Snell's Clinical Neuroanatomy

279

Functions of the Cerebellum

Efferent Cerebellar Pathways a

Table 7-2

Pathway

Function

Origin

Destination

Globose emboliform-rubral Dentatothalamic

Influences ipsilateral motor activity Influences ipsilateral motor activity

Globose and emboliform nuclei

To contralateral red nucleus, then via crossed rubrospi nal tract to ipsilateral motor neurons in the spinal cord

Dentate nucleus To contralateral ventrolateral nucleus of the thalamus, then to contralateral motor cerebral cortex; corticospi nal tract crosses midline and controls ipsilateral motor neurons in the spinal cord Fastigial nucleus Mainly to ipsilateral and to contralateral lateral ves tibular nuclei; vestibulospinal tract to ipsilateral motor neurons in the spinal cord Fastigial nucleus To neurons of reticular formation; reticulospinal tract to ipsilateral motor neurons to the spinal cord

Fastigial vestibular

Influences ipsilateral extensor muscle tone

Fastigial reticular

Influences ipsilateral muscle tone

a Note that each cerebellar hemisphere influences the voluntary muscle tone on the same side of the body.

the neurons of the cerebellar nuclei and the lateral ves tibular nuclei. Cerebellar output is conducted to the sites of origin of the descending pathways that influence motor activ ity at the segmental spinal level. In this respect, the cer ebellum has no direct neuronal connections with the lower motor neurons but exerts its influence indirectly through the cerebral cortex and brainstem. Physiologists have postulated that the cerebellum func tions as a coordinator of precise movements by continu ally comparing the output of the motor area of the cerebral cortex with the proprioceptive information received from the site of muscle action; it is then able to bring about the necessary adjustments by influencing the activity of the lower motor neurons (Fig. 7-13). This is accomplished by controlling the timing and sequence of firing of the α and γ motor neurons. The cerebellum can possibly send back information to the motor cerebral cortex to inhibit the agonist muscles and stimulate the antagonist muscles, thus limiting the extent of voluntary movement.

FUNCTIONS OF THE CEREBELLUM The cerebellum receives afferent information concern ing voluntary movement from the cerebral cortex and from the muscles, tendons, and joints. It also receives information concerning balance from the vestibular nerve and possibly concerning sight through the tec tocerebellar tract. All this information is fed into the cerebellar cortical circuitry by the mossy and climb ing fibers and converges on the Purkinje cells (see Fig. 7-8). The axons of the Purkinje cells project with few exceptions on the deep cerebellar nuclei. The output of the vermis projects to the fastigial nucleus, the intermediate regions of the cortex project to the globose and emboliform nuclei, and the output of the lateral part of the cerebellar hemisphere projects to the dentate nucleus. A few Purkinje cell axons pass directly out of the cerebellum and end on the lateral vestibular nucleus in the brainstem. Purkinje axons are generally believed to exert an inhibitory influence on

Copyright © 2021 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

Cerebral cortex

Feedback

Command center

Cerebellum

Comparator

Feedback

Muscle

Figure 7-13 Cerebellum serving as a comparator.