| 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 – Spinal Accessory |
| CN XII – Hypoglossal |
The hypoglossal nerve is the twelfth cranial nerve (XII), leading to the tongue. 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 to enter 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. It controls tongue movements of speech, food manipulation, and swallowing.[1]
The hypoglossal nerve is derived from the basal plate of the embryonic medulla oblongata.
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.[2]
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To test the function of the nerve, a person is asked to poke out his/her tongue. If there is a loss of function on one side (unilateral paralysis), the tongue will point toward the affected side, due to unopposed action of the genioglossus muscle (which pulls the tongue forward) on the side of the tongue that is usually innervated. This is the result of a lower motor neuron lesion (the damaged neuron directly innervates the skeletal muscle), and can lead to fasciculations and atrophy of the tongue.[3]
The strength of the tongue can be tested by getting the person to poke the inside of his/her cheek, and feeling how strongly he/she can push a finger pushed against the cheek - a more elegant way of testing than directly touching the tongue.
The tongue can also be looked at for signs of lower motor neuron disease, such as fasciculation and atrophy.
Paralysis/paresis of one side of the tongue results in ipsilateral curvature of the tongue (apex toward the impaired side of the mouth); i.e., the tongue will move toward the affected side.
Cranial Nerve XII is innervated by the contralateral cortex, so a purely upper motor neuron (cortex) lesion will cause the tongue to deviate away from the side of the cortical lesion. Additionally, the fasciculations and atrophy seen in lower motor neuron lesions are not present.[3]
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.[4]
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 proximal end of the facial nerve is not available, but the peripheral system is still viable. There are two options:
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