The human cranial nerves are the twelve pair of nerves that are part of the peripheral nervous system. Although most of them exit the skull, extending throughout the upper part of the body, they arise from grey nuclei in the brainstem, especially in the pons and medulla, branching off through special opening 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. The cranial nerves, unlike the spinal nerves, are not segmented and are highly specialized to innervate different muscles and organs.
The twelve pairs of cranial nerves
The first and second pairs of the cranial nerves are the olfactory (CN I) and optic nerve (CN II), 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 (CN III), trochlear (CN IV), and abducent nerve (CN VI); they innervate the muscles of the eyeball. The fifth pair, the trigeminal nerve (CN V), is associated with the region of the mandibular arch; this pair of 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 nerve (CN VII), innervates 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 nerve (CN VIII), 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 eleventh pairs (the glossopharyngeal (CN IX), vagus (CN X), and accessory nerve CN XI) are unequal in fiber composition and extent of spread. Therefore, the glossopharyngeal and vagus nerve have motor, sensory, and autonomic components. Branches of the vagus nerve also form the cardiac plexus, together with sympathetic trunk nerve fibers. In mammals, the accessory nerve arises from the vagus nerve. The accessory nerve is a motor nerve (efferent) that innervates the sternocleidomastoideus and trapezius muscles. The twelfth pair, the hypoglossal nerve (CN XII) is also motor and supply the muscles of the tongue.
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| The cranial nerves, from CN III to CN XII, as the come of the anterior side of brainstrem. Only the first two pairs are missing: the olfactory and the optic nerve, because they arise from center in the cerebrum (brain). |
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