Cerebellum and Its Motor Functions

The cerebellum, illustrated in Figures 56-1 and 56-2, has long been called a silent area of the brain, principally because electrical excitation of the cerebellum does not cause any conscious sensation and rarely causes any motor movement. Removal of the cerebellum, however, does cause body movements to become highly abnormal. The cerebellum is especially vital during rapid muscular activities such as running, typing, playing the piano, and even talking. Loss of this area of the brain can cause almost total incoordination of these activities even though its loss causes paralysis of no muscles.

But how is it that the cerebellum can be so important when it has no direct ability to cause muscle contraction? The answer is that it helps to sequence the motor activities and also monitors and makes corrective adjustments in the body's motor activities while they are being executed so that they will conform to the motor signals directed by the cerebral motor cortex and other parts of the brain.

The cerebellum receives continuously updated information about the desired sequence of muscle contractions from the brain motor control areas; it also receives continuous sensory information from the peripheral parts of the body, giving sequential changes in the status of each part of the body—its position, rate of movement, forces acting on it, and so forth. The cerebellum then compares the actual movements as depicted by the peripheral sensory feedback information with the movements intended by the motor system. If the two do not compare favorably, then instantaneous subconscious corrective signals are transmitted back into the motor system to increase or decrease the levels of activation of specific muscles.

The cerebellum also aids the cerebral cortex in planning the next sequential movement a fraction of a second in advance while the current movement is still

Figure 56-1

Figure 56-1

Anatomical lobes of the cerebellum as seen from the lateral side.

Hemisphere Vermis

Hemisphere Vermis

of hemisphere

Figure 56-2

of hemisphere

Figure 56-2

Functional parts of the cerebellum as seen from the posteroinfe-rior view, with the inferiormost portion of the cerebellum rolled outward to flatten the surface.

being executed, thus helping the person to progress smoothly from one movement to the next. Also, it learns by its mistakes—that is, if a movement does not occur exactly as intended, the cerebellar circuit learns to make a stronger or weaker movement the next time. To do this, changes occur in the excitability of appropriate cerebellar neurons, thus bringing subsequent muscle contractions into better correspondence with the intended movements.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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