Descending Motor Tracts and Cranial Nerve Nuclei

The pyramidal tracts carry efferent information from the motor cortex down through the brain to the spinal cord, where they synapse with cells of motoneurons, which supply skeletal muscle. But not all of these tracts travel to the spinal cord.

There are two main divisions of the motor tracts which descend from the corona radiata and which emerge from the internal capsule. These two divisions are the corticonuclear (corticobulbar) tracts and the corticospinal tracts (see also p. 2). The corticospinal tracts in turn separate in the uncrossed anterior corticospinal tracts, and the lateral, mainly crossed, corticospinal tracts.

The fibers of the corticonuclear (corti-cobulbar) tract leave the pyramidal tract at the level of the mesencephalon (mid-brain), and travel to the cranial nerve nuclei. Some of these fibers decussate, or cross to the contralateral side, while others remain ipsilateral, and travel to nuclei on the same side of the brain. The cranial nerve nuclei are involved in the control of facial and oral muscles (see also p. 222).

Cranial nucleus I is the olfactory nucleus receiving afferent inputs from the olfactory mucosa; II is the optic nerve mid-brain nucleus receiving afferents from the retina; III is the oculomotor nerve and sends efferents to muscles of the eye; IV is the trochlear nucleus, which receives afferents from somatic proprioceptors, and sends efferents to the superior oblique muscle of the eye.

The nuclei from V through XII are organized in seven columns within the brain stem, according to their embryological origin. Nucleus V is the trigeminal, which sends efferents to the masticatory muscles of the mouth, and receives afferents from proprioceptors and cutaneous afferents from face and mouth. Nucleus VI is the ab-ducens, which drives eye movements through the lateral rectus muscle; VII is the mixed sensory and motor nucleus, which innervates the lachrymal, salivary glands, and muscles of facial expression, and receives afferents from taste receptors of some mouth areas, as well as sensation from the external skin of the ear; VIII is the vestibulocochlear, which receives sensory afferents carrying information about balance, posture, hearing and head orientation in space; IX is the glossopharyngeal, a mixed nucleus, which drives swallowing and parotid gland secretion, and receives inputs from taste buds of part of the tongue, and from the carotid body; X is the vagus, a mixed nucleus, which innervates smooth muscle in heart, blood vessels, and many other types of smooth muscle and which receives several afferent inputs carrying visceral sensation; XII is the spinal accessory, a motor nucleus that drives the muscles of larynx and pharynx, and the sternocleidomastoid and trapezius muscles; XII is the origin of the hypoglos-sal nerve, which drives the muscles of the tongue.

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Corticonuclear Pathway

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  • Annibale
    Are motor tracts separate from cranial nerves?
    7 years ago

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