THE ROLE OF SYNAPTIC AND VOLTAGE-GATED CURRENTS IN THE CONTROL OF PURKINJE-CELL SPIKING - A MODELING STUDY

We have used a realistic computer model to examine interactions betwee n synaptic and intrinsic voltage-gated currents during somatic spiking in cerebellar Purkinje cells. We have shown previously that this mode l generates realistic in vivo patterns of somatic spiking in the prese nce of continuous background excitatory and inhibitory input (De Schut ter and Bower, 1994b). In the present study, we analyzed the flow of s ynaptic and intrinsic currents across the dendritic membrane and the i nteraction between the soma and dendrite underlying this spiking behav ior. This analysis revealed that: (1) dendritic inward current flow wa s dominated by a noninactivating P-type calcium current, resulting in a continuous level of depolarization; (2) the mean level of this depol arization was controlled by the mean rate of excitatory and inhibitory synaptic input; (3) the synaptic control involved a voltage-clamping mechanism exerted by changes of synaptic driving force at different me mbrane potentials; (4) the resulting total current through excitatory and inhibitory synapses was near-zero, with a small outward bias oppos ing the P-type calcium current; (5) overall, the dendrite acted as a v ariable current sink with respect to the soma, slowing down intrinsic inward currents in the soma; (6) the somato-dendritic current showed i mportant phasic changes during each spike cycle; and (7) the precise t iming of somatic spikes was the result of complex interactions between somatic and dendritic currents that did not directly reflect the timi ng of synaptic input. These modeling results suggest that Purkinje cel ls act quite differently from simple summation devices, as has been as sumed previously in most models of cerebellar function. Specific physi ologically testable predictions are discussed.