscs release acetylcholine; both transmitters increase sodium and potassium ion conductance. Most motoneurons respond to injury with axonal degeneration followed by regeneration which restores neuronal integrity. However, crustacean motor axons do not degenerate after isolation from their somata and when stimulated directly, release transmitter in a normal fashion. Certain molluscan muscles (catch muscles) maintain tension long after the end of neural activation. Relaxation is achieved through relaxing nerves which release 5-hydroxytryptamine.