Anatomical Organization of the Cerebellar Nuclei with Reference to the Afferents of Non-Cortical Origin

The cerebellum comprises two major portions, the cerebellar cortex and the cerebellar nuclei, the latter being conventionally given various terms (deep nuclei, central nuclei or intracerebellar nuclei). A number of anatomists have been making particular efforts to analyze the unique structure of the cerebellar cortex by means of classical histological techniques and by electron microscopy. Extensive physiological invetigations have also greatly contributed to elucidating the neuronal functions of the cerebellar cortex and provided an effective model to reveal the basic mechanisms of the brain function. Ito and his collaborators made an epochmaking discovery that the Purkinje cell in the cerebellar cortex is inhibitory in nature. This has become a breakthrough against the stalemated studies on the cerebellar anatomy and has drawn our attention to the significance of the excitatory input to cells of the cerebellar nuclei, which are under inhibitory control by the Purkinje cells. Our anatomical studies have been devoted to elucidating the origin of the excitatory afferents and analyzing the neuronal organization of the cerebellar nuclei. In this article recent advances have been reviewed primarily from the anatomical aspects.Projections to the cerebellar nuclei from the inferior olive, the lateral reticular nucleus and the spinal cord were established by experimental anatomical methods at the light and partly at the electron microscope levels. Additional projections have been reported to originate from the red nucleus, external cuneate nucleus and vestibular organ. Correspondingly, Golgi observations show that there is a terminal pattern specific to the afferents of extrinsic origin in the cerebellar nuclei. These fibers issue many coilaterals passing through the nuclei whereas the Purkinje cell axons terminate within a confined area with dense ramifications. Electron microscope studies by Chan-Palay suggest that the terminals of Purkinje cell axons have elliptical synaptic vesicles and make axosomatic synapses while those of extrinsic origin have round vesicles, making axodendritic synapses.Two types of neuronal element were identified in the nuclei, one is a large projection neuron and the other a small interneuron. The axon of the former type gives off recurrent collaterals, with which interneurons are connected. The interneurons project, in turn, onto the projection neurons and the other interneurons, presumably to establish a negative feedback circuit.At present it seems premature to systematize our knowledge since all the information available is still incomplete in many respects. For example, the origin of the excitatory afferents to the cerebellar nuclei is not fully disclosed, and the identification of axon terminals is not yet verified experimentally. However, it is expected that many difficulties will be surmounted by the recent technical innovations in the field of neuroanatomy, the methods which take advantage of anterograde and retrograde axonal transport.