EUTIMIA (BIENESTAR)
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CRANIAL NERVE XII HYPOGLOSSAL The rootlets of the hypoglossal nerve emerge from the anterior surface of the medulla in the ventrolateral sulcus between the py...ramid and the olive. They converge to form two groups of rootlets that pierce the dura and then converge to form the hypoglossal nerve, which exits the cranium through the hypoglossal (anterior condylar) foramen in the posterior cranial fossa. After exiting the skull, the nerve courses medial to CNs IX, X, and XI. It passes laterally and downward close to the posterior surface of the inferior ganglion of the vagus nerve to lie between the internal carotid artery and the internal jugular vein and deep to the posterior belly of the digastric muscle. Crossing lateral to the bifurcation of the common carotid artery, the nerve loops anteriorly above the greater cornu of the hyoid bone. After passing deep to the intermediate tendon of the digastric, stylohyoid, and free posterior border of the mylohyoid muscles, the hypoglossal nerve runs on the lateral surface of the hyoglossus muscle. It then passes forward on the lateral surface of the genioglossus muscle, dividing to supply its target muscles in the tongue. The hypoglossal nerve supplies all but one of the extrinsic muscles of the tongue, the exception being the palatoglossus muscle, which is supplied by CN X, and all the intrinsic tongue muscles. The extrinsic muscles act to pull the tongue forward and protrude it (genioglossus), elevate and retract the tongue (styloglossus), depress the tongue (hyoglossus), as well as move it from side to side. The intrinsic muscles act to change the shape of the tongue. The tongue has two very important functions. The phylogenetically old function is concerned with eating and initiating swallowing by the tongue pushing the bolus of food from the mouth into the oropharynx. These actions occur in response to sensory signals from the mouth carried in CN V. Gustatory (taste) and tactile (touch) signals carried in CNs V, VII, and IX pass from the mouth through the nucleus of the tractus solitarius, the trigeminal nucleus, and the reticular formation to act on the hypoglossal nucleus resulting in reflex activities such as swallowing, sucking, and chewing. The intricate and complex movements of the tongue in speech constitute the phylogenetically new function of the tongue. Cranial Nerves - Function and Dysfunctions, 3E (2010), Pauwels, Stewart, Akesson, Spacey See more