Cerebellum

The cerebellum is a large, semidetached region of the brain lying behind the brain stem, to which it is extensively attached. Even though the cerebellum receives substantial sensory input, it is considered a part of the motor system because of its primary involvement with coordination, postural control, equilibrium, and muscle tone.

In gross anatomic terms, the cerebellum can be divided into the body of the cerebellum (corpus cerebelli) and the much smaller flocculonodular lobe. A more useful functional distinction, however, can be made between the two lateral cerebellar hemispheres and the centrally located vermis (Fig. 9). This division is important because the cerebellar hemispheres are concerned with coordination of the limbs, and the vermis is devoted to postural adjustment. Thus,

Figure 8 Drawing of the ascending reticular activating system (ARAS), which is found in the upper pons and midbrain and sends projections rostrally to the thalamus. Reprinted with permission from Filley, C. M. (2001). Neurobehavioral Anatomy, 2nd ed., p. 42. University Press of Colorado, Boulder, CO.

Figure 8 Drawing of the ascending reticular activating system (ARAS), which is found in the upper pons and midbrain and sends projections rostrally to the thalamus. Reprinted with permission from Filley, C. M. (2001). Neurobehavioral Anatomy, 2nd ed., p. 42. University Press of Colorado, Boulder, CO.

damage to these regions causes, respectively, limb ataxia and postural instability (also known as trunkal ataxia).

Like the cerebrum, the cerebellum contains both gray and white matter. The gray matter includes billions of cerebellar cortical neurons, which are arranged in three layers: a superficial molecular layer, an intermediate Purkinje cell layer, and a deeper granular cell layer. In addition, there are four cell collections deep within the cerebellum called the dentate, globose, emboliform, and fastigial nuclei (Fig. 9). The white matter consists in part of axons traveling to and from the cerebellar cortex. There are also three prominent cerebellar peduncles—inferior, middle, and superior—that connect the cerebellum with the medulla, pons, and midbrain, respectively (Fig. 9).

The major role that the cerebellum plays in the motor system results from its intermediate position between multiple sensory inputs and its output to motor areas in the cerebral hemispheres. Through projections in the inferior and middle cerebellar peduncles, the cerebellum receives a variety of vestibular, spinal, and cerebral cortical inputs that reach the cerebellar cortex where extensive processing takes place. From this point, cerebellar output occurs mainly by relays in the four deep nuclei, which send axons through the superior cerebellar peduncle to the

Figure 9 The cerebellum, as depicted schematically from behind. The major divisions are the two hemispheres, situated laterally, and the single midline vermis. Reprinted with permission from Kandel, E. R., Schwartz, J. H., and Jessell, T. M., Eds. (1991). Principles of Neural Science, 3rd ed., p. 628. Elsevier, New York.

Figure 9 The cerebellum, as depicted schematically from behind. The major divisions are the two hemispheres, situated laterally, and the single midline vermis. Reprinted with permission from Kandel, E. R., Schwartz, J. H., and Jessell, T. M., Eds. (1991). Principles of Neural Science, 3rd ed., p. 628. Elsevier, New York.

midbrain and eventually the ventral anterior and ventral lateral nuclei of the thalamus. These thalamic connections allow the cerebellum to exert a powerful influence on motor regions of the cerebral cortex, providing fine-tuning of movements of the limbs and trunk. An important clinical point is that ataxia on one side of the body is ipsilateral to the side of the cerebellar lesion. Because the cerebellar output crosses to the opposite thalamus and then the subsequent corticospinal system crosses again, the bodily deficit is on the same side as the cerebellar damage.

Cerebellar lesions are most often associated with the clinical findings of ataxia, which may affect the limbs, trunk, or even speech (producing a specific type of dysarthria known as scanning speech), dysequilibrium as manifested by a wide-based gait, and muscular hypotonia. Findings have also suggested that the cerebellum has a contribution to higher motor and even cognitive functions. The acquisition of a motor skill such as playing the piano, an example of procedural learning, seems to depend in part on the cerebellum, and there are increasingly frequent reports of individuals with cerebellar lesions who develop cognitive deficits.

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