The little brain that tunes every skilled move
The cerebellum—Latin for “little brain”—anchors the vertebrate hindbrain. Usually smaller than the cerebrum, it can rival or exceed it in some mormyrid fishes. In humans it refines movement and underwrites sensorimotor learning, its finely grooved cortex tucked under the hemispheres.
Gross anatomy shows a tightly folded cortical sheet over white matter, a basal ventricle, and four deep nuclei buried inside. Unlike the cerebrum’s broad irregular folds, cerebellar surface grooves run in fine parallel ranks. It sits in the posterior cranial fossa behind the fourth ventricle, pons, and medulla, walled off from the cerebrum by the tough cerebellar tentorium; all traffic to the rest of the brain threads through the pons.
Motor control is the headline job, yet the structure is equally necessary for learning sensorimotor mappings. Theoretical models lean on synaptic plasticity inside cerebellar circuits to explain how the body calibrates aim, timing, and force. The same small set of neuronal elements repeats across cortical patches, a modular repeating motif.
Most of its bulk is one tightly pleated sheet of gray matter, each ridge called a folium; high-resolution MRI puts the adult human cerebellar cortex at about 730 square centimetres crammed into a block roughly 6 by 5 by 10 centimetres. Three paired peduncles wire it to the rest of the nervous system, the superior one mainly sending output toward the cerebral cortex via the thalamus. Surface fissures divide anterior, posterior and flocculonodular lobes. The flocculonodular lobe, the evolutionarily oldest part, handles balance and orientation through the vestibular nuclei; the medial spinocerebellum fine-tunes body and limb movement from proprioceptive input; and the lateral cerebrocerebellum, by far the largest in humans, takes input only from the cerebral cortex. The circuitry revolves around Purkinje and granule cells, fed by mossy and climbing fibres, and the cortex stacks a granular layer, a thin Purkinje layer and an outer molecular layer where parallel fibres cross Purkinje dendrites at right angles. The learning models behind it trace to David Marr and James Albus.
Source: Cerebellum
