Cerebellum · Grey Matter

The cerebellum is the folded structure at the back of the brain, below the occipital lobes and behind the brainstem, that makes movements smooth and accurate by predicting what each command will do and correcting it before the error shows.

Cerebellum. The cerebellum is the folded structure at the back of the brain, below the occipital lobes and behind the brainstem, that makes movements smooth and accurate by predicting what each command will do and correcting it before the error shows.

It is about a tenth of the brain's mass and holds about 69 billion neurons, roughly 80 % of all of them, most of them tiny granule cells. Its cortex is a regular, repeated circuit: inputs from the cortex (relayed through the pons) and from the spinal cord and inner ear reach Purkinje cells, whose output passes through the deep cerebellar nuclei back to the thalamus and the motor cortex, and down to the brainstem. Climbing fibres from the inferior olive deliver error signals that adjust the circuit, which is how it learns. Each cerebellar hemisphere serves the same side of the body.

It works by prediction. It compares a copy of the motor command with the sensory feedback that follows and adjusts the next command (a pianist's fingers corrected before the wrong note sounds, a throw corrected on the next attempt), a scheme modelled as an internal model.

Its damage gives cerebellar ataxia: unsteady gait, poorly aimed movements, a tremor that grows as the hand approaches its target, slurred speech.

It also serves thinking. Lesions of its posterior lobe impair planning, verbal fluency, spatial cognition and emotional regulation (the cerebellar cognitive affective syndrome), through loops with the prefrontal cortex.

Questions: Why does alcohol make walking unsteady and speech slurred? The cerebellum is especially sensitive to alcohol, which disturbs the signalling of its Purkinje cells and the circuits feeding them. With the predictive correction impaired, gait widens and sways, movements overshoot and speech loses its timing, the same signs a neurologist looks for in cerebellar disease (which is why the roadside test asks for walking heel to toe and touching the nose). Years of heavy drinking can make the change permanent by degenerating the front of the vermis, which leaves a broad, unsteady gait. What part does the cerebellum play in learning a new skill? The cerebellum learns the fine calibration of a movement: the force, timing and trajectory that make it land where intended. Each error is reported to its Purkinje cells by climbing fibres from the inferior olive, and those signals adjust the synapses that produced the error, so the internal model of the arm, the tool or the instrument improves with practice. The basal ganglia learn which actions to chain and repeat, and the cerebellum how to execute each one smoothly, which is why cerebellar damage makes adapting to new conditions (prism glasses, a heavier racket) particularly hard. Does the cerebellum do anything for thinking, or only for movement? Large parts of the cerebellar hemispheres are connected through the thalamus and pons with the prefrontal and parietal cortex, not with the motor areas. In 1998 Schmahmann and Sherman described patients with lesions of the posterior cerebellum who had trouble planning, switching tasks, finding words and judging space, and whose emotions had become blunted or disinhibited, the cerebellar cognitive affective syndrome. One hypothesis is that the cerebellum applies to thoughts the same predictive correction it applies to movements. How does the cerebellum correct a movement before the error shows? When the motor cortex sends a command, a copy reaches the cerebellum through the pons, and the cerebellum predicts what the movement will feel like and where the limb will end up. Sensory feedback is too slow to steer a fast movement, so the cerebellum compares the command with its prediction and with early feedback, and sends corrections back through the thalamus to the motor cortex. Errors that remain are signalled by climbing fibres and change the circuit, so the next attempt is better: a throw that missed left is aimed differently the next time. Which part of the brain holds most of its neurons? The cerebellum does, although it is only about a tenth of the brain's mass: counts by Azevedo, Herculano-Houzel and colleagues found about 69 billion of the brain's roughly 86 billion neurons there, about 80 %. Most of them are granule cells, the smallest neurons in the brain, packed in the cerebellar cortex at enormous density. The cerebral cortex, which makes up most of the mass, holds about 16 billion neurons, its bulk going into large cells, long connections and white matter.