Inhibitory synaptic potential of frog motor neurons

¹ Department of Physiology, University of Oregon Medical School, Portland, Oregon

Using isolated frog spinal cords treated lightly with pentobarbital, intracellular recordings from motor neurons revealed hyperpolarization of the membrane following selected dorsal root stimulation. The response showed 5.0 msec latency, 6.4 msec rise time, and 16.9 msec time constant of decay. Its reversal potential was 7.6 mv more negative than resting membrane potential. Threshold of the membrane for spike generation increased corresponding to the time course of the hyperpolarization. Spike potential generation following excitatory synaptic activation (dorsal root, lateral column, and adjacent ventral root) was depressed during the hyperpolarizing potential. Therefore, this hyperpolarization can be designated as inhibitory synaptic potential (IPSP). Pentobarbital sensitivity suggests a polysynaptic pathway. Evidence is presented which suggests that the IPSP is initiated from terminals on the somatic portion of the membrane.