Hypoglossal nerve

Nerve: Hypoglossal nerve
Hypoglossal nerve, cervical plexus, and their branches.
Inferior view of the human brain, with the cranial nerves labelled.
Latin	nervus hypoglossus
Gray's	subject #207 914
Innervates	genioglossus, hyoglossus, styloglossus
To	ansa cervicalis
MeSH	Hypoglossal+Nerve

Cranial nerves
CN I – Olfactory
CN II – Optic
CN III – Oculomotor
CN IV – Trochlear
CN V – Trigeminal
CN VI – Abducens
CN VII – Facial
CN VIII – Vestibulocochlear
CN IX – Glossopharyngeal
CN X – Vagus
CN XI – Accessory
CN XII – Hypoglossal

The hypoglossal nerve is the twelfth cranial nerve (XII), leading to muscles of the tongue. It is called hypoglossal nerve because it is below the tongue. It controls tongue movements of speech, food manipulation, and swallowing.

Trajectory

The nerve arises from the hypoglossal nucleus and emerges from the medulla oblongata in the preolivary sulcus separating the olive and the pyramid. It then passes through the hypoglossal canal. On emerging from the hypoglossal canal, it gives off a small meningeal branch and picks up a branch from the anterior ramus of C1. It spirals behind the vagus nerve and passes between the internal carotid artery and internal jugular vein lying on the carotid sheath. After passing deep to the posterior belly of the digastric muscle, it passes to the submandibular region, passes lateral to the Hyoglossus muscle, and inferior to the lingual nerve to reach and efferently innervate the tongue.

It supplies motor fibres to all of the muscles of the tongue, except the palatoglossus muscle, which is innervated by the vagus nerve (cranial nerve X) or, according to some classifications, by fibres from the glossopharyngeal nerve (cranial nerve IX) that "hitchhike" within the vagus.

The hypoglossal nerve is derived from the basal plate of the embryonic medulla oblongata.

Function

Swallowing to clear mouth of saliva and other involuntary activities completed by the tongue are controlled by the hypoglossal nerve; however, most functions are voluntary. Voluntary control requires conscious thought and nerve pathways occur in the corticobulbar region in the spinal cord.

The function of the hypoglossal nerve in manipulation for speech contributes to learning languages. Many languages require specific and sometimes unusual uses to create the desired sounds, hence why adults learning a new language may have trouble adjusting to the new movements.

Testing the hypoglossal nerve

To test the function of the nerve, a patient is asked to stick their tongue straight out. If there is a loss of innervation to one side, the tongue will curve toward the affected side, due to unopposed action of the opposite genioglossus muscle. If this is the result of a lower motor neuron lesion, the tongue will be curved toward the damaged side, combined with the presence of fasciculations or atrophy. However, if the deficit is caused by an upper motor neuron lesion, the tongue will be curved away from the side of the cortical damage, without the presence of fasciculations or atrophy.

The strength of the tongue can be tested by asking the patient to poke the inside of their cheek while feeling the pressure on the outside of their cheek.

Weakness of the tongue is displayed as a slurring of speech. The tongue may feel "thick", "heavy", or "clumsy." Lingual sounds (i.e., l's, t's, d's, n's, r's, etc.) are slurred and this is obvious in conversation.

Uses in nerve repair

Facial nerve paralysis is a difficult situation to fix, but new cranial nerve substitution techniques allow for some usage to be restored, to include hypoglossal-facial anastomosis.

This procedure is considered the standard for reanimating the face when the proximal end of the facial nerve is not available, but the peripheral system is still viable. There are two options:

• Hypoglossal nerve completely transected and connected to facial nerve.
• Hypoglossal nerve partially transected and connected to facial nerve. This may be accomplished with interposition cable grafts or jump grafts. An advantage of partial transection is minimizing tongue weakness and purported decrease in synkinesis. There are disadvantages though since there are then fewer nerve cells to drive the movement of features in the face.

Additional Images

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  Hypoglossal nerve

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  Hypoglossal nerve

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  Hypoglossal nerve

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  Hypoglossal nerve

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  Hypoglossal nerve

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  Hypoglossal nerve

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  Human pharynx.Posterior view.

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  Muscles, arteries and nerves of neck. Newborn dissection.

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  Muscles, arteries and nerves of neck. Newborn dissection.

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  Muscles, nerves and arteries of neck.Deep dissection. Anterior view.

External links

• NeuroNames hier-701
• MedEd at Loyola GrossAnatomy/h_n/cn/cn1/cn12.htm
• cranialnerves at The Anatomy Lesson by Wesley Norman (Georgetown University) (XII)
• Notes on Hypoglossal nerve