The Electrophysiology of the Cerebellar Purkinje Cell Revisited

Abstract

The electrophysiological properties of mammalian Purkinje cells has been a subject of study for more than 30 years (Eccles et al, 1967) and has produced a wealth of information relating to the ionic conductances in mammalian central neurons (cf. Llinás, 1988). Yet it is clear from the many examples of intracellular recordings obtained in vivo that the Purkinje cell is capable of many functional states. Such complexity is the expected product of the rich intrinsic electrical properties of these cellular elements and of the wealth of synaptic connectivity they receive. Indeed, if we consider that the tertiary spiny branchlets of mammalian Purkinje cells are contacted by many as 200,000 parallel fibers (Fox and Barnard, 1957; Fox et al, 1967) and that, unlike other central nervous system (CNS) spines (Peters et al, 1976), only one parallel fiber synapse is made onto each spine head (Paley and Chan-Paley, 1974), a large variation of dendritic-input patterns and thus, an enormous variability in the activation states, may be expected. Equally surprising is the other main synaptic input, the climbing-fiber system. In most adult cells only one climbing-fiber afferent contacts a given Purkinje cell. The contacts are made on groups of postsynaptic spines arising from the smooth surface of the primary and secondary dendrites (Larramendi and Victor, 1967). Clusters of six to eight spines are often organized in longitudinal rows along the dendrite and are contacted by en passant enlargements of the climbing fiber as it progresses over the surface of the dendrites. As many as 300 spines are contacted by a single afferent fiber (Llinas et al, 1969). Given this structure, it is not surprising that a stereotyped synaptic response to climbing fiber activity, the complex spike, is observed at all levels of cerebellar phylogeny (Llinas and Hillman, 1969).