The output of the cerebellum projects to all components of the voluntary and postural motor systems except the basal ganglia. The output is generated by the deep cerebellar nuclei and by the vestibular nuclei (the ‘deepest’ of the deep cerebellar nuclei); the output is excitatory. The cerebellum controls the activities of the many antagonist and synergist muscles that are used automatically in most normal movements. The cerebellum combines and coordinates their timing, duration, and amplitude of activity. It helps in the learning of new motor skills and in modifying old ones, such that they may be performed with perfect coordination and automatically.
Damage of the lateral cerebellar hemisphere cortex and dentate nucleus causes a curved trajectory, overshoot of endpoint, terminal tremor on the finger-nose-finger and heel-knee-shin tests, and irregularity of rapid alternating pronation-supination of the wrist and finger-to-thumb tests. Damage of the midline cerebellar cortex and fastigial nuclei causes falls to the side of the lesion, especially on heel-to-toe gait. There is increasing evidence that the extreme lateral, posterior, and inferior regions of cerebellar cortex and the lateral inferior portions of the dentate nucleus may control performance of so-called ‘cognitive’ tasks that do not involve overt movement (see also Chapter 9; Fiez et al., 1992).
Neuronal activity in the cerebellar nuclei
The output from the cerebellum is generated by its deep nuclei. Each deep cerebellar nucleus appears to have a separate somatotopic representation of the body, with the head caudal, tail rostral, trunk lateral, and extremities medial (Orioli and Strick, 1989; Asanuma et al., 1983a, 1983b, 1983c, 1983d).