The twelve pairs of cranial nerves which originate from the brain are remarkable for their functional diversity. They carry sensory information to the brain from the head and neck, as well as from VISCERA in the neck, thorax and abdomen and are especially significant because of the special senses of smell, vision, taste, hearing and balance. Mo tor neurons with axons in cranial nerves are responsible for the control of pupil diameter, tear formation, mastication, salivation, swallowing, movements of the facial musculature as well as cardiorespiratory and digestive functions. Collectively, cranial nerves have seven different functional sensory and motor components. These are: the general somatic afferent (GSA), general visceral afferent (GVA), general somatic efferent (GSE), and general visceral efferent (GVE) components that innervate somatic structures (skin and muscle) and viscera (internal organs and glands) and are analogous to the equivalent components found in spinal nerves. In addition, there are three special functional components unique to cranial nerves because of the special senses and structures associated with the branchial (gill) arches. These are the special somatic afferents (SSA) for vision, hearing and balance, the special visceral afferents (SVA) for olfaction and taste, and the special visceral efferents (SVE) for swallowing and phonation.
The following summary emphasizes the major functions and unique features of each cranial nerve. The OLFACTORY NERVE (I; olfaction; SVA) originates in bipolar sensory neurons located in the nasal mucosa of the nasal cavity. Olfactory chemoreceptors are present on the cilia, the distal processes of the bipolar olfactory neurons. The olfactory nerves are made up of numerous bundles of short, unmyelinated axons of the BIPOLAR NEURONS that project centrally to the olfactory bulb. OLFACTION is a very complex sensation and accounts for much of what we experience as taste (see GUSTATION). The OPTIC NERVE (II; vision; SSA) originates in ganglion cells of the RETINA. Optic nerve axons cross entirely or partially depending on the species (50% crossing in humans) at the OPTIC CHIASM and project to the SUPRACHIASMATIC NUCLEUS, LATERAL GENICULATE NUCLEUS, pretectal nucleus and SUPERIOR COLLICULUS. The OCULOMOTOR NERVE (III; eye movements, raising the eyelid, pupillary light reflex, visual accommodation; GSE, GVE) originates in motoneurons of the oculomotor nuclear complex in the MESENCEPHALON and innervates four eye muscles. Preganglionic parasympathetic neurons of the EDINGER-WESTPHAL NUCLEUS also travel in the oculomotor nerve to synapse in the CILIARY GANGLION. The postganglionic axons innervate the pupillary constrictor muscle (part of the iris) and the ciliary muscle. The TROCHLEAR NERVE (IV; superior oblique muscle; GSE) originates from motor neurons of the contralateral trochlear nucleus of the mesencephalon. The trochlear nerve is unusual in that its motor axons decussate completely and it exits dorsally from the BRAINSTEM.
The TRIGEMINAL NERVE (V; general sensory for face and head, muscles of mastication, proprioception; GSA, GVE) has three major sensory branches (ophthalmic, maxillary, mandibular) supplying corresponding areas of the face. The central axons of the sensory neurons project to the principal and spinal trigeminal sensory nuclei of the hindbrain and upper cervical spinal cord. Proprioceptive axons for the muscles of mastication originate from the mesencephalic nucleus of the trigeminal nerve. The motor root of the trigeminal nerve originates in the motor trigeminal nucleus in the PONS and innervates the muscles of mastication. The ABDUCENS NERVE (VI; lateral rectus muscle; GSE) originates in motor neurons of the abducens nucleus in the pons. The FACIALNERVE (VII; muscles of facial expression, stapedius muscle, digastric muscle, stylohyoid muscle, SVE; taste to anterior two-thirds of the tongue, SVA; salivation, tears, GVE) leaves the intracranial region through the internal acoustic meatus to travel in the facial canal in the skull. Special visceral efferent motor neurons for the muscles of facial expression originate in the facial nucleus. Parasympathetic preganglionic neurons are found in the superior salivatory nucleus. The somata of sensory neurons, primarily for taste sensations from the anterior two-thirds of the tongue, are found in the geniculate ganglion of the facial canal. Their distal processes form the chorda tympani nerve. The VESTIBULOCOCHLEAR NERVE (VIII; balance, hearing; SSA) also leaves the skull via the internal acoustic meatus. The vestibular component carries sensory information from specialized receptors in the inner ear to the vestibular complex of the lower pons and upper medulla. The auditory component carries sensory information from hair cells in the cochlea to the cochlear nuclei of the pons. The GLOSSOPHARYNGEAL NERVE (IX; SWallowing, salivation, taste, upper airway sensation, carotid sinus and carotid body pressor and chemoreceptors, SVE, GVE, SVA, GVA, GSA) has a motor component projecting from the nucleus ambiguus to the stylopharyngeus muscle and sensory neurons that transmit baroreceptor and chemoreceptor information from the carotid artery, general sensory input from the oropharynx, and taste from the posterior one-third of the tongue to the nucleus of the tractus solitarius. The VAGUS NERVE (X; swallowing, respiration, secretomotor, cardiomotor, taste, SVE, GVE, GVA, SVA) is widely distributed in the neck, thorax and abdomen. SVE motor neurons innervate muscles of the pharynx, larynx and esophagus. General visceral efferent motor neurons supply secretomotor axons to the stomach, small intestine and pancreas. Cardiomotor neurons project to parasympathetic ganglia on the heart. The ACCESSORY NERVE (XI; swallowing, head and shoulder movements, SVE) has cranial and spinal roots that innervate pharyngeal muscles via branches of the vagus and the sternocleidomastoid and trapezius muscles via the spinal accessory nerve. The HYPOGLOSSAL NERVE (XII; tongue movements; GSE) supplies intrinsic and extrinsic tongue muscles.
DAVID A.HOPKINS
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