Dynamic single unit simulation of a realistic cerebellar network model. II. Purkinje cell activity within the basic circuit and modified by inhibitory systems.

In continuation of earlier computer simulation studies 15 of the feedback inhibition exercised by the Golgi cells of the cerebellum, an attempt is made at modeling the behavior of Purkinje and basket cells under somewhat more complex functional circumstances. The simulation study is based on a realistic network model (with respect to numerical and metrical parameters, and consisting of more than 3 X 10(4) units) of the cat cerebellar cortex, derived from a recent quantitative histological and stereological analysis 9-12. As an input two identical pairs of foci of incoming mossy fiber activity are applied, separated from one another both spatially and temporally. The stimulation results showed that the assumption of a rigidly preaddressed wiring in the parallel fiber--Purkinje cell synaptic system (i.e. that parallel fibers would systematically select for contact Purkinje cells standing in register) would hardly be realistic because such a solution would offer no advantage whatever over a randomly connected synaptic system. The results would favor the contention that dendritic geometry of the Purkinje cells has a crucial significance in the sense of the concepts developed by W. Rall. The stimulation gives some important hints for the ways in which the specific inhibitory interneuron systems (Golgi and basket cells) add refinement to the operations of the network, particularly in securing integration of the influences of parallel fibers thrown into action by mossy input to spatially and temporally separated foci, especially at the level of Purkinje cells positioned in interfocal areas. Reducing the length of the parallel fibers to one-half of what was suggested by the stereological studies causes a serious breakdown of interfocal neighborhood interactions.