Nerve Pathways and Reflex Siphon Withdrawal in the Surf Clam

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Abstract
Neural pathways controlling reflex withdrawal of the siphons have been examined in the surf clam. Conduction-velocity measurements, coupled with intracellular recordings of postsynaptic responses, indicate that the pathways involved are among the fastest in the relevant nerve trunks. Cell bodies of neurons in the efferent limb of the reflex are located in specific paired regions of the visceral ganglion. Their axons are distributed to ipsilateral effectors via branches of the posterior pallial nerve and input to these cells is derived from a wide area of the periphery involving both sides of the animal. This pronounced sensory convergence can adequately account for symmetrical siphon withdrawal. It is concluded from several lines of evidence that fast reflex pathways within the ganglion are monosynaptic in nature. In addition, the prolonged time course of the postsynaptic response to single volleys, and the capabilities of the synaptic contacts for spatial and temporal summation, make it possible to account for after discharges in the efferent pathways lasting a second or more. Prolonged activity from these cells has not been observed in isolated visceral ganglion preparations, and it is possible that the sustained closure is controlled by pathways involving different groups of neurons.