M6 | Integrative action of the sensorimotor system – Cerebellum & Basal ganglia
M6 | Integrative action of the sensorimotor system – Cerebellum & Basal ganglia
Functional Organization of the Cerebellum
The cerebellar cortex overlies a core of white matter. At the center of the cerebellum is a collection of nuclei – the cerebellar deep nuclei (DCN). The deep cerebellar nuclei are the output areas of the cerebellum, receiving inputs from the cortex and projecting out to the thalamus, red nucleus and brainstem. From medial to lateral the cerebellar nuclei are the fastigial, interpositus (in man, this is subdivided into the globose and emboliform nuclei) and dentate nuclei.
The cerebellum receives two types of afferent inputs: climbing fibers (CF), which originate from the inferior olivary nucleus in the medulla, and mossy fibers (MF), which originate from multiple sources (spinal cord, pontine nuclei (PN), reticular formation, vestibular nuclei). Input fibers synapse on neurons in both CBX and DCN. The bulk of CBX output neurons (Purkinje cells) project first to DCN rather than directly out of CB.
Older studies of cerebellar connections suggested three major divisions of cerebellum based on a rough segregation of inputs. The flocculo-nodular lobe (phylogenetically oldest – archicerebellum) receives many vestibular inputs – hence vestibulocerebellum. The rostral and caudal parts of vermis (paleocerebellum) receive many spinal inputs – hence spinocerebellum. The cerebellar hemispheres and lobules VI and VII of the vermis (phylogenetically most recent – neocerebellum) receive many pontine inputs – hence pontocerebellum (cerebrocerebellum).
Each of the major sagittally oriented cerebellar subdivisions are compartmentalized into a series of parasagittal strips (for review see: Voogd and Glickstein 1998). Later studies have confirmed that cerebellar cortical afferents and efferents travel within these strips, or zones and, therefore, function relatively independently from one another.
Vestibulocerebellum (flocculonodular lobe)
Inputs derive directly from primary vestibular afferents (semicircular canals and otolith receptors) and indirectly from secondary afferents that arise from the vestibular nuclei. Main output targets are the vestibular nuclei. The main functions of the vestibulocerebellum are to 1) control of axial and proximal limb muscles that are used to maintain balance, and 2) control eye movements and coordinate movements of the head and eyes (vestibulo-ocular reflex).
The main body of the CB is divided into three longitudinally arranged subdivisions: medial, intermediate, and lateral. Together the medial and intermediate subdivisions are referred to as the spinocerebellum, the lateral subdivision is called the cerebrocerebellum.
Spinocerebellum
Medial and intermediate zones receive somatosensory information from spinal cord via spinocerebellar tracts: the dorsal and ventral spinocerebellar tracts (DSCT and VSCT, respectively) are the pathways from the trunk and legs; the cuneocerebellar tract (CCT) and rostral spinocereb