Compartmentalized cerebellar functions upon the stabilization of body posture.

This paper reviews the features and topodiagnostical meaning of pathological body posture in cerebellar disorders and presents knowledge as to the physiology and pathophysiology of postural disturbances attributed to the cerebellum. Lesions of the spinocerebellar (upper vermal and intermediate) part of the anterior lobe lead to spontaneous anterior-posterior body sway, with a frequency of about 3 Hz with mild or absent ataxia of the upper limbs, but prominent ataxia of the lower limbs. The tremor is provoked by eye closure and by body displacement. Increased gain and poor cerebellar control of the duration and gain of stabilizing reflexes may be the mechanisms of this oscillation. Lesions of the lower (vestibulocerebellar) vermis cause postural ataxia of the head and trunk while sitting, standing, and walking. Postural ataxia is omnidirectional, sometimes of excessive amplitude and contains frequency components below 1 Hz. Dysmetria of the upper and lower limbs is not prominent. Visual stabilization is less than in the other groups of cerebellar patients. Lesions of the cerebellar hemispheres alone, do not cause an increase of postural sway. Lesions of spinocerebellar afferents (Friedreich's disease) lead to a low frequency large amplitude lateral sway with the most of its power below 1.1 Hz. Visual stabilization is preserved. Latencies of early and late EMG responses to sudden displacements are normal in patients with cerebellar disorders. The cerebellum is, therefore, unlikely to be the primary generator of these reflexive responses. On the other hand, the temporal composition of the orderly time sequence of a complex motor program--be it a voluntary act or its postural balancing--is deranged in cerebellar lesions. It is hypothesized that the cerebellum helps to coordinate the timing not only within but also between the single components involved in each subunit of a complex movement in three-dimensional space and that is scales the size and duration of each muscular action. The cerebellum possibly specifies the cortical movement command and sends it back to the motor cortex. The basic structure of a motor program, however, does not seem to be generated within the cerebellum.