CEREBELLUM

A roughly spherical part of the hindbrain, located beneath the cerebral hemispheres (plate 1). The structure gets its name, which means “little brain,” from its resemblance to the cerebrum, since its outer surface consists of brain matter (cell bodies) and its inner core is largely made up of white matter (nerve fibers).The cerebellum is primarily an organ of motor co-ordination. It receives impulses from all the senses, but particularly from the muscles and inner ear, and relays messages that regulate the movements involved in posture, walking, manipulation, and balance.In the course of evolution the cerebellum was the first area of the nervous system which had the special function of co-ordinating sensory and motor impulses. It reached its greatest relative size in birds since it regulates the complex functions involved in flying. Its structure became differentiated in mammals, with a ventral or bottom portion receiving fibers from the sense organs for equilibrium in the inner ear, and anterior and posterior portions connecting with the spinal cord. In the higher mammals and especially in man, an additional structure called the neocerebellum developed in the rear or dorsal region. This area is primarily concerned with the co-ordination of impulses passing to and from the cerebral cortex. If the neocerebellum is damaged, the individual cannot properly put together the movements required for any complex activity, such as feeding himself or playing the piano. Other cerebellar disorders are cerebellar ataxia, a loss of muscular coordination which usually affects standing and walking (the wobbly “cerebellar gait”); intention tremor (a tremor occurring only in purposeful action, as in reaching for something); vertigo; and adiadochokinesis (inability to perform rapid alternating movements).