Neural command of electromotor output in mormyrids

The electric discharge of mormyrid fish has an irregular pattern controlled by the electromotor command nucleus in the medulla, Anatomical studies sug gest that much of the descending information integrated by the command nucl eus comes from the diencephalic precommand nucleus. But field potentials re lated to the motor command occur later in the precommand nucleus than in th e command nucleus, suggesting that they are a corollary rather than a cause of electromotor command initiation. Recorded extracellularly, certain prec ommand nucleus units fire spontaneously between electromotor commands but p ause briefly following each command; others units fire a burst of spikes on ly during the post-command pause. The firing frequency of the former is cor related with the duration of the interval between successive electromotor c ommands when the fish is discharging at more than approximately 5 Hz, The p ost-command pause in spontaneous firing is due to corollary-discharge-media ted feedback inhibition, probably generated by the activity of the bursting units that fire only during this period. Precommand nucleus neurons are ac tivated by electrosensory input, and stimulation of the precommand nucleus modulates the endogenous pattern of electromotor command. We propose that t he irregular rhythm of the motor command depends largely on the integration of descending information of various origins, conveyed via the precommand nucleus to the command nucleus, and that this process is regulated by corol lary discharge feedback inhibition to the precommand nucleus.