Overview of Hypoglossal Nerve XII
Hypoglossal nerve is a motor cranial nerve for the muscles of the tongue. It is the last cranial nerve. So,If the right CN 12 is dysfunctional, the tongue will deviate to the right. This is because the normally functioning left half will dominate as it no longer has opposition from the right. Similarly, the tongue would have limited or absent ability to resist against pressure applied from outside the left cheek.
|Type||General somatic efferent (GSE) (motor)|
|Origin||Hypoglossal nucleus in medulla oblongata|
|Branches||Meningeal branch, superior root of the ansa cervicalis, and terminal lingual nerves|
|Field of innervation||The intrinsic muscles of the tongue
Three extrinsic muscles of the tongue:
– the genioglossus muscle
– the hyoglossus muscle
– the styloglossus muscle
Hypoglossal Nerve Origin
The hypoglossal nerve (CN XII) is exclusively a motor nerve carrying general somatic efferent fibers (GSE). It innervates all intrinsic and almost all extrinsic muscles of the tongue, as well as one suprahyoid muscle, the geniohyoid muscle. The nerve originates from its motor nucleus, simply called the nucleus of the hypoglossal nerve, or hypoglossal nucleus.
The hypoglossal nucleus is a thin and long nucleus lies in the ventral portion of the medulla, near the midline. Its rostral portion belongs to an area called the hypoglossal trigone. The neurons of this nucleus are multipolar and they send efferent fibers that extend anteriorly and laterally. They pass between the medial lemniscus and the complex of the olivary nuclei, and in the form of a dozen fibers rather than together as one group, they leave the brainstem through the anterolateral sulcus of the medulla oblongata (between the pyramids and olives).
The nucleus of the hypoglossal nerve also gets a portion of its sensory fibers through many of its synaptic relations with the solitary nucleus and the sensory nuclei of the trigeminal nerve (CN V). In this way it is included in reflex movements of the tongue such as swallowing or chewing.
It is interesting that the fibers of the corticobulbar (or corticonuclear) tract connect the motor cortex with the nucleus of the hypoglossal nerve. The corticobulbar fibers intended for this connection to innervate the contralateral nucleus of the hypoglossal nerve. This means that the damage of the corticobulbar fibers manifests as paralysis of the muscles of the contralateral side of the tongue.
Hypoglossal Nerve Course and relations
First, let’s see which structures the hypoglossal nerve (CN XII) innervates, and then we’ll follow the nerve all the way from the brainstem to see how does it get to its target structure. The hypoglossal nerve supplies:
- All of the intrinsic muscles of the tongue
- Three extrinsic muscles of the tongue: the genioglossus, hyoglossus and styloglossus muscles
- As well as the geniohyoid muscle (a suprahyoid muscle)
The hypoglossal nerve leaves the brain stem through the anterolateral sulcus of the medulla oblongata. Its dozen roots pass across the posterior cranial fossa laterally before merging into the unique trunk of the nerve in the hypoglossal canal.
Shortly after the hypoglossal nerve leaves the cranium, about 2 centimeters after, the branches of the cervical plexus join it. These branches carry general somatic efferent (motor) fibers from spinal nerves C1 and C2, and general somatic afferent fibers from the spinal ganglion (dorsal root ganglion) of C2. These spinal nerves basically “use” the hypoglossal nerve to reach the suprahyoid and infrahyoid muscles.
When the nerve exits the cranium, it enters the retrostyloid space, which is a part of the lateral pharyngeal space. In this space, the nerve is closely related to all lateral pharyngeal elements, such as:
- internal carotid artery
- internal jugular vein
- glossopharyngeal nerve (CN IX)
- Vagus nerve (CN X)
- Accessory nerve (CN XI)
The hypoglossal nerve descends vertically through the retrostyloid space. At first, it is positioned medial to the internal carotid artery, but after that it crosses the artery and positions itself laterally to it. The hypoglossal nerve holds this relation until it reaches the beginning of the occipital artery. There, the nerve changes to a horizontal orientation, turning anterior, curving around the sternocleidomastoid branch of the occipital artery, and coursing towards the tongue. It first crosses over the carotid triangle, and then over the submandibular triangle of the neck.
The horizontal portion of the nerve first crosses the lateral side of the external carotid artery. After that, the nerve crosses the deep side of the posterior belly of the digastric muscle and the medial surface of the stylohyoid muscle.
After this, the hypoglossal nerve passes over the superficial side of the hyoglossus muscle, and passes above the greater horn of the hyoid bone. This segment of the pathway, from the hyoglossus muscle and the greater horn of the hyoid bone, is the part of the nerve that forms the superior edge of the Pirogov triangle (also known as the Pirogoff triangle), and it is covered by the submandibular gland.
Finally, the hypoglossal nerve together with the secretory duct of the submandibular gland passes through the intermuscular crack, between the anterior edge of the hyoglossus muscle and the posterior edge of the mylohyoid muscle, and enters the sublingual area where it splits into its terminal lingual branches. At this final part of the pathway, the lingual nerve is positioned superiorly to the hypoglossal nerve.
Hypoglossal Nerve Branches
The hypoglossal nerve (CN XII) has two lateral, and terminal lingual branches. The two lateral branches are the meningeal branch and the superior root of the ansa cervicalis, whereas the terminal branches are the terminal lingual nerves.
The meningeal branch carries fibers from the sensory spinal ganglion of the spinal nerve C2. This branch returns to the skull through the hypoglossal canal and innervates the dura mater on the floor of the posterior cranial fossa, and the posterior wall of the posterior cranial fossa. Want to recall the dura mater and cranial anatomy? We got you covered with great video tutorials.
The superior root of the ansa cervicalis actually carries the fibers from the cervical plexus that joined the nerve outside the cranium. It supplies three of the infrahyoid muscles: the sternohyoid, sternothyroid and the omohyoid muscles.When it comes to the terminal lingual branches, the nerve literally diverges into these branches after it passes through the aforementioned intermuscular crack between the hyoglossus muscle and the mylohyoid muscle.
These terminal lingual branches of the hypoglossal nerve are exclusively motor and innervate all of the intrinsic muscles of the tongue, three extrinsic muscles of the tongue (genioglossus, hyoglossus, and styloglossus), and the geniohyoid muscle additionally.
Assessment is performed as follows:
- Ask the patient to stick their tongue straight out of their mouth.
- If there is any suggestion of deviation to one side/weakness, direct them to push the tip of their tongue into either cheek while you provide counter pressure from the outside.
Penetrating injuries to the neck, and various lesions of the skull base may also affect the nucleus of the hypoglossal nerve. An injury of the nerve manifests as flaccid paralysis and atrophy of the ipsilateral muscles of the tongue followed with speech impairment.
To test the function of the hypoglossal nerve, a physician should ask their patient to protrude their tongue. The tongue should then be palpated to check the tone of the muscles as well as the ability of sensation.
Normally, simultaneous contraction of the paired genioglossus muscles cause the tongue to protrude straight and forward. So during examination of the patient it is important to remember that a unilateral lesion of the hypoglossal nerve will cause the tongue to deviate towards the side of the lesion (the impaired side) since the functional genioglossus muscle on the intact side is unopposed by the paralyzed and inactive genioglossus muscle on the lesion side.