Post‐tetanic potentiation, habituation and facilitation of synaptic potentials in reticulospinal neurones of lamprey

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Abstract
Synaptic potentials evoked by electrical stimulation of cranial nerves were recorded in giant reticulospinal neurons (Mueller cells) of lamprey. A variety of patterns of stimulation was employed to explore further the functional properties of the pathways intervening between the cranial nerve fibers and Mueller cells. Simultaneous low intensity stimulation of 2 different cranial nerves produced excitatory short-latency synaptic potentials whose amplitudes summed linearly. Tetanic (10/s) stimulation of a cranial nerve depressed the evoked short-latency synaptic response, but following the tetanus the synaptic response was potentiated above control amplitude for several minutes. Tetanic stimulation of one cranial nerve had no effect upon the synaptic responses evoked by stimulation of other cranial nerves. Low-frequency stimulation (1/s-1/20 s) of a cranial nerve produced a progressive decrease in the amplitude of the evoked short-latency synaptic response. This phenomenon was termed synaptic habituation because its characteristics were functionally similar to behavioral habituation in animals. Habituation of the synaptic response to stimulation of one cranial nerve had no effect on the synaptic responses produced by stimulation of other cranial nerves. Synaptic afterdischarges lasting from several seconds to several minutes were recorded in Mueller cells. They occurred both spontaneously and in response to strong electrical stimulation of cranial nerves. For several minutes following an afterdischarge the amplitudes of short-latency synaptic potentials produced by stimulation of any one of the cranial nerves were increased as much as 2-fold. This facilitation occurred equally well whether the short-latency synaptic responses were habituated or not. A theoretical cell-wiring diagram was proposed to account for the properties of short-latency evoked synaptic responses and synaptic afterdischarges and for the facilitation of short-latency responses by afterdischarges.