The pontocerebellum (neocerebellum, cerebrocerebellum) is involved in the planning and control of timing of precise movements that require dexterity of the extremities, particularly of the hand, arm, and forearm. The system comprises, mainly, the cerebral cortex, the pontine nuclei, the principal inferior olivary nucleus, the dentate nucleus, and the lateral cerebellar cortex.
The lateral cerebellar cortex receives major inputs from the contralateral basal pontine nuclei through the middle cere-bellar peduncle. The pontine nuclei receive inputs from the ipsilateral cerebral cortex, which explains why the pontocere-bellum is also called the cerebrocerebel-lum. The lateral cerebellar cortex also receives inputs from the contralateral principal inferior olivary nucleus, which sends collaterals to the dentate nucleus. The information in these fibers is organized topographically.
The major outputs of the pontocerebel-lum are corticonuclear fibers from the lateral zone of the cerebellar cortex to the dentate nucleus, and dentatofugal efferents through the decussation of the superior cerebellar peduncles. Fibers pass ros-trally to the contralateral red nucleus (dentatorubral fibers) and to the thalamus (dentatothalamic fibers), to the ventral lateral nucleus, and to the centromedian and intralaminar nuclei. From the ventral lateral nucleus, fibers project extensively to many regions of the cerebral pre-motor and motor cortex.
Descending dentatofugal fibers project caudally to the contralateral principal olivary nucleus (dentato-olivary fibers) and, to a lesser extent, to contralateral pontine and reticular formation nuclei. From the olivary nucleus, reciprocal outputs project to the contralateral lateral zone of the pontocerebellar cortex (olivo-cerebellar fibers), and also to the dentate nucleus (olivodentate fibers). Similarly, there are outputs from pontine nuclei to the contralateral pontocerebellum. There are also major descending projections from the motor cortex to the ipsilateral pons (corticopontine fibers).
The functional correlates of all these connections are not clear; there is, however, evidence that the dentato-ven-tral lateral nucleus-motor cortex connection drives the initiation of movement in response to a visual stimulus. It has also been shown that outputs from the ponto-cerebellum to the motor cortex are involved in the planning and execution of the timing and duration of muscle agonist-antagonist timing of excitation and inhibition.
Injury to the pontocerebellar module can result in severe disruption of movement. The ipsilateral side is affected because pontocerebellar efferents project to the contralateral motor cortex, which in turn projects caudally in corticospinal fibers that decussate in the pyramids. Injury is more serious if both cerebellar cortex and dentate nucleus are injured. These lesions result in a partial or complete loss of coordinated movement, termed dys-synergia or decomposition of movement. Patients may exhibit ataxia, or unsteady gait, hypotonia, which is a loss of muscle tone, and may tend to fall on the lesioned side. Tremor often accompanies the lesions, and is most noticeable during movement (intention tremor). (Note that in Parkinson's disease the tremor is more noticeable when the patient is not moving.) Tremor is also evident when the patient stretches out an arm i.e. works against gravity (static tremor). A diagnostic symptom is impaired check or rebound, when the patient cannot operate normal agonist-antagonist function. Other symptoms include dysarthria (slurred speech), and nystagmus.
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