Hypoglossal nerve
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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 the 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 loss of function on one side (unilateral paralysis,) the tongue will point towards the affected side.
The strength of the tongue can be tested by getting the person to poke at the inside of their cheek, and feeling how strongly they can push a finger pushed against their cheek - a more elegant way of testing than directly touching the tongue.
The tongue can also be looked at for signs of lower motorneuron disease, such as fasciculation and atrophy.