Afferent axons and their relations with neurons in the nucleus lateralis of the cerebellum: A light microscopic study

Summary The axons of Purkinje cells are the sole corticonuclear afferents to the lateral nucleus. The terminal arborizations of these axons consist of many (30–50) varicose branchlets, which issue from a thick, myelinated parent axon. Each terminal plexus fills a conical field which penetrates the lateral nucleus radially encompassing the cell bodies and parts of the dendritic trees of approximately 40 neurons. The fields of neighboring Purkinje axons overlap considerably. The non-cortical axons are simple, usally unbranched varicose fibers of three sizes: (1) thick, with large varicosities, (2) medium sized with smaller varicosities, or (3) fine, delicate threads with beadlike varicosities. These axons cross the dendritic trees of successive neurons as they penetrate into the nucleus in a radial fashion.The configuration of the dendritic trees of neurons in the various parts of the nucleus—the multipolar neurons and the columnar neurons—can be related to the conical shape of the Purkinje axonal plexus. It is suggested that the organization of converging Purkinje cell axonal fields determines the pattern of input to the cells of the lateral nucleus, rather than the topographical arrangement of Purkinje cells in the cortex. The terminal arborizations of Purkinje cell axons adjacent to one another in the lateral nucleus need not necessarily arise from neighboring Purkinje cells in the cerebellar cortex.The relationships between neurons in the central columnar zone and in the swirled zones of the lateral nucleus with the two classes of afferents are discussed. It is suggested that by virtue of their slender profiles, each of the large columnar neurons falls into the field of one Purkinje cell axonal cone whereas elsewhere, the multipolar neurons tend to share their well spread dendrites with neighboring Purkinje axonal fields. The small neurons that span columns in the central zone are oriented to sample larger numbers of axonal inputs than are adjacent columnar neurons.Supported in part by U.S. Public Health Service Research Grants NS10536, NS03659, Training Grant NS 05591 from the National Institute of Neurological Diseases and Stroke, and a William F. Milton Fund Award from Harvard University.