Cranial Nerves

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Cranial Nerves


the 12 pairs of nerves that branch off from the anterior (lower) surface of the brain stem in succession from front to back through special openings in the skull. The cranial nerves innervate the organs and tissues of the head and neck, with the exception of the vagus nerve, which descends into the thoracic and abdominal cavities. Cyclostomes, fish, and amphibians normally have only the first ten pairs of cranial nerves.

The first and second pairs of the cranial nerves are the olfactory and optic nerves, which, unlike the other cranial nerves, originate in the brain and serve as conducting pathways for the olfactory and visual analyzers. The third, fourth, and sixth pairs are the oculomotor, trochlear, and abducens nerves; they develop in association with cephalic myotomes and innervate the muscles of the eye. The fifth pair, the trigeminal nerves, is associated with the region of the mandibular arch; the nerves are the chief sensory nerves of the face and serve as the motor nerves of the muscles of mastication. The seventh pair, the facial nerves, innervates the organs of the lateral line and the musculature of the hyoid arch in fish, the superficial musculature of the neck and the muscle that lowers the lower jaw in terrestrial vertebrates, and the facial muscles in humans and simians. The facial nerves also contain secretory fibers to the lacrimal and salivary glands and sensory fibers to the mucous membrane of the tongue. The eighth pair, the vestibulocochlear nerves, evolved from the facial nerves. These nerves are purely sensory: They are responsible for linking the organs of hearing and equilibrium with the brain. The related ninth, tenth, and 11th pairs—the glossopharyngeal, vagus, and accessory nerves—are unequal in fiber composition and extent of spread. The ninth and tenth pairs have motor, sensory, and autonomic components. In mammals the 11th pair arises from the vagus nerve. The accessory nerve is motor to the sternocleidomastoid and trapezius muscles. The 12th pair, the hypoglossal nerves, is unusual in that it arose in amniotes as a result of fusion of the muscular branches of the spinal nerves.

The cranial nerves, unlike the spinal nerves, are not segmented and are highly specialized. Every muscle or group of functionally combined muscles (for example, the masseter muscle) and every organ is supplied only by a single source. Hence, disturbance of nerve conduction, as in paralysis of the facial nerve, cannot be compensated by adjacent nerves. The cranial nerves are nerves of highly specialized sense organs and are regulators of respiration, blood circulation, and digestion.