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The hypoglossal nerve is the twelfth cranial nerve. It nerve arises from the hypoglossal nucleus and emerges from the medulla oblongata between the olive and the pyramids. It then passes through the hypoglossal canal. It supplies motor fibres to all of the muscles of the tongue (except palatoglossus muscle).
Aside from the tongue, the hypoglossal nerve also controls, via the ansa cervicalis, thyrohyoid muscle, omohyoid muscle, sternothyroid muscle and sternohyoid muscle.
Hypoglossal nucleus
The hypoglossal nucleus extends the length of the medulla, and being a motor nucleus, is close to the midline. In the open medulla, it is visible as what is known as the hypoglossal trigone, a raised area (medial to the vagal trigone) protuding slightly into the fourth ventricle.
In the closed medulla, the gracial and cuneate nuclei lie posteriorly, which means the nucleus is less close to the back of the medulla (it is still close to the midline).
Testing the hypoglossal nerve
To test the function of the nerve, a person is asked to poke out their tongue. If there is a unilateral paralysis, the tongue will point towards the affected side.
The strength of the tongue can be tested by getting someone to poke at the inside of their cheek, and feeling how well they can push a finger placed on the cheek. (This avoids touching the tongue with fingers.)
The tongue can also be looked at for signs of lower motoneuron disease, such as fasciculation and atrophy.
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