Polyphasic responses evoked by multiaction interneurones in the buccal ganglia of Aplysia

Abstract

In the buccal ganglia of Aplysia depilans and A. fasciata, the small S₁ cells were found to affect synaptically not only the A, B, and C cells, as already reported, but also the middle-sized and large neurones which are placed in a central position in the caudal face of the ganglion, and which are labelled “D cells” here.

The responses of the D cells were quite varied: mono-, di-, tri-, and tetra-phasic responses were found. Most D cells were specialized towards a limited number of types of these responses.

By using standard criteria, the responses proved to be monosynaptic and chemically mediated.

Four monophasic responses were found to occur: a fast and a slow depolarization, and a fast and a slow hyperpolarization. The polyphasic responses appeared to originate from associations of the four monophasic PSPs at the various synaptic contacts. All the types of response which were possible according to such a mechanism were found in the experiments.

The modifications of the phases with imposed polarization of the postsynaptic membrane were studied. The equilibrium potentials of the two hyperpolarizing phases and the effects upon them of [K⁺] and [Cl^-] modifications in the perfusing medium were determined.

The results obtained suggest that the responses of the A, B, and C cells can equally well be explained by the intervention of the four monophasic components which have been identified in the responses of the D cells. We found, however, that the fast hyperpolarization present in responses of the B cells differed from the corresponding phase present in responses of the other types of postsynaptic cells as regards equilibrium potential, ionic mechanisms, degree of inversion with membrane hyperpolarization, susceptibility to d-tubocurarine, and rate of depression with repetition.