Physiology and ultrastructure of electrotonic junctions. II. Spinal and medullary electromotor nuclei in mormyrid fish.

Abstract

The spinal electromotor neurons and the medullary relay neurons are connected by thick dendrodendritic bridges where there are regions of membrane fusion involving obliteration of extracellular space. The middle of 3 layers is usually discontinuous, having striations with a periodicity of about 80 A. Terminals on the spinal and medullary relay neurons contain accumulations of vesicles and form desmosome-like junctions with the spinal neurons; however, the latter often have regions of membrane fusion with the relay neurons as well. The spinal electromotor neurons give 3 spikes preceding each organ discharge. The 1st postjunctional potential in the electroplaques is far below threshold but subsequent ones are facilitated until the 3rd is adequate to excite the cells. Three spikes are evoked by a single post-junctional potential or a brief direct stimulus. The 3rd spike is somewhat labile and in normal discharges a 2nd descending volley guarantees its occurrence. The medullary nuclear neurons fire twice preceding each organ discharge. The property of generating 2 spikes appears intrinsic to the cell membrane, since 2 spikes are evoked by brief direct stimuli or single antidromic volleys. The spinal and medullary relay neurons are electrotonically coupled. The junctions between them behave passively. A full-sized spike evoked in 1 cell propagates to all the other cells with little delay at the junctions between them. The medullary relay axons innervate the spinal cells. Each axon produces a postsynaptic potential (PSP) of several millivolts in each spinal neuron. The PSP''s in the spinal neurons tend to reduce the peak of the spike when evoked in the proper time relation. The PSP due to a single medullo-spinal fiber is facilitated by repetitive activation. Transmission at this junction is probably chemically mediated. The morphological and electrophysiological data indicate lowered membrane resistivity occurs in the junctional regions, presumably at the areas of membrane fusion.