Faculty Bibliography

Electrophysiological evidence for the existence of dendritic spikes in the Purkinje cells of pigeon cerebellar cortex is presented. Intradendritic records indicate that the electroresponsive properties of the dendrites linger after voltage-dependent sodium and potassium conductances are reduced by superfusion of the cortex with Ringer's solution containing 20 mug/ml tetrodotoxin and 5 mM 3-aminopyridine. Dendritic spikes could be evoked, in the complete absence of activity from all cerebellar afferents and from the soma and axon of the Purkinje cells, by direct electrical activation of the surface of the cerebellar cortex or by intracellular injection of current via the recording electrode. The denritic electroresponsiveness was blocked by superfusion with 20 mM Mn2+ or Co2+. It is thus concluded that dendrites of Purkinje cells in birds are capable of generating calcium-dependent spikes. The possible role of such a calcium current in neuronal function is discussed.

1. The electrophysiological properties of the EPSP generated in Purkinje cells by the activation of CFs were studies in the cat cerebellar cortex. 2. CF-EPSPs were evoked by electrical stimulation of the cerebellar white matter and recorded intracellularly from the soma of the Purkinje cells. 3. Current was injected into the Purkinje cells via the recording micropipette using a bridge amplifer in order to study the reversal properties of the EPSP. 4. The CF-EPSP reversal was biphasic with the early portion reversing first. 5. The reversed EPSP waveform was not a mirror image of the EPSP, but displayed a briefer time course. 6. A four-compartment computer stimulation showed that the reversal properities of the CF-EPSP were explicable in terms of a distributed synapse on a cable. 7. The biphasic reversal and asymmetry were shown to be due to the spatially nonuniform potential distribution created by the somatic current injection, which predominantly reversed the proximal part of the distributed synapse. Delayed rectification may also have contributed to the reversal asymmetry. 8. The advantages of a distributed synapse over a point synapse are discussed and the reversal properties of the CF-EPSP compared to those of the Ia-evoked EPSP in motoneurons.