The Climbing Fiber Input and its Excitation of Purkinje Cells

Abstract

The simple synaptic contacts made by a climbing fiber have been adequately described in Chapter II B and illustrated in Figs. 21, 23 and 24. Each climbing fiber has a primary synaptic relation to a single Purkinje cell. This synaptic contact is of very unusual extent, by reason of the long parallel relation between the branches of the climbing fiber and of the smooth dendrites of the Purkinje cell. The contact is not truly parallel or continuous, but is interrupted, so that there are probably hundreds if not thousands of separate contacts between any climbing fiber and its Purkinje cell. Even if one or the other of the short side branches terminating in a small knob were to touch a neighbouring Purkinje cell, as surmised by Scheibel and Scheibel (1954), this small contact would be insignificant functionally. Furthermore, as one can see on longitudinal or even better on oblique sections of folia with all Purkinje cells stained, there is no interlacing between the dendrites of neighboring Purkinje cells in the longitudinal direction of the folium. Between each of these dendritic arborizations there is a small space that is occupied by the dendritic arborizations of basket and stellate neurones (Fig. 110). Most of the side branches of the climbing fibers are so short that they can only make synaptic contacts on the same Purkinje cell. There are, however, also longer side branches of the climbing fiber that give synapses to basket and outer stellate cells, as described in Chapter II, Section B, and the existence of these axo-somatic synapses of climbing fiber collaterals has been confirmed by degeneration and EM analysis on Golgi neurones and basket cells (Fig. 36; Hámori and Szentágothai, 1966 a) — and their existence on stellate cells is highly probable.