The Nervous System - Advanced Version / The Cranial Nerves

submitted by Dr. Gary Farr Last Updated June, 24, 2002

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Cranial nerves can be thought of as modified spinal nerves, since the “general” functional fiber types found in spinal nerves also are found in cranial nerves but are supplemented by “special” afferent or efferent fibers. fibers conveying olfaction (in cranial nerve I) and taste (in cranial nerves VII, IX, and X) are classified as special visceral afferent, while the designation of special somatic afferent is applied to fibers conveying vision (cranial nerve II) and equilibrium and hearing (cranial nerve VIII). Skeletal muscles that arise from the branchial (pharyngeal) arches are innervated by fibers of cranial nerves V, VII, IX, and X; these are classified as special visceral efferent fibers.

The 12 pairs of cranial nerves are commonly identified either by name or by Roman or Arabic numeral.

Abducens nerve (CN VI or 6)

From its nucleus in the caudal pons, the abducens nerve exits the brain stem at the pons-medulla junction, pierces the dura, passes through the cavernous sinus close to the internal carotid artery, and exits the cranial vault via the superior orbital fissure. In the orbit the abducens nerve innervates the lateral rectus muscle, which turns the eye outward. Damage to the abducens nerve results in a tendency for the eye to deviate medially, or “cross.” Double vision may result on attempted lateral gaze.

Facial nerve (CN VII or 7)

The facial nerve is composed of a large root that innervates facial muscles and a small root (known as the intermediate nerve) that contains sensory and autonomic fibers.

From the facial nucleus in the pons, facial motor fibers enter the internal auditory meatus, pass through the temporal bone, exit the skull via the stylomastoid foramen, and fan out over each side of the face forward of the ear. fibers of the facial nerve are special visceral efferent; they innervate small muscles of the external ear, the platysma, the stapedius, the occipitofrontalis, the stylohyoid posterior belly of the digastric, the buccinator, and the muscles of facial expression.

The intermediate nerve contains autonomic (parasympathetic) as well as general and special sensory fibers. Preganglionic autonomic fibers, classified as general visceral efferent, project from the superior salivatory nucleus in the pons. Exiting with the facial nerve, they pass to the pterygopalatine ganglion via the greater petrosal nerve (a branch of the facial nerve) and to the submandibular ganglion by way of the chorda tympani nerve (another branch of the facial nerve, which joins the lingual branch of the mandibular nerve). Postganglionic fibers from the pterygopalatine ganglion innervate nasal and palatine glands and the lacrimal gland, while those from the submandibular ganglion serve submandibular and sublingual salivary glands. Among the sensory components of the intermediate nerve, general somatic afferent fibers relay sensation from the caudal surface of the ear, while special visceral afferent fibers originate from taste buds in the anterior two-thirds of the tongue, course in the lingual branch of the mandibular nerve, and then join the facial nerve via the chorda tympani branch. Both somatic and visceral afferent fibers have cell bodies in the geniculate ganglion, which is located on the facial nerve as it passes through the facial canal in the temporal bone.

Injury to the facial nerve at the brain stem produces a paralysis of facial muscles known as {bells_palsy} Bell's palsy as well as a loss of taste sensation from the anterior two-thirds of the tongue. If damage occurs at the stylomastoid foramen, facial muscles will be paralyzed but taste will be intact.

Vestibulocochlear nerve (CN VIII or 8)

This cranial nerve has a vestibular part, which functions in balance, equilibrium, and orientation in three-dimensional space, and a cochlear part, which functions in hearing. The functional component of these fibers is special somatic afferent; they originate from receptors located in the temporal bone.

Vestibular receptors are located in the semicircular canals, which provide input on rotatory movements (angular acceleration), and in the utricle and saccule, which generate information on linear acceleration and the influence of gravitational pull. This information is relayed by the vestibular fibers, whose bipolar cell bodies are located in the vestibular (Scarpa's) ganglion. The central processes of these neurons exit the temporal bone via the internal acoustic meatus and enter the brain stem alongside the facial nerve.

Auditory receptors of the cochlear division are in the organ of Corti and follow the spiral shape (about 2.5 turns) of the cochlea. Air movement against the eardrum produces mechanical actions by the ossicles of the ear, which, in turn, cause movement of fluid in the spiral cochlea. This fluid movement is transduced by the organ of Corti into nerve impulses interpreted as auditory information. The bipolar cells of the spiral (Corti's) ganglion give rise to central processes that course with the vestibular nerve. At the brain stem, cochlear fibers separate from vestibular fibers to end in the dorsal and ventral cochlear nuclei.

Lesions of the vestibular root result in eye movement disorders (nystagmus), unsteady gait with a tendency to fall toward the side of the lesion, nausea, and vertigo. Damage to the cochlea or cochlear nerve results in complete deafness, ringing in the ear (tinnitus), or both.

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