Coordinated and fluid movements of the limbs and trunk result from a neuronal organization in the cerebellum that permits an ongoing and almost instantaneous comparison between desired and actual movements while the movements are being carried out. An enormous number of neurons are committed to these tasks, as attested by the fact that the cerebellum contributes only 10 percent to the total weight and volume of the brain but contains half of the brain’s neurons. Also, it has been estimated that there are 40 times more afferent axons than efferent axons in the various cerebellar pathways—a reflection of the enormous amount of incoming (sensory) information that is required for the control of motor function.
The cerebellar cortex is configured as a stereotyped three-layered structure, the molecular, Purkinje, and granular layers, that together contain five types of neurons (Fig. 5-5). In its relatively regular geometry, it is similar to the columnar architecture of the cerebral cortex, but it differs in the greater degree of intracortical feedback between neurons and the convergent nature of input fibers. The outermost molecular layer of the cerebellum contains two types of inhibitory neurons, the stellate cells and the basket cells. They are interspersed among the dendrites of the Purkinje cells, the cell bodies of which lie in the underlying layer. The Purkinje cell axons constitute the main output of the cerebellar cortex, which is directed at the deep cerebellar and vestibular nuclei described above. Purkinje cells are, as mentioned earlier, entirely inhibitory and utilize the neurotransmitter gamma-aminobutyric acid (GABA). The innermost granular layer contains an enormous number of densely packed granule cells and a few larger Golgi interneurons. Axons of the granule cells travel long distances as parallel fibers, which are oriented along the long axis of the folia and form excitatory synapses with Purkinje cells. Each Purkinje cell is influenced by as many as a million granule cells to produce what has been termed physiologically a “simple spike,” in contrast to the complex spike noted below.
The predominant afferent input to the cerebellum is via the mossy fibers, which are the axons of the spinocerebellar tracts and via the projections from pontine, vestibular, and reticular nuclei. They enter through all three cerebellar peduncles, mainly the middle (pontine input) and inferior (vestibulocerebellar) ones. Mossy fibers ramify in the granule layer and excite Golgi and granule neurons through special synapses termed cerebellar glomeruli. The other main afferent input is via the climbing fibers, which originate in the inferior olivary nuclei (olives) and communicate somatosensory, visual, and cerebral cortical signals (Figs. 5-5 and 5-6). The climbing fibers, so named because of their vine-like configuration around Purkinje cells and their axons, preserve a topographic arrangement from olivary neuronal groups; a similar topographic arrangement is maintained in the Purkinje cell projections. The climbing fibers have specific excitatory effects on Purkinje cells that result in prolonged “complex spike” depolarizations. The firing of stellate and basket cells is facilitated by the same parallel fibers that excite Purkinje cells, and these smaller cells, in turn, inhibit the Purkinje cells. These reciprocal relationships form the feedback loops that permit the exquisitely delicate inhibitory smoothing of limb movements that are lost when the organ is damaged.
The uniform cortical structure of the cerebellum can reasonably lead to the notion that the organ has similar effects on all parts of the cerebrum to which it has projections (cortex, basal ganglia, thalamus, etc.).
It would follow that the activities of these cerebral structures (motor, cognitive, sensory) may be modulated in similar ways by cerebellar activity.