Descending tracts and corticopontine fibers

The descending pathways that have been described are shown, using the somewhat oblique posterior view of the brainstem (see Figure 10 and Figure 40), along with those cranial nerve nuclei that have a motor component. These pathways will be presented in summary form:

• Cortico-spinal tract (see Figure 45): These fibers course in the middle third of the cerebral peduncle, are dispersed in the pontine region between the pontine nuclei, and regroup as a compact bundle in the medulla, situated within the pyramids. At the lowermost part of the medulla (Figure 7), most of the fibers decussate to form the lateral cortico-spinal tract of the spinal cord (see Figure 68 and Figure 69). A small portion of the tract continues ipsilaterally, mostly into the cervical spinal cord region, as the anterior (ventral) cortico-spinal tract.

• Cortico-bulbar fibers (see Figure 46): The cortical fibers that project to the cranial nerve nuclei of the brainstem are shown in this diagram. The term also includes those cortical fibers that project to the reticular formation and other brainstem nuclei. These are also located in the middle third of the cerebral peduncle and are given off at various levels within the brain-stem.

• Rubro-spinal tract (see Figure 47): This tract from the lower portion of the red nucleus decussates in the midbrain region and descends through the brainstem. In the spinal cord, the fibers are located anterior to the lateral cortico-spinal tract (see Figure 68).

Cortico-Pontine Fibers

The cortico-pontine fibers are part of a circuit that involves the cerebellum. The cortical fibers arise from the motor areas as well as from widespread parts of the cerebral cortex. The fibers are located in the outer and inner thirds of the cerebral peduncle (see also Figure 46): the fronto-pontine fibers in the inner third, and fibers from the other lobes in the outer third. They terminate in the nuclei of the pons proper (see Figure 6), and the information is then relayed (after crossing) to the cerebellum via the massive middle cerebellar peduncle (discussed with Figure 55; see also Figure 6 and Figure 7). The role of this circuit in motor control will be explained with the cerebellum (see Figure 54-Figure 57).

The motor cranial nerve nuclei and their function have been discussed (see Figure 7 and Figure 8A), and their location within the brainstem will be described (see Figure 64-Figure 67). Only topographical aspects will be described here:

• CN III — Oculomotor (to most extra-ocular muscles and parasympathetic): These fibers traverse through the medial portion of the red nucleus, before exiting in the fossa between the cerebral peduncles, the interpeduncular fossa (see Figure 65A).

• CN IV — Trochlear (to the superior oblique muscle): The fibers from this nucleus cross in the posterior aspect of the lower midbrain before exiting posteriorly (see Figure 10 and Figure 66A). The slender nerve then wraps around the lower border of the cerebral peduncles in its course anteriorly.

• CN V — Trigeminal (to muscles of mastication): The motor fibers pierce the middle cere-bellar peduncle in the mid-pontine region, along with the sensory component.

• CN VI — Abducens (to the lateral rectus muscle): The anterior course of the exiting fibers could not be depicted from this perspective.

• CN VII — Facial (to muscles of facial expression): The fibers to the muscles of facial expression have an internal loop before exiting. The nerve loops over the abducens nucleus, forming a bump called the facial colliculus in the floor of the fourth ventricle (see Figure 10). It should be noted that the nerve of only one side is being shown in this illustration.

• CN IX — Glossopharyngeal and CN X-Vagus (motor and parasympathetic): The fibers exit on the lateral aspect of the medulla, behind the inferior olive.

• CN XI — Spinal Accessory (to neck muscles): The fibers that supply the large muscles of the neck (sternomastoid and trapezius) originate in the upper spinal cord and ascend into the skull before exiting.

• CN XII — Hypoglossal (to muscles of the tongue): These fibers actually course anteriorly, exiting from the medulla between the inferior olive and the cortico-spinal (pyramidal) tract.

Fronto-pontine fibers

Cortico-spinal and cortico-bulbar fibers

Temporo-parieto-occipito-

pontine fibers pontine fibers

Red i

Rubro-spinal tract

Trigeminal nerve (CN V)

Facial

Glossopharyngeal nerve (CN IX) Vagus nerve (CN X)

Hypoglossal nerve (CN XII) Hypoglossal n.

Accessory nerve (CN XI)

Rubro-spinal tract

Cortico-spinal and cortico-bulbar fibers

Rubro-spinal tract

Glossopharyngeal nerve (CN IX) Vagus nerve (CN X)

Hypoglossal nerve (CN XII) Hypoglossal n.

Accessory nerve (CN XI)

Rubro-spinal tract

Oculomotor nerve (CN III) Oculomotor n.

Trochlear n.

Trochlear nerve (CN IV)

Pyramidal decussation

Anterior cortico-spinal tract Lateral cortico-spinal tract

Cervical spinal cord

Oculomotor nerve (CN III) Oculomotor n.

Trochlear n.

Trochlear nerve (CN IV)

Pontine nuclei

Middle cerebellar peduncle

Cortico-bulbar fibers

Abducens n. Facial nerve (CN VIII)

Cortico-bulbar fibers Ambiguus n.

Pyramidal decussation

Anterior cortico-spinal tract Lateral cortico-spinal tract

Cervical spinal cord

FIGURE 48: Descending Tracts and Cortico-Pontine Fibers

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