Neuromuscular analysis of closing in the dimorphic claws of the lobster Homarus americanus

The mechanical and electrical activity of the closer muscle is compared in the cutter and crusher claws of the lobster Homarus americanus. Two excitor axons innervate the closer muscle. In the cutter the fast axon produces twitch contractions in response to single stimuli while its counterpart in the crusher gives twitches only to paired stimuli. The twitches fatigue more rapidly in the cutter than in the crusher. The slow axon in both claws produces tonic contractions which are fatigue resistant. The electrical events in the muscle fibers correlate closely with the mechanical activity. Thus the cutter fast EPSPs show poor facilitation and fatigue rapidly while the slow EPSPs have high facilitation values and are less prone to fatigue. The slow and fast axon EPSPs in the crusher are qualitatively similar to their counterparts in the cutter. In both claws the postsynaptic response was considerably enhanced by paired stimuli.