CN XII: Hypoglossal nerve

In this note, we are going to discuss the anatomy and function of the hypoglossal nerve (n. hypoglossus), or the 12th cranial nerve (CN XII).

It innervates all intrinsic and almost all extrinsic tongue muscles, except the palatoglossus.

This is a motor nerve. According to the generally accepted classification of functional components, it contains general somatic efferent fibers (GSE).

They arise from the hypoglossal nucleus (nucleus nervi hypoglossi), located in the medulla oblongata.

They leave the brainstem through the anterolateral sulcus (sulcus anterolateralis) of the medulla oblongata in the form of 10–15 rootlets.

Hypoglossal nerve leaves the brainstem from the anterolateral sulcus

Then they merge into the common trunk, which exits the cranial cavity through the hypoglossal canal (canalis nervi hypoglossi) of the occipital bone.

There is data indicating that the hypoglossal nerve gets a portion of sensory fibers from the sensory nuclei of the trigeminal nerve, providing reflex movements such as swallowing and chewing.

Shortly after leaving the cranium, the hypoglossal nerve receives fibers from the cervical plexus, taking part in the formation of the so-called ansa cervicalis (ansa cervicalis).

Hypoglossal nerve passes along the ansa cervicalis

The ansa cervicalis is closely related to the internal jugular vein and innervates the infrahyoid muscles. Those fibers that run along the hypoglossal nerve innervate the geniohyoid muscle (m. geniohyoideus) and the thyrohyoid muscle (m. thyrohyoideus).

When the nerve exits the cranium, it lies between the internal carotid artery and the internal jugular artery.

Hypoglossal nerve between the blood vessels

Around the angle of the mandible, it curves anteriorly and travels deep to the posterior belly of the digastric muscle.

Nerve travels deep to the digastric muscle

After this, the nerve passes over the external surface of the hyoglossus muscle and along the internal surface of the mylohyoid muscle, inferiorly to the lingual nerve.

This segment of the nerve pathway located between the hyoglossus muscle and the greater horn of the hyoid bone is the superior border of the so-called lingual triangle or Pirogov triangle (trigonum linguale), which is used in surgery to access the lingual artery (a. lingualis).

Pirogov’s lingual triangle (trigonum linguale)

Within the sublingual area, the hypoglossal nerve splits into its terminal lingual branches, supplying the muscles of the tongue.

These include:

All of the intrinsic muscles of the tongue: the superior longitudinal (m. longitudinalis superior), inferior longitudinal (m. longitudinalis inferior), transverse (m. transversus linguae) and vertical (m. verticalis linguae) muscles of the tongue.

And the three extrinsic muscles of the tongue: the styloglossus (m. styloglossus), genioglossus (m. genioglossus), and hyoglossus (m. hyoglossus) muscles. The palatoglossus muscle is innervated by the vagus nerve.

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