Cranial Nerve XII


The hypoglossal nerve is classified as a general somatic efferent (GSE) nerve because it innervates striated muscle that arises embryologically from the somites. [©i , y The nucleus of CN XII is located in the medulla, ventral to the floor of the fourth ventricle. The hypoglossal triangle or trigonum hypoglossi, in the floor of the fourth ventricle, serves as an external landmark for the rostral portion of the hypoglossal nucleus. y As with some of the other pure motor cranial nerves, the nucleus is situated medially in the brain stem, which is a result of embryological anatomical relationships. The hypoglossal nucleus, formed by a column of large cells, is approximately 1.8 cm in length. It transverses much of the length of the medulla. M , y Numerous fibers cross the midline of the medulla to connect the paired hypoglossal nuclei. y Several discrete smaller nuclear groups surround the hypoglossal nucleus in the medulla, including the nucleus intercalatus, nucleus prepositus, and the nucleus of Roller. M


Upon exiting the hypoglossal nucleus, the hypoglossal nerve travels ventrolaterally, passing through the medullary reticular formation, just lateral to the lateral margin of the medial longitudinal fasciculus and the medial lemniscus, and through the medial segments of the inferior olivary nucleus. y The nerve exits the ventral aspect of the medulla in the pre-olivary or ventrolateral sulcus, between the medullary pyramid and the inferior olive, and medial to the ninth, tenth, and eleventh cranial nerves.[6 , y When the nerve exits the medulla, it is comprised of 10 to 15 separate rootlets on each side. y These rootlets coalesce to form two larger bundles, and these perforate the dura mater separately, uniting after their passage through it. y

The hypoglossal nerve exits the posterior fossa through the hypoglossal canal. y It descends in the neck close to the angle of the mandible. It passes beneath the internal carotid artery and the internal jugular vein, then travels forward between these two blood vessels and crosses the external carotid artery. y The branches of the hypoglossal nerve are the meningeal (sending branches to the dura mater in the posterior cranial fossa), descending (sending branches to the omohyoid, sternohyoid, and sternothyroid muscles), thyrohyoid (sending branches to the thyrohyoid muscle), and muscular (sending branches to the extrinsic and intrinsic muscles of the tongue) (.Fig 14-2 ).y , y The extrinsic muscles of the tongue include the genioglossus, styloglossus, hyoglossus, and chondroglossus muscles, whereas the intrinsic muscles of the tongue include the superior and inferior longitudinales, transversus, and verticalis muscles. y The emphasis in the standard neurological examination, with regard to hypoglossal nerve function, is on the function of the extrinsic muscles of the tongue. y , y


The supranuclear input for the hypoglossal nerve is better understood than that for the spinal accessory nerve. As with the latter, the supranuclear input is derived from the lowest portion of the precentral gyrus. y The corticobulbar afferents, which travel by way of the genu of the internal capsule and the middle of the cerebral peduncle, are predominantly contralateral, although some are also ipsilateral. y , y The supranuclear input for the genioglossus muscle may be strictly contralateral. y


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