Evidence for electrotonic coupling between frog motoneurons in the in situ spinal cord.

A recurrent EPSP was observed on antidromic stimulation of motoneurons in the in situ spinal cord of Rana temporaria and R. esculenta at 20-22C. The EPSP was finely graded and not refractory following full or partial antidromic spike components in a given neuron. The EPSP amplitude varied in parallel with the antidromic field potential under different conditions, suggesting transmission of the EPSP to the recorded motoneuron depended on invasion of the somadendritic membrane or neighboring motoneurons by the antidromic spike. The latency of the EPSP with respect to antidromic invasion of the local motoneuron pool was too short for the EPSP to be mediated by chemical transmission. It was concluded the EPSP was electrically transmitted between the somadendritic membranes of the motoneurons. Under certain conditions, the EPSP magnitude could be made to vary with membrane potential in a direction opposite to that expected from a chemical EPSP. Dendritic spikes were sometimes associated with the EPSP.