The cerebellum is an impressive brain structure lying in the back and the base of the skull. It has an extensive cortical surface with dense connections to and from the cerebral cortex. While the cerebellum is about 10% of the total brain volume, it contains 80% of the brain's neurons. The basal ganglia and cerebellum work together to control movement. A simple schema would have the motor cortex sending signals to both, with feedback via the thalamus. The output of the cerebellum is excitatory. The basal ganglia are inhibitory. Correct balance between the two systems produces coordinated movements. Disturbances in either system will cause movement disorders. The neurological understanding of the cerebellum focused on movement, balance and muscle tone. Disease and damage to the cerebellum can lead to: 1) loss of coordination of motor movement (asynergia), 2) impaired movement tracking (dysmetria), 3) the inability to perform rapid alternating movements (adiadochokinesia), 4) tremors (action tremor), 5) staggering, wide based walking (ataxic gait), 6) falling, 7) weak muscles (hypotonia), 8) slurred speech (ataxic dysarthria), and 9) abnormal eye movements (nystagmus).
The cerebellum is essential to learning and implementing motor skills. New skills require effort and much practice to acquire and then become automatic. Since all interactions with the outside world are movements, the cerebellum assumes great importance. Movements include speaking, writing and reading, key components of acquired cognitive skills. The role of the cerebellum in cognition and emotion is a research priority and Springer is publishing a journal devoted to cerebellar neuroscience. Manto wrote: “Research on the cerebellum is evolving rapidly. The exquisiteness of the cerebellar circuitry with a unique geometric arrangement has fascinated researchers from numerous disciplines. Advances in genetic studies, detailed molecular and cellular analyses, profusion of brain imaging techniques, emergence of behavioral assessments, and reshaping of models of cerebellar function are generating an immense amount of knowledge. Simultaneously, a better definition of cerebellar disorders encountered in the clinic is emerging. The essentials of a trans-disciplinary blending are expanding. The launch of the society for research on the cerebellum (SRC) illustrates how cerebellar research is burgeoning. This special issue gathers the contributions of the inaugural conference of the SRC dedicated to the mechanisms of cerebellar function. Contributions were brought together around five themes: (1) cerebellar development, death, and regeneration; (2) cerebellar circuitry: processing and function; (3) mechanisms of cerebellar learning; (4) cerebellar function: timing, prediction, and/or coordination; (5) anatomical and disease perspectives on cerebellar function.”
Schmahmann wrote: ”The cerebellum is a critical node in the distributed neural circuits subserving not only motor function but also autonomic, limbic and cognitive behaviors. Dysmetria of movement that characterizes the cerebellar motor syndrome results from lesions of the motor cerebellum, mostly in lobules III–V in the anterior lobe and the secondary sensorimotor region in lobule VIII. Dysmetria of thought in the realms of intellect and emotion manifests as the cerebellar cognitive affective syndrome which occurs following lesions of the cognitive and limbic cerebellum in the posterior lobe, represented in lobules VI, VIIA (including lobules VIIAf and VIIAt at the vermis, and crus I and crus II in the hemispheres) and VIIB, and possibly in lobule IX as well. The vestibular cerebellum is localized to lobule X and adjacent parts of lobule IX . The deep cerebellar nuclei are also incorporated into this motor-cognitive dichotomy.”
Brown et al studied human dance to music processes in the brain to identify what systems were involved. They suggested that three hypotheses of cerebellar function were pertinent to their findings: 1. The cerebellum embodies internal forward-inverse model pairs needed to integrate the sensory aspects of movement, the movements per se and perception of the auditory beat. 2. The cerebellum generates timing processes in the preparation and coordination of motor responses (vermal and anterior cerebellum) and the sensory perception of duration on the order of hundreds of milliseconds (lateral cerebellum). 3. The cerebellum optimizes the acquisition of sensory data to synchronize the execution of movement with the auditory rhythm. The basal ganglia and cerebellum receive input from and send output to the cerebral cortex forming multisynaptic loops. Bostana et al identified communication between the basal ganglia and cerebellum.
Projections from the subthalamic nucleus (STN) go to the cerebellar cortex and the dentate nucleus of the cerebellum projects to the striatum. The anatomy suggests an integrated functional network involving cerebellum, basal ganglia and cerebral cortex. They suggested that abnormal increases in cerebellar activity are found in Parkinson's Disease patients; stimulation of the STN improves the motor signs and normalizes cerebellar activation. Resting tremors can be stopped by stimulating or lesioning the ventral intermediate nucleus of the thalamus, a main target of cerebellar efferents.