The Accessory Hypoglossal and Vagus Nerves

The vagus, accessory, and hypoglossal nerves are X, XI, and XII, respectively. The vagus is the main parasympathetic nerve, having very extensive motor and sensory components (see also p. 240: the auto-nomic nervous system). The accessory nerve is purely motor, and consists of cranial and spinal divisions. The cranial part has a linear set of rootlets that project bilaterally from the medulla, and lie just caudal to the vagal roots. The spinal part projects from the five most rostral segments of the spinal cord. The hypoglossal nerve is also a purely motor nerve, which supplies the extrinsic and intrinsic muscles of the tongue.

The motor efferents of the vagus originate in the nucleus ambiguus of the medulla, and innervate the muscles of the larynx, pharynx, soft palate, and the upper esophagus. Therefore, damage to the nucleus ambiguus affects speech and swallowing. Note that the caudal vagal efferents leave the brain stem in the cranial roots of the accessory nerve, but join the other vagal fibers at the jugular foramen. The parasympathetic efferents originate in the dorsal motor nucleus of the vagus, which lies beneath the floor of the fourth ventricle in the medulla. They innervate large parts of the cardiovascular, GIT, and respiratory systems. Sensory afferents of the vagus join the brain stem just caudal to the glossopharyngeal nerve, and carry inputs from the abdominal and thoracic viscera, from baroreceptors in the aortic arch, and chemoreceptors in the aortic bodies. They also carry general sensory inputs from part of the concha of the external ear, the tympanic membrane, external auditory meatus, the larynx, pharynx, and esophagus. Vagus visceral afferents outnumber efferents by at least 4:1 (see also p. 172).

Cranial accessory fibers become incorporated into the vagus at the jugular fora-

Greenstein, Color Atlas of Neuroscience © 2000 Thieme men. The efferent cranial accessory division consists of visceral efferents from the nucleus ambiguus. These are the motor fibers of the vagus, which supply the larynx, pharynx, and upper third of the esophagus. The spinal fibers originate also in the nucleus ambiguus and travel with the vagus. They innervate the trapezius and sternomastoid muscles, which move head and shoulders.

The hypoglossal nerve originates in the hypoglossal nucleus, and travels through the medulla, emerging from the brain stem as a group of rootlets between the pyramid and the olive. The efferents mediate movement and shape of the tongue. The hypoglossal nucleus receives afferent inputs from the contralateral motor cortex, which are involved in voluntary tongue movements during speech. It also receives afferents from the trigemi-nal sensory nucleus and from the solitary nucleus (nucleus solitarius). These afferents are part of the reflex control of swallowing, chewing, and sucking.

If the hypoglossal nucleus nerve is le-sioned unilaterally, this results in tongue paralysis on the ipsilateral side. The tongue atrophies and becomes distorted, mainly because the mucous membranes of the tongue are too large to be accommodated by the reduced tongue volume. Paresis of the tongue, which means muscular weakness, results.

nucleus of solitary tract nucleus ambiguus dorsal vagus .nucleus nucleus of solitary tract nucleus ambiguus dorsal vagus .nucleus

Nucleus Dorsalis Motoris Vagus

pyramid motor trigeminal cortex sensory nucleus pyramid transverse section of medulla, showing hypoglossal and vagal nuclei, and nerves motor trigeminal cortex sensory nucleus tongue atrophy

Paralysis Hypoglossal

tongue deviates to right in peripheral paralysis of hypoglossal nerve on right side vagus nerve to muscles of pharynx and larynx spinal root of accessory nerve inferior salivatory nucleus (efferent)

solitary nucleus (afferent)

hypoglossal nucleus (efferent)

dorsal nucleus ofvagus (efferent)

nucleus ambiguus (efferent)

tongue atrophy tongue deviates to right in peripheral paralysis of hypoglossal nerve on right side jugular foramen accessory nerve to ^ trapezius and sternomastoid muscles vagus nerve to muscles of pharynx and larynx accessory, hypoglossal and vagus nerves and roots Greenstein, Color Atlas of Neuroscience © 2000 Thieme spinal root of accessory nerve

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