The mode of synaptic linkage in the cerebro-ponto-cerebellar pathway investigated with intracellular recording from pontine nuclei cells of the cat.

1. Intracellular potentials of pontine nuclei (PN) cells were recorded in cats anesthetized with pentobarbitone sodium. 2. Stimulation of the cerebellar nuclear regions or the brachium pontis induced an antidromic action potential composed of IS-SD spikes, after-depolarization and after-hyperpolarization. 3. Cerebellar stimulation produced EPSPs only in a few PN cells. 4. Activation of the corticopontine or pyramidal tract produced in all PN cells an EPSP built up from the unitary components with variable amplitudes and time courses. Paired or repetitive activation revealed a property of the frequency potentiation of the EPSP. 5. Unitary EPSPs also occurred spontaneously. A great majority of these spontaneous EPSPs were cerebral in origin, and had amplitudes and time courses comparable with those evoked by stimulation of the corticopontine or pyramidal tract. 6. The half-width versus time of peak relationship of these unitary EPSPs suggested a dendritic location of the synapses with variable distances from the soma. It is assumed that large, proximal synapses serve for efficient relay of signals while small, distal synapses for their integration.