Single-Unit Activity Patterns in Nuclei That Control the Electromotor Command Nucleus during Spontaneous Electric Signal Production in the MormyridBrienomyrus brachyistius

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Abstract
Mormyrid fish generate weak electric organ discharges (EODs) used for communication and navigation. EODs are initiated in the medullary command nucleus (CN), which receives dense projections from the mesencephalic precommand nucleus (PCN) and the adjacent thalamic dorsal posterior nucleus (DP), plus a minor projection from the ventral edge of the toral ventroposterior nucleus (VPv). The dorsal region of the ventroposterior nucleus (VPd) projects to DP-PCN and receives input from the electric organ corollary discharge pathway. I recorded extracellularly from single units within DP-PCN and VPd and correlated their activity patterns with electromotor output to generate hypotheses on electromotor control mechanisms. DP-PCN neurons show an oscillatory pattern of activity, firing within a window of 10-200 msec before each EOD, while remaining silent for 50-150 msec after each EOD. VPd neurons only fire during the silent period of DP-PCN neurons, suggesting that they provide recurrent inhibition to DP-PCN. During “scallops”, only DP-PCN neurons with high baseline firing rates increase their activity, whereas during “accelerations”, only neurons with low baseline firing rates show a strong increase in activity. Thus, the generation of different displays likely results from the activation of different groups of neurons projecting to CN. The activity of VPd neurons decreases during both displays, suggesting that disinhibition plays an important role in their generation. The mormyrid electromotor network shares many functional properties with central pattern generators (CPGs) found in relatively simple motor systems, indicating that it may be an excellent model system for studying CPG function in vertebrate communication.