Hypoglossal Nerve

The hypoglossal nerve (cranial nerve XII) contains somatic efferent fibers for the supply of the extrinsic and intrinsic muscles of the tongue, except palatoglossus (supplied by the cranial accessory nerve). Its nucleus lies close to the midline in the floor of the fourth ventricle and extends almost the full length of the medulla (Figure 18.1). The nerve emerges as a series of rootlets in the interval between the pyramid and the olive. It crosses the subarachnoid space and leaves the skull through the hypoglossal canal. Just below the skull, it lies close to the vagus and spinal accessory nerves (Figure 18.2). It descends on the carotid sheath to the level of the angle of the mandible, then passes forward on the surface of the hyoglossus muscle where it gives off its terminal branches.

Figure 18.1 (A) Sensory nuclei (left) and motor nuclei (right) serving cranial nerves IX–XII. (B) The special visceral efferent cell column giving a contribution to the glossopharyngeal nerve and forming the cranial accessory nerve. (C) The general visceral efferent cell column contributing to the glossopharyngeal and vagus nerves. (D) The somatic efferent cell column giving rise to the hypoglossal nerve.

Figure 18.2 Semischematic illustration of the lowest four cranial nerves and the internal carotid branch of the superior cervical ganglion.

Afferent impulses from about 100 muscle spindles in the same side of the tongue travel from the hypoglossal to the lingual nerve and are then relayed to the mesencephalic nucleus of the trigeminal nerve.