Because the cerebellum modulates many aspects of motor performance, it is not surprising that its impairment leads to many abnormalities of behavior. These symptoms permit an examiner to identify with considerable accuracy which part of the cerebellum is damaged (Dichgans, 1984).
The cerebellum has three major functional divisions (see Figure 1), and a unique pattern of impairment results from damage to each. The uppermost part of the cerebellum, or spinocerebellum (consisting mostly of the vermis and anterior lobe), receives sensory information about the current spatial location of the parts of the body and anticipates subsequent movement. The spinocerebellum has rich connections with descending motor pathways, which it modulates. Damage here produces characteristic abnormalities of gait and posture, especially ataxia (loss of coordination) of the legs. Long-term alcoholism can cause degeneration of the anterior lobe of the cerebellum, resulting in characteristic weaving and erratic gait.
The lowermost part of the cerebellum (consisting especially of the lateral parts of each cerebellar hemisphere) is called the cerebrocerebellum in recognition of its close relationship with the cerebral cortex. The cerebrocerebellum is implicated in planning complex movements, so damage here can cause diverse motor problems, such as decomposition of movement, in which gestures are broken up into individual segments instead of being executed smoothly. Because the cerebrocerebellum also functions in higher-level cognition, such as motor learning, damage here can also cause cognitive deficits.
Sandwiched between the two major divisions of the cerebellum is the vestibulocerebellum, made up of small and somewhat primitive structures called the nodule and flocculus. As its name suggests, the vestibulocerebellum has close connections with the vestibular nuclei of the brainstem, through which it receives information about body orientation. Its outputs help the motor systems to maintain posture and appropriate orientation toward the external world; for example, damage to this system produces errors in gaze and difficulty with tracking visual objects as the head moves.
With more research, we are coming to realize that the cerebellum plays a crucial role in an astonishingly wide variety of behaviors. So maybe we shouldn’t be too surprised by the report that this compact structure contains more than half of all the neurons in the human nervous system (Andersen et al., 1992).
As we learn more about the cerebellum and other motor centers in the brain, we can look forward to new and better treatments for a wide variety of movement disorders.
Andersen, B. B., Korbo, L., and Pakkenberg, B. (1992). A quantitative study of the human cerebellum with unbiased stereological techniques. Journal of Comparative Neurology 326: 549–560.
Dichgans, J. (1984). Clinical symptoms of cerebellar dysfunction and their topodiagnostical significance. Human Neurobiology 2: 269–279.