Chapter 27 – The Cerebellum

The cerebellum receives, but does not interpret, sensory information and profoundly influences, but does not initiate, motor activity. The adult cerebellum is composed of three main lobes (anterior, posterior, flocculonodular) connected to the brainstem and spinal cord by three cerebellar peduncles (superior, middle, inferior). Based on its afferent/efferent connections, the cerebellar cortex is organized into rostral-caudally oriented functional zones (medial, intermediate, lateral) related to particular cerebellar nuclei (medial, interposed, lateral), respectively. The cerebellar cortex is composed of molecular, Purkinje, and granular cell layers; all receive afferent fibers, but Purkinje cells are the only efferent neurons of the cerebellum. Fibers entering the cerebellum (via peduncles) send collaterals to a particular nucleus and a main branch to the cortical zone overlying that specific nucleus. Purkinje cells project to the cerebellar nucleus in their corresponding zone modifying the messages to extracerebellar targets. Sensory input to the cerebellum is processed in the nuclei and cortex. The vestibulocerebellar module (medial nucleus plus medial cortical zone) receives input that influences balance and equilibrium; damage to this system results in truncal ataxia, titubation, and nystagmus. The spinocerebellar module (globose and emboliform nuclei plus intermediate zone) receives input from spinal cord and inferior olivary nuclei; damage to these pathways generally results in deficits related to the medial or lateral zones. The pontocerebellar module (lateral zone plus lateral nucleus—both large) receives input from the cerebral cortex via the basilar pontine nuclei; damage to this system results in dyssynergia, dysmetria, intention tremor, dysdiadochokinesia, and failure of the heel-to-shin test.