cerebellum (sĕrˌəbĕlˈəm) [key], portion of the brain 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.