ABSTRACT
Neuromuscular control in the Mollusca presents numerous interesting problems to the comparative physiologist. Among these are the extent of polyneural and multi-terminal innervation, the possibility of peripheral inhibition, and the anatomical basis for wholly peripheral reflex activities. Perhaps in no area are the problems more intriguing—and the experimental results more equivocal—than with regard to the physiology of lamellibranch tonic smooth muscle. These muscles are unique, not only for the fact that they are able to exert the largest known force for weight of muscle tissue, but also for their well-known ability to remain in the contracted state for prolonged periods of time with very little expenditure of metabolic energy (for a review, see Hoyle, 1964). While these muscles can also contract in a phasic manner, relaxing quickly after a brief twitch-like response, in some cases there is no histological differentiation to complement the functional dichotomy. It is therefore logical to inquire whether the different states of activity may have their bases in different types of excitatory synapses made with the muscle fibres by specific ‘tonic’ and ‘phasic’ motoneurons. Another suggestion (Takahashi, 1960) is that phasic responses occur following simultaneous activity in excitatory and inhibitory nerve fibres, the latter drastically limiting the duration of the contracted state of the muscle.
