Like the larger cerebrum, the cerebellum has two hemispheres. The cerebellar hemispheres are located to either side of a small area called the vermis that separates the two hemispheres down the middle. Each of the cerebellar hemispheres is further divided into five lobes. Deep folds in the cerebellum separate the ten lobes. Positioned at the lower rear of the brain, the cerebellum sits below the cerebrum and behind the pons.
From the exterior, the cerebellar hemispheres are distinguished by tight, parallel folds. This is different than the cerebrum, which has large, surface folds that appear convoluted. The cortex of the cerebellum is actually one continuous layer of brain tissue tightly folded upon itself. Although the cerebellar hemispheres represent only ten percent of the volume of the brain, they contain more neurons than the rest of the brain. This is possible because of the tightly folded nature of the cerebellum.
Located deep within each of the cerebellar hemispheres are the deep nuclei. All outgoing signals are sent through these nuclei. The cortex of the cerebellar hemispheres evaluates incoming messages transmitting sensory and other data from the cerebrum. Fine-tuning of movement is coordinated by this part of the brain as sensory messages are received and signals to muscles are sent. Damage to the cerebellum causes a lack of coordination and timing of movements, and other issues, including speech problems.
Functionally, the cerebellum is divided into three sections. The pontocerebellum consists of most of the cortex of the cerebellum. Damage to this portion of the cerebellum is usually exhibited as a lack of coordination on the same side of the body as the damage. Injury or disease of the vestibulocerebellum is characterized by dizziness and an uncoordinated walking gait. If the spinocerebellum is damaged by alcohol, injury or disease, the walking gait becomes staggering and posture is affected.
Several disorders affect the cerebellum. Spinocerebellar ataxia is a degenerative genetic condition characterized by progressive loss of coordination. Over ten different types of spinocerebellar ataxia are known, depending on the specific gene mutation involved. Research has revealed a connection between autism and a reduction in the size of the cerebellar hemispheres. Strokes and brain injuries also affect the functioning of the cerebellum and result in a lack of control of movement.