ELECTRIC ORGAN COROLLARY DISCHARGE PATHWAYS IN MORMYRID FISH .1. THE MESENCEPHALIC COMMAND ASSOCIATED NUCLEUS

Corollary discharge signals associated with the motor command that elicits the electric organ discharge are prominent in the electrosensory lobe of mormyrid fish (Gnathonemus petersii). Central pathways and structures that convey these signals from the motor command nucleus to the electrosensory lobe are known anatomically, but these structures and their contributions to the various corollary discharge phenomena have not been examined physiologically. This study examines one such structure, the mesencephalic command associated nucleus (MCA). Recordings from MCA cells show a highly stereotyped two spike response. The first spike of the response has a latency of about 2.5 ms following the initiation of the electric organ discharge (EOD) motor command which is about 5.5 ms before the occurrence of the EOD. Results from stimulation and lesion experiments indicate that MCA is responsible for: 1) the gate-like corollary discharge-driven inhibition of the knollenorgan pathway; 2) the gate-like corollary discharge-driven excitation of granule cells in the mormyromast regions of the electrosensory lobe; and 3) various excitatory effects on other cells in the mormyromast regions. Some corollary discharge phenomena are still present after MCA lesions, including the earliest corollary discharge effects and the plasticity that follows pairing with electrosensory stimuli. These phenomena must be mediated by structures other than MCA.