Cranial Nerves: Glossopharyngeal & Vagus

Carotid Sheath

• Lies in contact with the prevertebral fascia on the pharynx's side wall, with the styloid apparatus muscles sloping down laterally.

Carotid Sheath Composition and Location

• Extends from the aortic arch to the skull base.
• The upper part is attached to the carotid canal margins in the petrous bone and the tympanic part of the temporal bone's inferior border, blending with the parotid fascia's deep layer.
• Contains the internal carotid artery, internal jugular vein, and the ninth to twelfth cranial nerves.
• Medial to the pharynx, lateral to the parotid gland's deepest part, and superior to the styloid process and its muscles.
• Anteriorly bounded by the infratemporal fossa and posteriorly by the cervical sympathetic trunk on the prevertebral fascia.
• The internal jugular vein is behind the internal carotid artery at the skull base, sloping laterally as it descends to lie lateral to the common carotid artery.
• The vagus nerve is located in the groove between the internal jugular vein and internal carotid artery, within the carotid sheath.
• Glossopharyngeal and accessory nerves emerge between the artery and vein at the skull base, curving away from each other.
• The hypoglossal nerve emerges from the hypoglossal canal, passes through the sheath behind the inferior vagal ganglion, and emerges between the artery and vein.
• Arteries lie deep, and veins are superficial to the hypoglossal nerve throughout its course to the tongue.
• The internal carotid artery enters the skull, while the external carotid artery divides into terminal branches within the parotid gland.
• Structures passing between the internal and external carotid arteries include the styloid process, stylohyoid ligament, styloglossus, stylopharyngeus, glossopharyngeal nerve (with its branch to stylopharyngeus and pharyngeal branch), and pharyngeal branch of the vagus nerve.
• Stylohyoid and hypoglossal nerve pass superficial to the external carotid artery.
• The superior laryngeal nerve and its internal and external laryngeal branches pass deep to both carotid arteries.

Glossopharyngeal Nerve

• Originates as the nerve of the third pharyngeal arch.
• Emerges from the jugular foramen's anterior part on the inferior petrosal sinus's lateral side.
• The inferior ganglion bulges the nerve at a deep notch in the petrous bone's inferior border.
• Contains cell bodies of most sensory fibers and passes down on the internal carotid artery.
• Curves forward around the lateral side of stylopharyngeus.
• Parallels the lower border of styloglossus and passes behind the hyoglossus, reaching the tongue.
• The glossopharyngeal nerve has six branches: tympanic, motor branch to stylopharyngeus, carotid sinus nerve, pharyngeal branches, tonsillar branch, and lingual branch.

Glossopharyngeal Nerve Branches

• Tympanic Branch (Jacobson's Nerve): Supplies the middle ear, mastoid air cells, and auditory tube with sensory fibers; also carries parasympathetic fibers to the otic ganglion for parotid gland innervation.
• Motor Branch to Stylopharyngeus: Arises as the nerve spirals around the muscle's posterior border; cell bodies of motor axons are in the nucleus ambiguus.
• Carotid Sinus Nerve: Supplies the carotid sinus and carotid body (baroreceptors and chemoreceptors), connecting to vasomotor and autonomic centers via the nucleus of the tractus solitarius.
• Pharyngeal Branches: Join the pharyngeal plexus on the middle constrictor muscle, supplying sensation and parasympathetic fibers to the oropharynx's mucous membrane.
• Tonsillar Branch: Supplies the mucous membrane over the palatine tonsil, anastomosing with lesser palatine nerves.
• Lingual Branch: Supplies the posterior one-third of the tongue with sensory and secretomotor fibers to the glands, relaying in small ganglia in the mucous membranse

Vagus Nerve

• Emerges through the jugular foramen's middle compartment; possesses a superior ganglion.
• An elongated inferior ganglion dilates the trunk just below the skull base.
• Ganglia contain cell bodies of afferent fibers: the superior ganglion for meningeal and auricular branches, and the inferior ganglion for sensory fibers.
• The inferior ganglion receives a branch from the accessory nerve, carrying nucleus ambiguus fibers for striated muscles of the pharynx, soft palate, esophagus, and larynx.
• Runs down the neck within the carotid sheath between the carotid artery and jugular vein.
• Passes in front of the subclavian artery in the root of the neck, entering the mediastinum to supply thoracic and abdominal viscera.
• Branches in the neck include meningeal, auricular, carotid body, pharyngeal, superior laryngeal, cervical cardiac, and recurrent laryngeal nerves.

Vagus Nerve Branches

• Meningeal Branches: Supply the dura mater of the posterior fossa from the superior ganglion.
• Auricular Branch: Supplies the posteroinferior quadrant of the tympanic membrane's outer surface, the external acoustic meatus, and the skin behind the auricle.
• Carotid Body Branch: Forms a plexus with the glossopharyngeal nerve's carotid sinus branch.
• Pharyngeal Branch: Joins the pharyngeal plexus on the middle constrictor muscle, supplying pharyngeal and soft palate muscles (except tensor palati).
• Superior Laryngeal Nerve: Divides into the internal laryngeal nerve (pierces the thyrohyoid membrane to reach the piriform recess) and the external laryngeal nerve (supplies cricothyroid).
• Cervical Cardiac Branches: Two on each side; pass down to the deep part of the cardiac plexus (right) or to both deep and superficial parts (left).
• Recurrent Laryngeal Nerve: Hooks around the ligamentum arteriosum (left) or subclavian artery (right), ascending near the trachea to supply the trachea, esophagus, possibly the inferior constrictor, and laryngeal muscles (except cricothyroid).

Accessory Nerve

• Formed by the union of cranial and spinal roots in the posterior cranial fossa.
• Occupies the middle compartment of the jugular foramen, lateral to the vagus nerve.
• All fibers of the cranial root join the vagus nerve.
• The remaining cervical fibers slope down on the internal jugular vein, passing deep to the styloid process and posterior belly of the digastric.
• Gives off a branch to sternocleidomastoid, piercing the muscle near the motor branch entrance.
• Crosses the posterior triangle to supply trapezius.
• Proprioceptive fibers from the muscles reach the spinal cord via C2-C4.

Hypoglossal Nerve

• Emerges from the hypoglossal canal.
• Picks up a branch from the anterior ramus of C1 and spirals behind the inferior ganglion of the vagus.
• Emerges between the internal carotid artery and internal jugular vein, lying on the carotid sheath deep to the styloid muscles and posterior belly of the digastric.
• Curls forward beneath the digastric tendon, passing across the arteries and deep to the veins to reach the tongue.
• Non-lingual branches (meningeal branch, branch to ansa cervicalis, branches to thyrohyoid and geniohyoid) are formed by C1 fibers.
• True hypoglossal fibers supply only tongue muscles, with cell bodies in the hypoglossal nucleus in the medulla.
• No sensory fibers present.

Overview of the glossopharyngeal and vagus nerves, including their origin, branches, and functions. Focus on the glossopharyngeal nerve's role in vasomotor control and parasympathetic supply to the parotid gland. Also, addresses the vagus nerve's association with sensory fibers and the contribution of the accessory nerve.