The Pons

The pons (Fig. 14) is a thick bridge of nerve fibers, crossing the midline anteriorly. With a massive arm reaching into the cerebellum on either side, it binds that organ (its derivative) to the brain stem like a backpack strapped around one's waist. But it does more than hold the cerebellum in place. It provides a key link between the cerebral cortex and the cerebellum. The brachium pontis, or middle cerebellar peduncle, is a huge cable by which the contralateral cerebral cortex, through the liaison of 20 million pontine neurons, accesses the cerebellar movement computer. This cross-link in cerebral-cerebellar function enables movement to be performed in a smooth, coordinated, directed manner.

Caudal to the middle cerebellar peduncle is the inferior peduncle, in which sensory information from spinal cord and messages from the cortex sent through the inferior olivary nucleus of the medulla reach the cerebellum. Rostral to it is the superior peduncle. It distributes cerebellar output, via a thalamic nucleus, back to the contralateral motor cortex (thus completing a neural "double-cross"), as well as to various motor centers in the brain stem (red nucleus, reticular formation, and others).

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