Neuronal circuitry in the nucleus lateralis of the cerebellum.

Abstract

This study summarizes the nervous circuitry of the lateral nucleus from evidence presented in the previous papers in this investigation. The large and small neurons receive three types of extracerebellar afferent fibers—collaterals of climbing fibers and mossy fibers on their way to the cerebellar cortex, and fluorescent CAT fibers of two varieties. The large projection neurons receive a large inhibitory corticonuclear input via synapses of Purkinje cell axons upon their somatic and dendritic surfaces, as well as upon initial axonal segments. On their way into the superior cerebellar peduncle, the axons of these neurons send recurrent collateral branches, which provide a positive feedback circuit within the nucleus. The small inhibitory interneurons depend to a large extent on the activating input upon their dendrites; their corticonuclear input from Purkinje cells is less numerous than that of large neurons. Their myelinated axons provide a rapid system of internal inhibitory control. In cases in which the axons are long and leave the nucleus, like those emitted by the small neurons in the medial hilus zone, they may provide a route of internuclear communication. The arrangements of neurons in the columnar zone are reviewed. The large neurons are oriented at an angle to incoming extracerebellar fibers and present a profile to the Purkinje cell axon. The interneuronsare tilted at ninety degrees to the dendritic trees of the large neurons, and they receive a larger extracerebellar afferent input. Whereas the corticonuclear input to the large columnar neurons probably expresses a ratio of one Purkinje cell to one large neuron, the tilt assumed by the small neurons may increase this ratio. Electron microscopy has shown that small neurons receive a smaller corticonuclear input than large neurons. Elsewhere in the swirled zones of the nucleus, these influences are less precisely oriented, and therefore less restrictive interplay exists between corticonuclear influences and the large multipolar neurons.