Heterogeneity in the pattern of distribution of the axonal collaterals of Purkinje cells in zone b of the cat's vermis: An intracellular HRP study

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Abstract
Intracellular recordings were obtained from Purkinje cells located in zone b of the vermis; all of the cells were located within lobule V. The ulnar and sciatic nerves were stimulated bilaterally in order to determine the input characteristics of each impaled neuron. Subsequent to recording physiological data the neurons were intracellularly labeled with horseradish peroxidase (HRP). At the conclusion of the experiment the animals were perfused and their brains were processed for HRP histochemistry. Zone b was divided histologically into five 200‐μm intervals and the location of each intracellularly injected Purkinje cell relative to these intervals was determined. The input characteristics of Purkinje cells within a single 200‐μm interval are not identical. Some Purkinje cells respond exclusively to input from a specific limb, whereas other Purkinje cells within the same interval are activated by both forelimb and hindlimb stimulation. The collaterals of a zone b Purkinje cell form a dense plexus at the Purkinje cell‐granule cell junction; branches also distribute to the superficial and deep granule cell layer, as well as to the lower molecular layer. Two patterns of collateral distribution were observed. In one pattern, the plexus has a single arbor confined to the vicinity of the cell body of origin. In the second pattern, at least two arborizations are present. One arbor remains within the vicinity of the cell body of origin whereas other arbors are distant from the parent cell; the two arbors are separated by areas devoid of collaterals. An analysis of the distribution of axonal varicosities shows that they are not uniformly distribution around the cell body of origin in either the sagittal or transverse plane. Cortical areas that contain numerous beads are interspersed between regions that contain few or no varicosities. The discontinuous pattern of distribution and the presence of varicosities in all three cortical laminae suggest that Purkinje cells in zone b have multiple discrete foci of influence on diverse populations of cerebellar neurons.These anatomical findings suggest that individual Purkinje cells may play different functional roles in the local circuitry of zone b. Cells with the spatially confined arbors may be involved with the integration of information within restricted sagittal intervals (approximately 200 μm in width) of the cerebellum. Purkinje cells with the dual arbors have the potential to integrate information over a greater extent of zone b or, in some cases, between zone b and adjacent cortical zones (e.g., c1 or x).