OverviewThe branchial motor component of CN X provides voluntary control of the following:
- Striated muscle of the pharynx.
- Striated muscle of the larynx, except for the stylopharyngeus muscle (CN IX) and the tensor veli palatini muscle (CN V).
- Palatoglossus muscle of the tongue (the rest of the muscles of the tongue are innervated by CN XII).
Origin and Central Course
Fibers leaving the nucleus ambiguus travel anteriorly and laterally to exit the medulla posterior to the olive as a series of 8 - 10 rootlets.
Upon emerging from the lateral aspect of the medulla the branchial motor component travels with the fibers of the accessory nerve (CN XI) into the jugular foramen of the skull. The remaining components of the vagus nerve also enter the jugular foramen and give rise to two ganglia (the superior and inferior vagal ganglia) within the jugular foramen. The branchial motor fibers join with the rest of the vagus nerve just below the inferior vagal ganglion.
All fibers of CN X exit the skull via the jugular foramen.
Extracranial Course and Final Innervation
Upon exiting the skull the vagus nerve travels between the internal jugular vein and internal carotid artery within the carotid sheath.
The branchial motor fibers leave the vagus nerve as three major branches:
- Pharyngeal branch
- Superior laryngeal nerve
- Recurrent laryngeal nerve
The pharyngeal nerve is the principle motor nerve of the pharynx.
It branches from the vagus nerve just below the inferior ganglion and travels inferiorly and medially to pass between the internal and external carotid arteries. The nerve enters the middle constrictor muscle of the pharynx where it spreads out to form the pharyngeal plexus to innervate all muscles of the pharynx and soft palate (except the stylopharyngeus and tensor veli palatini muscles which are innervated by CNs IX and V, respectively).
The muscles innervate by the pharyngeal nerve include:
- Superior, middle, and inferior constrictor muscles
- Levator palatini muscle
- Salpingopharyngeus muscle
- Palatopharyngeus muscle
- Palatoglossus muscle of the tongue
Superior Laryngeal Nerve
The superior laryngeal nerve branches from the vagus nerve just below the pharyngeal nerve. The nerve descends in the neck adjacent to the pharynx and splits to form the internal and external laryngeal nerves.
The external laryngeal nerve supplies the inferior constrictor muscle before piercing it to supply the cricothyroid muscle which is involved in the controlling the movements of the vocal folds.
The internal laryngeal nerve pierces the thyrohyoid membrane and is a sensory nerve of the larynx.
Recurrent Laryngeal Nerve
The path of the recurrent laryngeal nerve differs on the right and left sides of the body.
The left recurrent laryngeal nerve separates from the vagus nerve at the level of the aortic arch. The nerve loops posteriorly around the aortic arch and ascends through the superior mediastinum to enter the groove between the esophagus and trachea.
The right recurrent laryngeal nerve splits from the vagus before entering the superior mediastinum at the level of the right subclavian artery. The nerve hooks posteriorly around the subclavian artery and also ascends in the groove between the esophagus and trachea.
Both recurrent laryngeal nerves pass deep to the lower margin of the inferior constrictor muscle to innervate the intrinsic muscles of the larynx responsible for controlling the movements of the vocal folds.
Voluntary Control of the Muscles of the Pharynx and Larynx
Signals for the voluntary movement of the muscles innervated by CN X originate in the pre-motor and motor cortex (in association with other cortical areas) and pass via the corticobulbar tract in the posterior limb of the internal capsule to the nucleus ambiguus.
The ambiguus nuclei are bilaterally innervated by the higher centers.
Lower Motor Neuron (LMN) Lesion
Unilateral damage to the vagus nerve is indicated by:
- Hoarseness (due to paralysis of the intrinsic muscles of the larynx on the affected side).
- Difficulty in swallowing due to the inability to elevate the soft palate on the affected side (due to paralysis of the levator palatini muscle).
On examination the soft palate droops on the affected side and the uvula deviates opposite the affected side due to the unopposed action of the intact levator palatini muscle.
Unilateral damage to the recurrent laryngeal nerve can occur during surgical procedures in the neck, resulting in hoarseness due to unilateral weakness or paralysis of the muscles controlling the vocal fold on the affected side.
Enlargement of the paratrachial lymph nodes, as can be seen in metastatic lung cancer, can lead to compression of one of the recurrent laryngeal nerves with similar results.