Physiology and ultrastructure of electrotonic junctions. I. Supramedullary neurons.

Abstract

The axons of the supramedullary neurons of the puffer and Sargassum fish run in compact bundles. Adjacent axons show regions where their membranes are fused together, occluding the extracellular space. The cell bodies do not show such junctions. There are many typical synapses on the axons. In the puffer the supramedullary neurons are electronically coupled. Hyperpolarization and depolarization spread from cell to cell. Electrotonic coupling is mediated, at least primarily, by junctions between axons. An impulse is able to propagate part way along the axonal path connecting two cells. The supramedullary neurons of the Sargassum fish are electrotonically coupled. As in the puffer, responses to indirect stimuli are synchronized and an impulse may propagate from a directly excited cell to all other cells in the cluster. The morphological and electrophysiological data indicate lowered membrane resistivity occurs in the junctional regions, presumably at the areas of membrane fusion.