Rhythmicity as an intrinsic property of the mormyrids electromotor command system

Abstract

The timing of electromotoneuron discharges was investigated in weak-electric mormyrid fish ( Gnathonemus petersii) after suppression of electrosensory feedback by injection of FLAXEDIL. FLAXEDIL, a paralytic agent, momentarily silenced the electric organs, removing the autostimulation of the fish's electroreceptors by its own discharge. It is shown that in such conditions: (1) the electromotor output was cyclic; (2) the time intervals separating discharges belonged mostly to two distinct categories; (3) the first category was centred in all fish around 100 msec; (4) the second category fluctuated depending on the individuals between 250 and 400 msec; (5) the different types of time intervals did not follow each other at random; (6) their serial ordering presented inter-individual differences. A rhythmic pattern was thus demonstrated in the absence of a feedback time-locked to the motor command, bringing evidence that rhythmicity results from the intrinsic activity of the electromotor command system. This pattern showed many similarities with that recently described in immobile and undisturbed discharging fish [26], leading us to postulate that the mormyrid electromotor output is organised by a central pattern generator (CPG). It is suggested that peripheral control, achieved through the use of electrosensory feedback, serves to increase the variability of the rhythms of discharge, so that they are adapted to the situation in which the fish participates.