Neuroanatomy of the mormyrid electromotor control system

Mormyrid fish produce a diverse range of electric signals that are under the control of a central electromotor network. The anatomical organization of this network was delineated by injecting biotinylated compounds into neurophysiologically identified nuclei. Previous work using retrograde labeling with horseradish peroxidase indicated that the medullary command nucleus (CN) receives inputs from the precommand nucleus (PCN) at the mesencephalic-diencephalic border and the ventroposterior nucleus (VP) in the torus semicircularis. This study confirms these projections and identifies the dorsal posterior nucleus (DP) in the thalamus as an additional input to CN. DP and PCN form a bilateral column of cells extending ventrolaterally and caudally from the dorsal thalamus. The primary input to DP/PCN is from VP, which is identified as having two distinct subdivisions. A small group of large, multipolar cells along the ventral edge projects to DP/PCN and to CN, whereas a dorsal group of small, ovoid cells projects to DP/PCN but not to CN. VP receives input from the tectum mesencephali and the mesencephalic command-associated nucleus (MCA). As in all vertebrates, the tectum mesencephali receives input from several sources and likely provides multimodal sensory input to the electromotor system. MCA is part of the electromotor corollary discharge pathway, and its projection to VP suggests a feedback loop. These results, combined with recent physiological studies and comparisons with other taxa, suggest that modifiable feedback to DP/PCN plays a critical role in electromotor control and that the different inputs to CN may each be responsible for generating distinct electric signals.