Morphology and physiology of the brainstem nuclei controlling the electric organ discharge in mormyrid fish.

The motoneurons to the mormyrid electric organ are driven from the medullary relay nucleus. This nucleus is in turn innervated by an adjacent cell group, nucleus C. The goals of this study were to characterize the morphology and physiology of neurons in these two nuclei, and to test the hypothesis that nucleus C is the command nucleus responsible for initiating the electric organ discharge. Medullary relay neurons and nucleus C neurons were recorded intracellularly and labeled with horseradish peroxidase. Medullary relay neurons have a richly branched dendritic arborization, confined mainly to the nucleus itself, and somatosomatic, dendrosomatic, and presynaptic dendro-axonal gap junctions have been observed. Medullary relay neuron axons descend to the spinal cord without branching. Nucleus C dendrites extend far into the ventral reticular formation. Axons of nucleus C neurons have one branch that ramifies densely within the medullary relay nucleus, forming large club endings on the medullary relay neuron soma. Two additional branches project bilaterally toward the bulbar command associated nuclei. Both medullary relay neurons and nucleus C neurons fire a double action potential that precedes each electric organ discharge. Activity in nucleus C precedes that in the medullary relay nucleus by 100-300 microseconds. Postsynaptic activity is recorded in nucleus C neurons but not in medullary relay neurons. Hyperpolarization of a single nucleus C neuron can lower the frequency of the electric organ discharge. Both morphological and physiological data indicate that nucleus C is an integrating center where the electric organ command is initiated.