Pathways of the electric organ discharge command and its corollary discharges in mormyrid fish.

The motoneurons which innervate the mormyrid electric organ are driven by a descending volley from the medullary relay nucleus. This nucleus does not initiate the electric organ discharge (EOD) but is driven in an obligatory manner by another center, a command nucleus. One goal of the present study was to identify this command nucleus anatomically. A second goal was to determine the pathways by which corollary discharges of the EOD motor command exert their effects on sensory input to the electroreceptive lateral line lobe. Horseradish peroxidase (HRP) was injected into the medullary relay nucleus and other EOD command-related centers. Placement was guided by recording the electrical activity preceding the EOD. A nucleus of smaller cells is found immediately beneath the large cells of the medullary relay nucleus. This nucleus, nucleus C, projects densely to the medullary relay nucleus and is hypothesized here to be the command nucleus. Nucleus C appears to receive input from the mesencephalon and from unspecified sources of input to the nearby reticular formation. Nucleus C projects to the medullary relay nucleus and to a lateral nucleus, the bulbar command-associated nucleus. This nucleus is probably the source of the corollary discharge signals. It projects to the medullary relay nucleus and to the paratrigeminal and mesencephalic command-associated nuclei. The latter two nuclei project to separate regions which in turn project to the electroreceptive lateral line lobe. There are thus at least two different paths by which the presumed EOD command nucleus, nucleus C, can affect the electroreceptive lateral line lobe.