cerebellum(redirected from Cerebellar hemispheres)
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cerebellum(sĕr'əbĕl`əm), portion of the brainbrain,
the supervisory center of the nervous system in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought. Anatomy and Function
..... Click the link for more information. that coordinates movements of voluntary (skeletal) muscles. It contains about half of the brain's neurons, but these particular nerve cells are so small that the cerebellum accounts for only 10% of the brain's total weight. The cerebellum operates automatically, without intruding into consciousness; motor impulses from the cerebrum are organized and modulated before being transmitted to muscle. As the muscle tissue responds, its sensory nerve cells return information to the cerebellum. Thus, throughout periods of muscular activity, the cerebellum adjusts speed, force, and other factors involved in movement. The overall effect is a smooth, balanced muscular activity. If the cerebellum is injured, an activity like walking becomes spasmodic: the muscles involved contract too much or too little and operate out of sequence. Maintaining muscle tone is also a function of the cerebellum. Filling most of the skull behind the brain stem and below the cerebrum, the human cerebellum approximates an orange in size and consists of two hemispherical lobes. The grooved surface of the cerebellum is gray matter, composed chiefly of nerve cells. The interior, dense with nerve fibers, makes up the white matter. Five different nerve cell types make up the cerebellum: stellate, basket, Purkinje, Golgi, and granule cells. The Purkinje cells are the only ones to send axons out of the cerebellum. Three main nerve tracts link the cerebellum with other brain areas. Injury to the cerebellum usually results in disruption of eye movements, balance, or muscle tone.
in vertebrates and man, the division of the brain responsible for the coordination of movement and the maintenance of the body’s posture, tone, and balance and functionally involved in the regulation of autonomic, sensory, adaptive, tropic, and conditioned reflexive activities.
The cerebellum develops from a thickening of the dorsal wall of the neural tube. Evolutionarily, it first appears in cyclostomes, such as lampreys and hagfish, as “auricles” (the archicerebellum) that receive information primarily from the vestibular apparatus and lateral line organs. In addition to the archicerebellum, rays and sharks have a paleocerebellum that receives impulses chiefly from receptors of the muscles, tendons, joints, and sense organs. Mammals, unlike the lower classes of vertebrates, have a cerebellum with distinct hemispheric structures (the neocerebellum) that receives information primarily from the cerebral cortex and the visual and acoustic receptors. The degree of cerebellar development varies mainly with the level of development of motor activity. Therefore, in mobile animals such as birds, the cerebellum is comparatively large.
In man, the cerebellum is situated in the posterior cranial fossa under the occipital lobes of the cerebrum and above the medulla oblongata. It has two hemispheres connected by a median section called the vermis. The bodies of nerve cells form its gray surface layer (the cerebellar cortex). Paired nuclei of the gray matter are distributed in the cerebellum’s white matter, which consists of nerve fibers. Three pairs of cerebellar peduncles connect the cerebellum to the higher and lower divisions of the brain (the corpora quadrigemina, the pons, and the medulla oblongata).
The cortex of the cerebellum is more or less the same in all vertebrates. It consists of three layers made up of five types of cells, of which four are inhibitory. The surface (molecular) layer covers the Purkinje (gangliar) layer, which, in turn, covers the granular (deep) layer. Neural impulses reach the cortex mainly along the mossy fibers and, to some extent, along the climbing fibers. The axons of the Purkinje cells originating in the ganglionic layer are the only exit from the cortex, and they end at the cerebellar nuclei.
Data on cerebellar functions have been obtained primarily by complete or partial removal of the cerebellum, by cerebellar stimulation, and, in recent years, by electrophysiological methods. In man, congenital developmental anomalies or injuries disrupt equilibrium, decrease muscle tone, and impair the coordination of the force, degree, and rate of muscle contractions. They may cause tremors when voluntary movements are performed, or they may cause the patient to tire easily. These disturbances are less pronounced in mammals than in other animals. Mammals also compensate for the loss or impairment of these functions more quickly and more completely than other animals.
Removal of the cerebellum alters conditioned reflexive activity. Electric stimulation of certain regions of the cerebellum elicits motor reactions in different muscle groups of the eyes, head, and extremities and decreases the tone of the extensor muscles. It also produces shifts in physiological processes associated with the autonomic nervous system. These shifts are manifested by changes in the activities of the alimentary canal and the cardiovascular and respiratory systems, as well as by changes in thermoregulation and metabolism.
The bioelectric activity of the cerebellum is characterized by rapid and slow potentials. The slow rhythms are associated with the cerebral cortex; the rapid rhythms are an internal property of the cerebellum. Bioelectric potentials arise in certain regions of the cerebellar cortex in response to the stimulation of various regions of the cerebrum; the proprioreceptors of the muscles, the tendons and ligaments; and the receptors of the viscera, skin, eyes, and ears. All of these phenomena are indications of the complex and varied functions of the cerebellum, which may be regarded as a universal regulator of the body’s somatic and autonomic functions.
REFERENCESGrigor’ian, R. A., and V. V. Fanardzhian. “Mozzhechok.” In Obshchaia i chastnaia fiziologiia nervnoi sistemy. Leningrad, 1969.
Fiziologiia cheloveka. Moscow, 1972.
Dow, R. S., and G. Moruzzi. The Physiology and Pathology of the Cerebellum. Minneapolis, Minn., 1958.
Eccles, J. C, M. Ito, and J. Szentagothai. The Cerebellum as a Neuronal Machine. Berlin, 1967.
I. V. ORLOV