Comparative analysis of cerebellar unit discharge patterns in the decerebrate cat during passive movements

Summary

1) The present experiments were undertaken to study how information about the parameters of a passive movement is processed at different neuronal levels of the cat cerebellar cortex. The analysis has been performed by recording extracellularly in the intermediate part of the cerebellar anterior lobe from presumed mossy fibres, presumed granule cells, and Purkinje cells with simple spikes and complex spikes. 2) The discharge patterns obtained during passive movements of the cat's forepaw were characterized by components which could be related to dynamic or static parameters of the movement. With respect to the occurrence of dynamic responses, patterns were classified according to a statistically derived measure in three different types. By using the same statistical measure, discharge patterns were additionally classified into two subgroups according to their response components reflecting static parameters. Within the patterns a clearcut relationship between dynamic and static components was observed. The corresponding distributions are shown and discussed. 3) A very interesting result of the classification of cerebellar discharge patterns is that the distribution of the different types depended on the level of integration within the cerebellar cortex. Patterns of the low scale integrated cerebellar input (mossy fibre-system), as well as those of granule cells (the first cerebellar computational niveau), reflected both static and dynamic movement parameters. At the Purkinje cell level (a level with a high degree of convergence) the discharge patterns are characterized predominantly by dynamic responses. 4) The interrelationship between complex- and simple spikes of Purkinje cells was tested by different methods: a) By analyzing the paired values of the mean complex-(CS) and simple spike (SS) discharge probabilities of 110 Purkinje cells a scatter was obtained, indicating an underlying hyperbolic relation (prob(CS) = a/(prob(SS))^b). Thus, a high CS discharge probability is accompanied by a low SS probability and vice versa, b) The timelocked complex- and simple spike responses were studied by comparing the similarity of their responses. All combinations of complex- and simple spike patterns were observed, ranging from a sign correct similarity to a mirror image similarity. The distribution of the measure for similarity shows that the mirror image predominated, c) The individual simple spike discharge probability is characterized by a pause evoked by the occurrence of a complex spike event. The simple spike discharge probabilities during an interval preceeding and following a complex spike event were compared. A post climbing pause coefficient was defined as a measure for the effectiveness of the complex spike event. No relationship between these coefficients and the above mentioned measure for similarity was found. Hence, for the Purkinje cell discharging with the simple spikes independent spike generating processes have to be assumed. 5) From these results it can be derived that cerebellar discharge patterns can be classified with respect to responses to static and dynamic parameters of passive limb movements. Based on this classification it appears that the distribution of responses to static and dynamic parameters depends on the computational level within the cerebellar cortex. If both static and dynamic parameters are conveyed by a single unit, a clear relationship between the response components could be observed. However, this effect was independently found at all cerebellar cortical computational levels indicating a functional principle of processing a pair of movement parameters. The interrelation of complex- and simple spike responses to passive movement was further studied. Since transients of complex- and simple spike patterns were observed ranging from two almost identical patterns to mirror image like patterns, it is assumed that under physiological conditions one of the tasks of the climbing fibre system consists of tuning the simple spike discharge according to the peripheral requirements.