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The hypoglossal nerve is the twelfth cranial nerve (CN XII) (TA: nervus hypoglossus or nervus cranialis XII). It arises from the medulla and passes through the hypoglossal canal to the neck, where it travels to the sublingual space to supply somatic motor innervation to all of the intrinsic and extrinsic muscles of the tongue, with the exception of the palatoglossus muscle, which is innervated by the vagus nerve.
In the neck, the hypoglossal nerve sheath receives a hitch-hiking branch of the cervical plexus containing fibers from the C1 and C2 spinal roots. These fibers distribute from the hypoglossal nerve at a number of different points, supplying numerous other muscles of the neck.
Nucleus and intraparenchymal portion
The hypoglossal nucleus lies near the midline in the tegmentum of the upper medulla, immediately deep to the hypoglossal trigone. Fibers from the hypoglossal nucleus travel anteriorly, just lateral to the medial lemniscus, before passing anterolaterally between the medullary pyramid and inferior olivary nucleus. The hypoglossal nerve exits the brainstem in the form of a multitude of rootlets through the anterolateral sulcus of the medulla, between the pyramid and the olive 1.
The emerging rootlets converge to form the two roots of the hypoglossal nerve, which travel anterolaterally within the premedullary cistern to enter the hypoglossal canal, located in the posterior cranial fossa, between the occipital condyle and jugular tubercle of the occipital bone 1.
Within the hypoglossal canal, the two hypoglossal roots converge to form a single nerve. The hypoglossal nerve travels obliquely forwards (posteromedial to anterolateral) within the canal, emerging from the inferior skull base in the upper neck.
A branch of the cervical plexus conducting fibers from the C1 and C2 spinal roots passes inside the sheath of the hypoglossal nerve just after it exits the cranium, without converging with the nerve itself. These fibers have numerous distinct functions, to be discussed later (see branches) 2.
The hypoglossal nerve descends through the neck in the retrostyloid space, initially medial to the internal carotid artery. In this location, the hypoglossal nerve is in close relation to other lateral pharyngeal structures, including the internal jugular vein, and the glossopharyngeal, vagus and spinal accessory nerves 1,3.
The hypoglossal nerve passes across the internal carotid artery on its descent through the neck, such that by the time it reaches the level of the occipital artery, it is situated lateral to the internal carotid artery. At this level, the hypoglossal nerve begins to travel anteriorly, passing lateral to the sternocleidomastoid branch of the occipital artery and lateral to the uppermost loop of the lingual artery on its course towards the tongue. In doing so, the nerve passes through the carotid triangle, and subsequently, the submandibular triangle 1,2.
The nerve travels along the superficial surface of the hyoglossus muscle, entering the sublingual space by passing between the anterior edge of this muscle and the posterior edge of the mylohyoid muscle. It does so alongside the submandibular duct. Inferior to the lingual nerve, the hypoglossal nerve divides into terminal lingual branches to supply all of the intrinsic and extrinsic muscles of the tongue (with the exception of the palatoglossus muscle) 3,4.
The terminal lingual branches of the hypoglossal nerve supply all four of the intrinsic muscles of the tongue 1. These branches also supply three of the four extrinsic muscles of the tongue, with the exception being the palatoglossus muscle, which receives innervation from the vagus nerve via the pharyngeal plexus.
The C1 and C2 fibers traveling within the hypoglossal sheath diverge from the hypoglossal nerve proper at a number of points.
- superior root of the ansa cervicalis (descending hypoglossal nerve): C1 motor fibers innervating the infrahyoid (strap) muscles diverge from the hypoglossal nerve just lateral to the occipital artery, as the nerve lies between the internal jugular vein and internal carotid artery 2
- nerve to thyrohyoid: C1 motor fibers diverge as the hypoglossal nerve lies on the loop of the lingual artery
- nerve to geniohyoid: C1 motor fibers diverge as the hypoglossal nerve passes near the mylohyoid muscle
- meningeal branch: C2 sensory fibers pass retrogradely along the course of the hypoglossal nerve, to innervate a region of dura mater lining the floor and posterior wall of the posterior cranial fossa
The hypoglossal nerve is argued to be best evaluated using a segmental approach, with the choice of MRI or CT modalities best informed by dividing the nerve into its constituent segments 1. MRI is the preferred modality to directly visualize a given segment of the nerve itself, whereas CT is generally superior in displaying the course of the nerve through the skull base 5. Authors recommend tailoring the imaging technique to suit both the anatomical segment to be studied and the main pathological entities commonly affecting that segment 5.
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