Analysis of Compound Postsynaptic Potentials in the Central Nervous System of the Surf Clam

Compound postsynaptic potentials, comprising graded excitatory-inhibitory sequences, are the characteristic mode of response to afferent input exhibited by a population of cells in the visceroparietal ganglion of Spisula. Experimentally induced interaction between the phases of the response indicates that the observed sequential invasion represents differences in individual component latencies, rather than the physiological resultant of two separate processes having simultaneous onset but different rates of decay. Excitation is depressed by changes in membrane conductance throughout the duration of the inhibitory phase; moreover, since similar pathways from the periphery initiate both phases, excitatory events are limited to a duration roughly equal in length to the latency for the inhibition. Within this interval repetitive volleys can evoke summation of excitatory events. The inhibitory mechanism is temporally stable, however, and dominates the membrane during repetitive trains of volleys at 1 to 100 per sec. Artificially generated increases in the membrane potential decrease the IPSP while increasing the amplitude of the EPSP. Thus, both phases of the compound response appear to result from events occurring at chemically transmitting synaptic loci. Evidence is presented that these events are driven via collaterals of the same afferent fibers. The behavioral role of these response sequences is uncertain. Analogies, in terms of some observed reflex activity in these clams, appear to exist but presently lack experimental verification.