Nucleus and brain exit
The hypoglossal nerve has a single nucleus — the motor nucleus (nucleus n. hypoglossi). It lies in the posterior region of the medulla oblongata. When viewing the projection of this nucleus from the rhomboid fossa, it is located in the area of the corresponding hypoglossal trigone (trigonum n. hypoglossi).
The formation of the trunk begins with nerve fibers emerging from the brain tissue. This does not occur as a single large bundle, but rather as multiple small rootlets. Specifically, from 10 to 15 rootlets emerge from the medulla oblongata along the anterolateral sulcus (the groove strictly between the pyramid and the olive of the medulla).
Topography and skull exit
After emerging from the brainstem, the numerous rootlets combine to form the main trunk of the hypoglossal nerve. This trunk leaves the cranial cavity through a specialized bony opening — the hypoglossal canal (canalis n. hypoglossi).
Immediately after exiting the skull, the nerve begins its complex cervical course, characterized by close syntopy with the major blood vessels. The course of the nerve is as follows:
- First, the trunk runs downwards, lying in the space between the vagus nerve and the internal jugular vein.
- Then it gently loops around the lateral side of the internal carotid artery.
- Continuing its path, the nerve passes between the internal carotid artery and the internal jugular vein.
- Lower down, it crosses the external carotid artery.
- Finally, the trunk passes deep to the posterior belly of the digastric muscle, after which it heads directly toward the muscles of the tongue.
Branches of the hypoglossal nerve
Along its course, the nerve does not remain an isolated trunk, but gives off several significant groups of branches:
- Lingual branches (rr. linguales) are the terminal branches of the twelfth pair. These are the ones that penetrate the tongue and provide innervation to both its intrinsic and extrinsic muscles.
- Meningeal branches are small branches that arise from the main trunk at the very beginning of its path, while the nerve is still in the region of the hypoglossal canal.
- Communicating branches — form a network of connections with other neural structures. The hypoglossal nerve connects with:
- the superior cervical sympathetic ganglion;
- the inferior ganglion of the vagus nerve;
- the lingual nerve (which is a branch of the trigeminal nerve — n. trigeminus);
- the ansa cervicalis (ansa cervicalis), which receives nerve fibers from the C1–C3 spinal segments.
Briefly: answers to topical questions
What are the symptoms of a unilateral lesion of the hypoglossal nerve?
Symptoms of a unilateral hypoglossal nerve lesion include motor impairments of the corresponding half of the tongue and speech disorders. In peripheral or nuclear lesions, the following signs are observed:
- Tongue deviation — deviates toward the lesion side upon protrusion.
- Muscle atrophy — of the ipsilateral half of the tongue.
- Fasciculations — muscle twitching.
- Dysarthria — impaired speech articulation.
- Weakness in pushing the tongue — on the affected side (paresis of the genioglossus muscle).
At the same time, the act of swallowing remains intact. Unilateral supranuclear lesions do not cause pronounced impairment of tongue mobility due to bilateral cortical innervation.
Which nerve roots contribute to the ansa cervicalis?
The deep ansa cervicalis (ansa cervicalis) is formed by the superior and inferior roots, which consist of fibers from the upper cervical spinal nerves and the hypoglossal nerve.
- Superior root (radix superior) — formed by transient motor fibers from the C1 segment running within the hypoglossal nerve (n. hypoglossus).
- Inferior root (radix inferior) — formed by motor branches from the C2 and C3 segments (occasionally from C3–C4).
Joining together, these roots form the ansa cervicalis, the branches of which innervate the infrahyoid muscles of the neck.
To remember the site of exit of the nerve from the brain, use the mnemonic: The hypoglossal nerve emerges from the sulcus Between the Pyramid and the Olive.
Class and exam questions
What functional type does the hypoglossal nerve belong to?
The hypoglossal nerve is strictly a motor nerve. It contains no primary sensory or parasympathetic nuclei.
Where is the motor nucleus of the XII pair located?
The motor nucleus (nucleus n. hypoglossi) is located in the posterior region of the medulla oblongata and projects onto the rhomboid fossa in the area of the hypoglossal trigone.
How does the trunk of the hypoglossal nerve leave the cranial cavity?
After coalescing from 10–15 rootlets, the main trunk of the nerve exits the cranial cavity through the eponymous hypoglossal canal (canalis n. hypoglossi).
What else to study on this topic
- Projection of the hypoglossal nerve nucleus onto the rhomboid fossa (trigonum n. hypoglossi).
- Detailed topography of the nerve relative to the internal and external carotid arteries.
- Formation of anastomoses and communicating branches (ansa cervicalis, C1–C3 fibers).
- Features of the innervation of intrinsic and extrinsic tongue muscles by the terminal branches (rr. linguales).
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