Synaptic responses of medullary neurons of turtles to stimulation of the locomotor region

Responses of medullary neurons to microstimulation of the locomotor region by a current of up to 30 μA were studied by intracellular recording in turtles. The resting potential recorded in these neurons was from 22 to 42 mV. Depolarization PSPs (EPSPs) were recorded in 43 neurons, hyperpolarization PSPs (IPSPs) in 12, and mixed in 36. Synaptic discharges were observed in 29 neurons. Of these cells 11 generated action potentials without visible PSPs. The latent period of the PSPs was most frequently between 2 and 8 msec. The time from the beginning of the EPSP to the beginning of the action potential was 1-3 msec if the response index was high, but in the case of weaker stimulation, it began to fluctuate strongly and lengthened. Unitary EPSPs were recorded in 15 neurons and IPSPs in three. Their amplitude was 0.6-0.8 mV, from 2 to 12 times less than the depolarization threshold (1-7 mV). These results, together with those obtained previously by extracellular recording of medullary unit activity in turtles and cats, are used to discuss the possible mechanism of spread of locomotor activity.