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The 275 Purkyně cells identified by the criteria of the previous paper have been investigated with respect to their role as units integrating the input to the anterior lobe from various limb nerves. The discharges from single Purkyně cells have been studied in lightly anesthetized (pentothal) or in decerebrate unanesthetized cats, there being averaging usually of 128 responses in the form of post-stimulus time histograms and cumulative frequency distributions. Single Purkyně cells exhibited a wide variation in their responses to the diverse inputs from the various afferent nerves. Attention was focussed on excitatory and inhibitory responses evoked by mossy fibers with a short latency, usually 10-15 msec for hindlimb afferents. With most Purkyně cells these responses were predominantly evoked from cutaneous nerves, low threshold fibers being particularly effective. A few Purkyně cells were preponderantly excited by afferent volleys from muscle nerves, but there was a large group with a mixed input from cutaneous and muscle nerves. Graded strengths of stimulation of muscle nerves showed that sometimes group I volleys were prepotent, but other Purkyně cells were selectively excited by group II volleys. Though sometimes the afferent volleys from antagonistic muscles had a reciprocal action on a Purkyně cell, as on a motoneurone, it was more common to find similar actions. Also convergence of inputs from forelimb and hindlirnb nerves, both cutaneous and muscular, was not uncommon, particularly in marginal areas between hindlimb and forelimb zones. A special design feature is the convergence onto a Purkyně cell of mossy fiber and climbing fiber inputs evoked by the same afferent volley. This convergence was of particular interest along the parasagittal strip of hindlimb climbing fiber distribution in lobule V. It was not possible to translate the observations into some map of the cerebellar cortex on which are marked the territorial distributions from the various limb afferent nerves. Rather, there was an ill-defined patchy character, closely adjacent Purkyně cells often receiving very different subsets of the total input from the various limb nerves. The unitary integrations accomplished by the individual Purkyně cells are further integrated when their axons converge onto and inhibit the neurones of the cerebellar nuclei, and this integration by convergence would occur in each successive relay on the output pathways from the cerebellum. It is pointed out that the experimental findings on the integrative action of the individual Purkyně cells provide basic information for attempts to construct models simulating cerebellar performance and control. [ABSTRACT FROM AUTHOR]

The preceding two papers gave accounts of mossy fiber (MF) or of climbing fiber (CF) inputs to Purkyně cells under conditions where the other input was depressed by the experimental procedure. By utilizing either chloralose anesthesia or decerebration with sparing of the pyramidal tracts it has been possible to study the convergence of MF and CF inputs onto single Purkyně cells. The stimulation of cutaneous mechanoreceptors, the recording procedures for unitary Purkyně cell discharges and the computer averaging techniques were as previously described. Testing by taps to the footpads evoked a combined MF and CF response more commonly than either response alone, and often both inputs were very effective. There was a tendency for such phasic CF responses to be more frequently observed than the tonic responses to pad pressure, but such responses did occur. Purkyně cells were located by the usual procedure along the microelectrode tracks later identified in serial sections. Those cells activated by the fast MF inputs from the pad receptors were found to be closely associated in groups or colonies. The delayed MF inputs probably via spino-reticular pathways were more widely dispersed. The topographical relationships of these colonies are displayed on maps of the unfolded cerebellar cortex for lobules II to VI of both vermis and pars intermedia. In general these distributions of Purkyně cells activated from forefoot and hindfoot appear as islands in the larger fields that degeneration procedures exhibit for the cuneocerebellar and dorsal spinocerebellar tracts respectively. The CF inputs from the footpads also project to these same colonies, so that there are conjoint MF and CF colonies. The several modalities of the cutaneous mechanoreceptors of the forefoot or hindfoot often participate in the receptive fields of individual Purkyně cells. Such a field may be restricted to one or other side of the foot, all tested cutaneous mechanoreceptors then sharing approximately in the same restriction. Finally it is shown how these experimental findings relate to the theories of cerebellar function, particularly to the dynamic loop hypothesis. [ABSTRACT FROM AUTHOR]