Electrical Transmission among Neurons in the Buccal Ganglion of a Mollusc, Navanax inermis

Abstract

The opisthobranch mollusc, Navanax, is carnivorous and cannibalistic. Prey are swallowed whole by way of a sudden expansion of the pharynx. The buccal ganglion which controls this sucking action was isolated and bathed in seawater. Attention was focused upon 10 identifiable cells visible on the ganglion's rostral side. Two cells were observed simultaneously, and each was penetrated with two glass microelectrodes, one for polarizing the membrane and the other for recording membrane potential variations. The coupling coefficients for direct current flow and action potentials of several identified cells were tabulated. Attenuation was essentially independent of the direction of current flow, but depended upon the relative size of the directly and indirectly polarized cells. The attenuation of subthreshold sinusoidally varying voltages increased with frequency above about 1 Hz. The coupling coefficient for spikes was lower than for DC due to greater high frequency attenuation. There is considerable similarity in the spontaneous PSP's of all cells, which is not due to the electrical coupling but to input from a common source. The 10 cells were not chemically interconnected but some were electrically connected to interneurons which fed back chemically mediated PSP's. The feedback can be negative or positive depending upon the membrane potential of the postsynaptic cell. We conclude that electrical coupling among the 10 cells plays a minor role in sudden pharyngeal contractions but that the dual electrical-chemical coupling with interneurons may be important in this respect.