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The hypoglossal nerve is a motor nerve that controls all of the muscles of the tongue
(except for one, the palatoglossus, which is controlled by the vagus nerve).
The tongue muscles consist of what are known as intrinsic muscles, which control the shape
of the tongue, and extrinsic muscles, which act to protrude, retract, elevate, and move
the tongue side to side.
Healthy function of the hypoglossal nerve is thus critical for things like eating, swallowing,
and speaking.
The hypoglossal nuclei are found in the medulla.
Hypoglossal nerve fibers leave the hypoglossal nuclei on each side of the brainstem and descend
to travel to the same side of the tongue to stimulate the muscles of the tongue from below,
hence the term hypoglossal, which comes from the Greek for under the tongue.
There are three other branches that extend from the hypoglossal nerve to supply various
other muscles in the neck as well as the dura mater at the back of the head.
Only the fibers that supply the tongue originate in the hypoglossal nucleus, however, and thus
they are often considered the true hypoglossal nerve.
Damage to the hypoglossal nerve can cause tongue weakness and impair tongue dexterity.
It may also lead to small muscle twitches, or fasciculations, in the tongue as well as
atrophy of the tongue--especially at the tip or borders of the organ.
If the hypoglossal nerve on only one side is damaged, then atrophy will typically be
seen in the tongue muscles on that same side.
If the patient is asked to protrude his or her tongue, the tongue will often deviate
toward the side of the damaged nerve.
Patients with damage to only one hypoglossal nerve, however, are often able to compensate
for the deficiency of the tongue muscles on one side.
But if both nerves are damaged, the patient may be unable to protrude the tongue at all
and may experience severe problems with speech and swallowing.