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GLOSSOPHARYNGEAL NERVE (CN IX)



The glossopharyngeal nerve provides parasympathetic innervation to the

parotid gland The glossopharyngeal nerve, one of the smallest cranial nerves, carries five functional components. These are: (i) SVA (taste) and (ii) GVA sensation from the posterior

one-third of the tongue, the adjacent pharyngeal wall, and the carotid sinus (a baroreceptor or blood pressure receptor located near the bifurcation of the common carotid artery), (iii) GSA sensation from the external ear, (iv) SVE (branchiomotor) innervation to the stylopharyngeus muscle, and (v) GVE parasympathetic innervation to the parotid gland. The glossopharyngeal nerve exits the brainstem as a group of rootlets posterior to the olive in the dorsolateral sulcus. These rootlets immediately collect to form the main trunk of the glossopharyngeal nerve, which shortly exits the cranial vault via the jugular foramen where it presents two swellings, its superiorand inferior ganglia(Fig. 15.15). The superior ganglion contains GSA, and the inferior ganglion contains GVA and SVA, cell bodies of first order pseudounipolarneurons.

The inferior ganglion of the glossopharyngeal nerve houses the cell bodies of the SVA (taste)neurons. Their peripheral processes course with the trunk of the glossopharyngeal nerve to the tongue where they supply the posterior one-third of the tongue and adjacent pharyngeal wall with taste sensation. The central processes of the SVA neurons pass into the brainstem via the glossopharyngeal nerve root, join the solitary tractand terminate in the solitary nucleus(Fig. 15.16A).

GVAfirst order nerve cell bodies reside in the inferior ganglionof the glossopharyngeal nerve. Their peripheralprocesses terminate in the mucosa of the posterior one-third

of the tongue, tonsil and adjacent pharyngeal wall, tympanic cavity, and auditory tube (Fig. 15.16B). Unilateral stimulation of the pharyngeal wall elicits a bilateral contraction of the

pharyngeal muscles and soft palate (gag reflex). The glossopharyngeal nerve serves as the afferent limb (GVA peripheral fibers whose cell bodies are housed in the inferior ganglion), whereas the vagus nerve provides the efferent limb of the reflex arc. The central processes of the afferent fibers enter the solitary tract and synapse in the nucleus ambiguus. The nucleus ambiguus sends motor fibers via the vagus nerve to the muscles of the palate and pharynx. Many individuals in the general population do not have a gag reflex.

Baroreceptor fibers terminate in the carotid body and sinus, which form the afferent limb of the reflex arc that controls blood pressure. The central processes of the GVA neurons enter the brainstem via the glossopharyngeal nerve root, join the solitary tract and terminate in the solitary nucleus. The solitary nucleus relays sensory input to the reticular formation, the brainstem GVE (autonomic) motor nuclei, and the intermediolateral horn (containing preganglionic sympathetic neurons) of the spinal cord for reflex activity related to the control of arterial lumen diameter and blood pressure. The glossopharyngeal nerve also provides GSAtouch, pain, and temperature innervation to the pinna of the ear and the external auditory meatus. The cell bodies of these sensory neurons are located in the superior ganglion of the glossopharyngeal nerve. The central processes of these neurons course in the glossopharyngeal nerve root, enter the brainstem, and join the spinal tract of the trigeminal nerve to terminate and synapse in the spinal nucleus of the trigeminal nerve. Recent clinical evidence supports that fibers transmitting nociceptive sensory input from the pharyngeal wall and posterior one-third of the tongue enter the brainstem and descend in the spinal tract of the trigeminal and terminate in the spinal nucleus of the trigeminal. Furthermore, sensation from oral structures is transmitted via the glossopharyngeal afferent terminals to the main sensory nucleusof the trigeminal. The nucleus ambiguus contains the SVE branchiomotornerve cell bodies whose axons emerge from the brainstem along with rootlets of the glossopharyngeal nerve, and course with the trunk of the glossopharyngeal nerve (Fig. 15.16C).

These axons then leave the glossopharyngeal nerve as the nerve to the stylopharyngeus muscle, the only muscle innervated by the glossopharyngeal nerve.

The inferior salivatory nucleus, located in the medulla, contains the GVEcell bodies of preganglionic parasympathetic neurons whose axons exit the brainstem as part of the glossopharyngeal nerve (Fig. 15.16D). These fibers then branch off as the tympanic nerve and subsequently spread out to form the tympanic plexus in the tympanic cavity.

The preganglionic parasympathetic fibers course to the otic ganglion, the parasympathetic ganglion of the glossopharyngeal nerve (located in the infratemporal fossa), where they synapse with postganglionic parasympathetic neurons whose fibers join the auriculotemporal branch of the trigeminal nerve to reach the parotid gland, providing it with secretomotor innervation.

CLINICAL CONSIDERATIONS

A unilateral lesion to the glossopharyngeal nerve near its exit from the brainstem, damaging all of its fibers, will result in damage to the SVA fibers relaying taste sensation and will cause ipsilateral loss of taste sensationfrom the posterior one-third of the tongue. Damage to the GVE parasympathetic fibers will cause a reduction in salivary secretionof the parotid gland; and damage to the GVA fibers will result in diminished visceral sensationfrom the pharyngeal mucous membrane, loss of the gag reflex(due to damage of the afferent limb of the reflex arc), and loss of the carotid sinus reflex. The stylopharyngeus muscle, which elevates the pharynx during swallowing, will be paralyzed.

 

VAGUS NERVE (CN X)

 

The vagus nerve has the most extensive distribution in the body, innervating structures in the head but also the neck, thorax, and abdomen The vagus(L., “wanderer”) nerve(Fig. 15.17) is a large cranial nerve that has the most extensive distribution in the body. Although it is a cranial nerve, its innervation is not limited to the structures in the head, but also extends into the neck, thorax, and abdomen. The vagus nerve carries five functional components: (i) SVA; (ii) GVA; (iii) GSA; (iv) SVE; and (v) GVE (the same functional components carried by the facial and glossopharyngeal nerves). A group of fine rootlets surface in the medulla in the dorsolateral sulcus, inferior to the glossopharyngeal nerve and superior to the spinal accessory nerve. The rootlets join to form two distinct bundles—a smaller inferior and a larger superior that collectively form the vagus nerve. The inferior bundle joins the spinal accessory nerve and accompanies it for a short distance, but then the two diverge to go their separate ways. The smaller vagal bundle joins the main trunk of the vagus to exit the cranial vault via the jugular foramen. Inferior to the jugular foramen, the vagus nerve displays two swellings, the superior (jugular) and inferior (nodose) ganglia. The superior ganglionhouses the cell bodies of pseudounipolar first order sensory neurons carrying GSA information from the pinna of the ear and external auditory meatus and the dura of the posterior cranial fossa. The inferior ganglioncontains the pseudounipolar first order nerve cell bodies transmitting GVA sensory innervation from the mucosa of the soft palate, pharynx, and larynx, and a minor SVA (taste) sensation from the epiglottis.

SVA (taste) pseudounipolarneuron cell bodies located in the inferior ganglion of the vagus nerve send their peripheral fibers to terminate in the scant taste buds of the epiglottis. Their central processes enter the brainstem along with the other vagal fibers to terminate in the solitary nucleus(Fig. 15.18A).

GVA pseudounipolarneuron cell bodies housed in the inferior ganglion distribute their peripheral processes in the mucous membranes of the soft palate, and those lining the pharynx, larynx, esophagus, and trachea. Chemoreceptor fibers (GVA also) terminate in the carotid body where they monitor blood carbon dioxide concentration. The central processes of all of the GVA neurons enter the brainstem, course in the solitary tractand terminate in the solitary nucleus (Fig. 15.18A).

GSA pseudounipolarneuron cell bodies conveying pain, temperature, and touch sensation reside in the superior ganglion and send their peripheral processes to the pinna, external auditory meatus, skin of the ear, and tympanic membrane.





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