The cerebellum coordinates volitional movement by comparing the motor plan with movements as they occur, and providing feedback to the motor cortices for correction. The cerebellum is posterior to the brainstem and connected to it by bilateral structures called the cerebellar peduncles, which are divided into inferior, middle, and superior pairs. The medial cerebellum coordinates muscles involved in gait and movements of the torso and neck. The lateral cerebellum coordinates movements of the ipsilateral limbs. Both medial and lateral cerebellar areas also coordinate muscles involved in speech and eye movements.
The motor plan, developed in association motor cortices, is simultaneously sent to the primary motor cortex, to execute the plan, and to the cerebellum, via the corticopontine tract. The corticopontine tract descends through the deep cerebrum and upper brainstem to synapse in ipsilateral nuclei about midway up the brainstem called the basal pontine nuclei. Axons of these nuclei form the pontocerebellar tract, which decussates across the brainstem to bring motor plan information to the contralateral cerebellum via the middle cerebellar peduncle. For limb movements, unconscious position sense information about movements as they occur reaches the ipsilateral cerebellum via the spinocerebellar tracts, which mainly travel through the inferior cerebellar peduncle. The cerebellar cortex then compares the motor plan to the movement that is occurring.
If correction is needed, cerebellar cortical neurons send feedback information along axons projecting through the cerebellar white matter to deep cerebellar nuclei. The largest of these is the dentate nucleus, which receives feedback information from the lateral cerebellar cortex about ipsilateral limb movements. Dentate nucleus axons form the dentatothalamic tract, which travels through the superior cerebellar peduncle, decussating in the upper brainstem prior to synapsing in the contralateral thalamus. Thalamic neurons then project to the motor cortices, where the cerebellar feedback is used to improve coordination and accuracy of movements. The medial cerebellum appears to mostly interconnect with multiple brainstem areas, rather than sending feedback to the motor cortices.
Dysfunction of the cerebellum or its connections may cause ataxia (incoordination), dysarthria, dysdiadochokinesia (abnormal rapid alternating movements), or vertigo and nystagmus. Limb ataxia is examined with finger-to-nose and heel-to-shin testing. The finger to nose test involves the patient moving their finger rapidly back and forth between touching the examiner’s finger and the patient’s nose. To increase the difficulty of the test, the examiner may move their finger each time. The heel-to-shin test involves the patient dragging their heel up and down their contralateral shin from their ankle to their knee and back repeatedly. This test should be done with the patient supine, because if the patient is sitting they may use gravity to mask incoordination of the leg.
The limb may move in an incoordinated way, which is called limb ataxia; it may miss the target, which is called dysmetria; or there may be a tremor that appears when approaching a target, which is called intention tremor. Gait ataxia is unsteady, staggering, and has a wide base, meaning the feet are far apart laterally. Dysdiadochokinesia is tested with rapid alternating movements, such as rapidly supinating and pronating the hands.