SPATIAL-DISTRIBUTION OF THE MEDULLARY COMMAND SIGNAL WITHIN THE ELECTRIC ORGAN OF GYMNOTUS-CARAPO

The pacemaker nucleus of Gymnotus carapo contains two types of neurons : pacemaker cells which set up the frequency of the electric organ dis charge (EOD) and relay cells which convey the command signal to the sp inal cord. Direct activation of a single relay cell provides enough ex citation to discharge a pool of spinal electromotor neurons and electr ocytes, generating a small EOD (unit EOD). Different relay cells gener ate unit EODs of variable size and waveform, indicating the involvemen t of different groups of electrocytes. A special technique of EOD reco rding (multiple air-gap) was combined with intracellular stimulation o f relay cells to study the spatial distribution within the electric or gan (EO) of the command signal arising from different relay cells. Thr ee types of relay cells could be identified: type I commanding the ros tral 10% of the EO, type II which distribute their command all along t he EO and type III driving the caudal 30%. Waveform analysis of unit E ODs indicates that doubly innervated electrocytes which are the most r elevant for attaining the specific EOD waveform, receive a favored com mand from the pace-maker nucleus.