The cerebellum's role in the central nervous system

The present article aims to describe the cerebellum's role in the function of the brain and thus explain why the cerebellum is often referred to as the "little brain". The cerebellum is a fist-sized structure which is located at the lower back of the brain. Formerly this structure was thought to have only a motor function, which it performed by helping other motor regions of the brain to do their work effectively. The cerebellum is recognized to play an important role in the integration of sensory perception and motor control. The large number of neural pathways linking the cerebellum with the cerebral motor cortex (which sends information to the muscles causing them to move) and the spinocerebellar tract (which provides proprioceptive feedback on the position of the body in space), coordinate motor control. The cerebellum regulates these pathways, using the constant feedback on body position to fine-tune motor movements. In other words it compares what the person is thinking to do (according to motor cortex) with what is actually happening in the limbs (according to proprioceptive feedback) and corrects the movement if there is a problem. Because of this 'updating' function, lesions within the cerebellum are not so debilitating as to cause paralysis, but rather present as feedback deficits resulting in disorders in fine movement, equilibrium, posture and motor learning. Initial observations by physiologists indicated that patients with cerebellar damage have problems with motor coordination and movement. Research into cerebellar function was also done via lesion and ablation studies in animals. Physiologists that conducted such studies noted that lesions led to animals presenting strange movements, awkward gait and muscular weakness. These observations and studies, resulted in the conclusion that the cerebellum is a motor control structure. However during the past decade, a broader view of its function has emerged as a result of new research and its function is now being reappraised. The cerebellum is now regarded as a structure that helps not only motor but also nonmotor regions to do their work effectively. What was once thought to be primarily a motor/sensory integration region, is now proving to be involved in many diverse cognitive functions. Modern research shows that the cerebellum has a broader role in a number of key cognitive functions which we refer to in this article. The cerebellum receives an enormous amount of information from the highest level of the human brain including information from sensory areas of the cerebral cortex, from motor areas, from cognitive areas, from language areas and even from areas involved in emotional functions, which are connected to the human cerebellum by approximately 40 million nerve fibers. The cognitive functions in which the cerebellum is involved are also mentioned. These include motor learning (such as riding a bicycle), adaptation, attention (more specifically switching of attentional focus), memory (retaining phonological information), the processing of language (we refer to its connection with articulation, mutism, reduction in verbal fluency and difficulties in constructing long and complex sentences) and other sensory temporal stimuli (more specifically we refer to problems in discriminating sensory information). Furthermore we discuss cerebellar injury. Damage to the cerebellum can lead to: loss of coordination of motor movement (asynergia), the inability to judge distance and when to stop (dysmetria), the inability to perform rapid alternating movements (adiadochokinesia), movement tremors (intention tremor), staggering, wide based walking (ataxic gait), tendency toward falling, slurred speech (ataxic dysarthria), abnormal eye movements (nystagmus) and reduced emotional and behavioral control. The arguments used in the present article are based on recent research and medical textbooks. In the final section of our review we present our conclusions that justify why the cerebellum can be referred to as the "little brain".

Key words: Cerebellum, brain, cognitive functions, motor function.