PERIPHERAL INHIBITION IN SKELETAL MUSCLE OF INSECTS

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Abstract
Fibers of grasshopper (Romalea microptera) and locust (Sclustocerca gregaria) metathoracic extensor tibiae muscles generate inhibitory postsynaptic potentials (i.p.s.p.''s) which are excited by an axon coursing in nerve 3b. The i.p.s.p.''s are normally hyperpolarizing, are generated through electrically inexcitable Cl-activation and their reversal potential (ECl) is about -70 mv. The electrogenesis is also activated by gamma-aminobutyric acid (GABA) and is inactivated by picrotozin. Only fibers which are innervated by the "slow" exciter axon, but not all of this group, also receive inhibitory innervation. Fibers not innervated by the inhibitor axon do not repond to the activator or inactivator drugs. Whether neurally evoked or by GABA, the inhibitory electrogenesis diminshes or may block mechanical responses evoked by stimulating the "slow" exciter, but has little or no effect on the twitches evoked through the "fast" exciter. I.p.s.p.''s have been observed in other muscles of these insects and also in Bladerus giganteus and B. craniifer, and thus may have a role in controlling movements. The finding of inhibitory activity and the pharmacological similarities of both excitatory and inhibitory synapses with those of Crustacea remove a hitherto emphasized difference between insect and crustacean neuromuscular systems.

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