The hypoglossal nerve is the twelfth cranial nerve, which means that it is located the closest to the back of the head. Its primary purpose is to control the movement of the tongue. In fact, it innervates all of the intrinsic muscles of the tongue, as well as three out of four of the extrinsic muscles of the tongue. The motor branches of the hypoglossal nerve exist directly below the tongue - a fact which can be seen by the etymology of the word hypoglossal (hypo = under, glossal = tongue).
The nerve's nucleus resides in the medulla oblongata. When it exits the medulla, it has several sections, made from individual fibers. These sections combine into two roots, each of which passes through the hypoglossal canal at the base of the skull. The two roots then join together in the hypoglossal canal, run down the neck, near the accessory nerve and the vegus nerve (the two preceding cranial nerves). Eventually, the nerve curves around and ends up on the floor of the mouth. It divides into sections again in order to innervate various areas of the tongue.
Damage to the hypoglossal nerve will weaken the side of the tongue that is parallel to the affected portion of the nerve, causing reduced movement of the tongue. For example, if the left branch of the cranial nerve is damaged, the left side of the tongue will be affected. Some lesions on the nerve are transient, which means that symptoms only last a given amount of time and then wear off. Some lesions, though, especially those located on or near the nucleus of the nerve, can cause more severe symptoms. Symptoms may include extreme weakness, paralysis, or fasciculation (cramps) on the affected side of the tongue. With time, that side of the tongue will atrophy, causing the tongue to drift towards that side during protrusion.