Pharmacology notes

Apr 15th, 2015
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The lowest four cranialnervesHYPOGLOSSAL NERVEThe hypoglossal nerve (cranial nerve XII) contains somaticefferent fibers for the supply of the extrinsic and intrinsicmuscles of the tongue. Its nucleus lies close to the midline inthe floor of the fourth ventricle and extends almost the fulllength of the medulla (Figure 15.1). The nerve emerges as aseries of rootlets in the interval between the pyramid and theolive. It crosses the subarachnoid space and leaves the skullthrough the hypoglossal canal. Just below the skull, it liesclose to the vagus and spinal accessory nerves (Figure 15.2). Itdescends on the carotid sheath to the level of the angle of themandible, then passes forward on the surface of the hyoglossus muscle where it gives off its terminal branches.In the neck, proprioceptive fibers enter the nerve from thecervical plexus, to accept afferents from about 100 musclespindles in the same half of the tongue.Phylogenetic noteIn reptiles, the lingual muscles, the geniohyoid muscle, andthe infrahyoid muscles develop together from the uppermostmesodermal somites. The somatic efferent neurons supplying this hypobranchial muscle sheet form a continuous ribbonof cells extending from lower medulla to the third cervicalspinal segment. In mammals, the hypoglossal nucleus islocated more rostrally and its rootlets emerge separatelyfrom the cervical rootlets. However, the caudal limit of thehypoglossal nucleus remains linked to the cervical motor cellcolumn

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