Experimental observations suggest that the cerebellar cortex is somatotopically organized, but not in the same manner as the motor and sensory cortices of the cerebral hemispheres. Stimulation of the cerebellar cortex does not produce movement of body parts. Moreover, motor function of body parts is not represented in a continuous fashion in the cerebellar cortex and instead corresponds to small discontinuous patches. There is an approximate motor specificity in that the legs, trunk, and gait are affected in a restricted fashion with lesions of the vermis and coordination of limb movements is affected by lesions of the body of the cerebellar hemispheres but this does not simulate the refined organization observed in the cerebral cortex. However, a sensory organization that reflects the topography of the body has been confirmed by mapping local peripheral sensory stimuli to corresponding sites of the cerebellar cortex in animals, and by analyzing the subtle motor effects produced by stimulation of parts of the cerebellar cortex. Many published diagrams of the cerebellum with an overlaid homunculus reflect sensory, not motor, topographic organization. The interesting history of ideas regarding cerebellar localization and current understanding are reviewed by Manni and Petrosini.
Role of the Deep Cerebellar Nuclei
The physiologic studies of Allen and Tsukahara and of Thach and colleagues produced some of the most important data pertaining to the role of the deep cerebellar nuclei. These investigators studied the effects of cooling the deep nuclei during a projected movement in the macaque monkey. Their observations, coupled with established anatomic data, permit the following conclusions.
The dentate nucleus receives information indirectly from the premotor and supplementary motor cortices via the pontocerebellar system and helps to initiate volitional movements. These movements are accomplished via efferent projections from the dentate nucleus to the ventrolateral thalamus and motor cortex. The dentatal neurons have been shown to fire just before the onset of volitional movements, and inactivation of the dentatal neurons delays the initiation of such movements. The interpositus nucleus also receives projections from the cortex via the crossed pontocerebellar fibers; in addition, it receives spinocerebellar projections via the intermediate zone of the cerebellar cortex. The spinocerebellar projections convey information from Golgi tendon organs, muscle spindles, cutaneous afferents, and spinal cord interneurons involved in movement. The interpositus nucleus fires in relation to a movement once it has started. Also, the interpositus nucleus appears to be responsible for making volitional oscillations (alternating movements). Its cells fire in tandem with these actions and the regularity and amplitude of movements become impaired when these cells are inactivated. In addition, Thach pointed out that the nucleus interpositus normally damps physiologic tremor and suggested that this may play a part in the genesis of the intention tremor described further on. The fastigial nucleus receives projections from the spinocerebellar fibers and, like the interpositus, projects to the vestibular nuclei. It controls antigravity and other muscle synergies in standing and walking; ablation of this nucleus greatly impairs these motor activities.
There are complex physiologic relationships among the deep nuclei, and cerebellar cortical regions generally have inhibitory influences on the deep nuclei. The flocculonodular lobe has among the most complex influences, being inhibitory to the fastigial nucleus but also projecting directly in an inhibitory manner to the lateral vestibular (Deiters) nucleus. The vestibular nuclei, and in particular the lateral one, may be considered, in effect the equivalent of a deep cerebellar nucleus.
Neuronal Organization of the Cerebellar Cortex