The Cranial Nerves

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.

Olfactory nerve (CN I or 1)

Bipolar cells in the nasal mucosa give rise to axons that enter the cranial cavity through foramina in the cribriform plate of the ethmoid bone. These cells and their axons, totaling about 20–24 in number, make up the olfactory nerve. Once in the cranial cavity, the fibers terminate in a small oval structure resting on the cribriform plate called the olfactory bulb. As stated above, the functional component of olfactory fibers is special visceral afferent. Injury or disease of the olfactory nerve may result in anosmia, an inability to detect odours; this may also dull the sense of taste.

Optic nerve (CN II or 2)

Rods and cones in the retina of the eye receive information from the visual fields and, through intermediary cells, convey this input to retinal ganglion cells. Ganglion cell axons converge at the optic disc, pass through the sclera, and form the optic nerve. The optic nerve from each eye enters the skull via the optic foramen and joins its opposite to form the optic chiasm. At the chiasm, fibers from the nasal halves of each retina cross, while those from the temporal halves remain uncrossed. In this way the {eyes} optic tracts, which extend from the chiasm to the thalamus, contain fibers conveying information from both eyes. Injury to one optic nerve therefore results in total blindness in that eye, while damage to the optic tract on one side results in partial blindness in both eyes.

Optic fibers also participate in accommodation of the lens and in the pupillary light reflex. Since the subarachnoid space around the brain is continuous with that around the optic nerve, increases in intracranial pressure can result in papilledema, or damage to the optic nerve, as it exits the bulb of the eye.

Oculomotor nerve (CN III or 3)

The oculomotor nerves arise from two nuclei in the rostral midbrain. These are (1) the oculomotor nucleus, the source of general somatic efferent fibers to superior, medial, and inferior recti muscles, to the inferior oblique muscle, and to the levator palpebrae superious muscle; and (2) the Edinger-Westphal nucleus, which projects general visceral efferent preganglionic fibers to the ciliary ganglion.

The oculomotor nerve exits the ventral midbrain, pierces the dura, courses through the lateral wall of the cavernous sinus, and exits the cranial cavity via the superior orbital fissure. Within the orbit it branches into a superior ramus (to the superior rectus and levator muscles) and an inferior ramus (to the medial and inferior rectus muscles, the inferior oblique muscles, and the ciliary ganglion). Postganglionic fibers from the ciliary ganglion innervate the sphincter pupillae muscle of the iris as well as the ciliary muscle.

With the exception of the levator palpebrae superioris muscle, which is innervated bilaterally, oculomotor neurons project primarily to orbital muscles on the same side of the head. A lesion of the oculomotor nerve will result in paralysis of the three rectus muscles and the inferior oblique muscle (causing the eye to rotate downward and slightly outward), paralysis of the levator palpebrae superious muscle (drooping of the eyelids), and paralysis of the sphincter pupillae and ciliary muscles (so that the iris will remain dilated and the lens will not accommodate).

The fourth cranial nerve is unique for three reasons. First, it is the only cranial nerve to exit the dorsal side of the brain stem. Second, fibers from the trochlear nucleus cross in the midbrain before they exit, so that trochlear neurons innervate the contralateral (opposite side) superior oblique muscle of the eye. Third, trochlear fibers have a long intracranial course before piercing the dura.

The trochlear nucleus is located in the caudal midbrain; the functional component of these cells is general somatic efferent. After exiting at the dorsal side of the midbrain, the trochlear nerve loops around the midbrain, pierces the dura, and passes through the lateral wall of the cavernous sinus. It then enters the orbit through the superior orbital fissure and innervates only the superior oblique muscle, which rotates the eye downward and slightly outward. Damage to the trochlear nerve will result in a loss of this eye movement and may produce double vision (diplopia).

The trigeminal nerve is the largest of the cranial nerves. It has both motor and sensory components, the sensory fibers being general somatic afferent and the motor fibers being special visceral efferent. Most of the cell bodies of sensory fibers are located in the trigeminal ganglion, which is attached to the pons by the trigeminal root. These convey pain and thermal sensations from the face, oral and nasal cavities, and parts of the dura and nasal sinuses, sensations of deep pressure, and information from sensory endings in muscles. Trigeminal motor fibers, projecting from nuclei in the pons, serve the muscles of mastication.

Lesions of the trigeminal nerve result in sensory losses over the face or in the oral cavity. Damage to motor fibers results in paralysis of the masticatory muscles; as a result, the jaw may hang open or deviate toward the uninjured side when opened. Trigeminal neuralgia, or tic douloureux, is an intense idiopathic pain originating mainly from areas supplied by sensory fibers of the maxillary and mandibular branches of this nerve.

The trigeminal ganglion gives rise to three large nerves, the ophthalmic, maxillary, and mandibular.

The ophthalmic nerve passes through the wall of the cavernous sinus and enters the orbit via the superior orbital fissure. Branches in the orbit are (1) the lacrimal nerve, serving the lacrimal gland, part of the upper eyelid, and the conjunctiva; (2) the nasociliary nerve, serving the mucosal lining of part of the nasal cavity, the tentorium cerebelli and some of the dura of the anterior cranial fossa, and skin on the dorsum and tip of the nose; and (3) the frontal nerve, serving the skin on the upper eyelid and the forehead and scalp above the eyes up to the vertex of the head.

The maxillary nerve courses through the cavernous sinus below the ophthalmic nerve and passes through the foramen rotundum into the orbital cavity. Branches of the maxillary nerve are (1) the meningeal branches, which serve the dura of the middle cranial fossa; (2) the alveolar nerves, serving the upper teeth and gingiva and the lining of the maxillary sinus; (3) the nasal and palatine nerves, which serve portions of the nasal cavity and the mucosa of the hard and soft palate; and (4) the infraorbital, zygomaticotemporal, and zygomaticofacial nerves, serving the upper lip, the lateral surfaces of the nose, the lower eyelid and conjunctiva, and skin on the cheek and the side of the head behind the eye.

The mandibular nerve exits the cranial cavity via the foramen ovale and serves (1) the meninges of middle and parts of the anterior cranial fossae (meningeal branches); (2) the temporomandibular joint, skin over part of the ear, and skin over the sides of the head above the ears (auriculotemporal nerve); (3) oral mucosa, the anterior two-thirds of the tongue, gingiva adjacent to the tongue, and the floor of the mouth (lingual nerve); and (4) the mandibular teeth (inferior alveolar nerve). Skin over the lateral and anterior surfaces of the mandible and the lower lip is served by cutaneous branches of the mandibular nerve.

Trigeminal motor fibers exit the cranial cavity via the foramen ovale along with the mandibular nerve. They serve the muscles of mastication (temporalis, masseter, medial and lateral pterygoid), three muscles involved in aspects of swallowing (anterior portions of the digastric muscle, the mylohyoid muscle, and the tensor veli palatini), and a muscle that has a damping effect on loud noises by stabilizing the tympanic membrane (tensor tympani).

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.

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.

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 {ears} 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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The ninth cranial nerve, which exits the skull through the jugular foramen, has both motor and sensory components. Cell bodies of motor neurons, located in the nucleus ambiguus in the medulla, project as special visceral efferent fibers to the stylopharyngeal muscle. The action of the stylopharyngeus is to elevate the pharynx, as in gagging or swallowing. In addition, the inferior salivatory nucleus of the medulla sends general visceral efferent fibers to the otic ganglion via the lesser petrosal branch of the ninth nerve; postganglionic otic fibers distribute to the parotid salivary gland.

Among the sensory components, special visceral afferent fibers convey taste sensation from the back third of the tongue via lingual branches of the nerve. General visceral afferent fibers from the pharynx, the back of the tongue, parts of the soft palate and eustachian tube, and the carotid body and carotid sinus have their cell bodies in the superior and inferior ganglia, which are situated, respectively, within the jugular foramen and just outside the cranium. Sensory fibers in the carotid branch detect increased blood pressure in the carotid sinus and send impulses into the medulla that ultimately produce a reduction in heart rate and arterial pressure; this is known as the carotid sinus reflex.

The vagus nerve has the most extensive distribution in the body of all the cranial nerves, innervating structures as diverse as the external surface of the eardrum and internal organs of the abdomen. The root of the nerve exits the cranial cavity via the jugular foramen. Within the foramen is the superior ganglion, containing cell bodies of general somatic afferent fibers, and just external to the foramen is the inferior ganglion, containing visceral afferent cells.

Pain and temperature sensations from the eardrum and external auditory canal, and pain fibers from the dura of the posterior cranial fossa, are conveyed on general somatic afferent fibers in the auricular and meningeal branches of the nerve. Taste buds on the root of the tongue and on the epiglottis contribute special visceral afferent fibers to the superior laryngeal branch. General visceral afferent fibers conveying sensation from the lower pharynx, larynx, trachea, esophagus, and organs of the thorax and abdomen to the left (splenic) flexure of the colon converge to form the posterior (or right) and anterior (or left) vagal nerves. Right and left vagal nerves are joined in the thorax by cardiac, pulmonary, and esophageal branches. In addition, general visceral afferent fibers from the larynx below the vocal folds join the vagus via the recurrent laryngeal nerves, while comparable input from the upper larynx and pharynx is relayed by the superior laryngeal nerves and by pharyngeal branches of the vagus. A vagal branch to the carotid body usually arises from the inferior ganglion.

Motor fibers of the vagus nerve include special visceral efferent fibers arising from the nucleus ambiguus of the medulla and innervating pharyngeal constrictor muscles and palatine muscles via pharyngeal branches of the vagus as well as the superior laryngeal nerve. All laryngeal musculature (excluding the cricothyroid but including the muscles of the vocal folds) are innervated by fibers arising in the nucleus ambiguus. Cells of the dorsal motor nucleus in the medulla distribute general visceral efferent fibers to plexuses or ganglia serving the pharynx, larynx, esophagus, and lungs. In addition, cardiac branches arise from plexuses in the lower neck and upper thorax, and, once in the abdomen, the vagus gives rise to gastric, celiac, hepatic, renal, intestinal, and splenic branches or plexuses.

Damage to one vagus nerve results in hoarseness and difficulty in swallowing and speaking. Injury to both nerves results in increased heart rate, paralysis of pharyngeal and laryngeal musculature, atonia of the esophagus and intestinal musculature, vomiting, and loss of visceral reflexes. Such a lesion is usually life-threatening, as paralysis of laryngeal muscles can result in asphyxiation.

The accessory nerve is formed by fibers from the medulla (known as the cranial root) and by fibers from cervical levels C1–C4 (known as the spinal root). The cranial root originates from the nucleus ambiguus and exits the medulla below the vagus. Its fibers join the vagus and distribute to some muscles of the pharynx and larynx via pharyngeal and recurrent laryngeal branches of that nerve. For this reason, the cranial part of the accessory nerve is, for all practical purposes, part of the vagus nerve.

fibers that arise from spinal levels exit the cord, coalesce and ascend as the spinal root of the accessory nerve, enter the cranial cavity through the foramen magnum, and then immediately leave through the jugular foramen. The accessory nerve then branches into the sternocleidomastoid muscle, which tilts the head toward one shoulder with an upward rotation of the face to the opposite side, and the trapezius muscle, which stabilizes and elevates (or shrugs) the shoulder.

The hypoglossal nerve innervates certain muscles that control movement of the {taste} tongue. From the hypoglossal nucleus in the medulla, general somatic efferent fibers exit the cranial cavity through the hypoglossal canal and enter the neck in close proximity to the accessory and vagus nerves and the internal carotid artery. The nerve then loops down and forward into the floor of the mouth and branches into the tongue musculature from underneath. Hypoglossal fibers end in intrinsic tongue muscles, which modify the shape of the tongue (as in rolling the edges), as well as in extrinsic muscles that are responsible for changing its position in the mouth.

A lesion of the hypoglossal nerve on one side of the head would result in paralysis of intrinsic and extrinsic musculature on the same side. The tongue would atrophy and, on attempted protrusion, would deviate toward the side of the lesion.