The Hypoglossal is the only Cranial Nerve that is responsible for motor innervation only.. no sensory responsibilities at all. It innervates all intrinsic and extrinsic muscles of the tongue, except for the palatoglossus (which is innervated by…you guessed it.. the Vagus-we will get to that diva next episode). It takes a really twisty route from the medulla, through the hypoglossal canal in the occipital bone, down through the neck and up and over the tongue to all of the muscles it directs there. (Because of this route the hypoglossal is at risk for traumatic damage during carotid endarterectomy surgery.) The hypoglossal allows us voluntary control over tongue movements-sticking out our tongue, moving it side to side (or if you’re really fancy, twisting it over or doing that crazy fold over). It’s relatively easy to assess clinically, asking the patient to stick out their tongue-damage will result in the tongue deviating to one side (the weaker side).
An upper motor neuron injury will cause weakness to the opposite side, a lower motor neuron damage to the same side as the lesion. Assessing for equality of strength from side to side, bulk of the tongue, and fasciculations as well as appearance at rest can also give you information about the healthiness of the hypoglossal. Patients with damage to the hypoglossal will show impairment to the physiology of the tongue, so will have difficulty with bolus manipulation and formation, as well as moving the bolus back into the pharynx. They may also demonstrate impaired clearance of the bolus through the pharynx as the movement of the tongue contributes to the pressure generated for this to happen.
See the links below for more on this relatively straightforward and super important to swallowing cranial nerve.