Chapter 17. Cranial Nerves

Big Picture

Cranial nerves (CNN) emerge through openings in the skull and are covered by tubular sheaths of connective tissue derived from the cranial meninges. There are 12 pairs of cranial nerves, numbered I to XII, from rostral to caudal, according to their attachment to the brain. The names of the cranial nerves reflect their general distribution and function. Like spinal nerves, cranial nerves are bundles of sensory and motor neurons that conduct impulses from sensory receptors and innervate muscles or glands.

Rapid Review of the Nervous System

To best understand the cranial nerves, it is helpful to remember the following information:

• Neuron versus nerve. A neuron is a single sensory or motor nerve cell, whereas a nerve is a bundle of neuronal fibers (axons). Cranial nerves have three types of sensory and three types of motor neurons, known as modalities. Therefore, a nerve may be composed of a combination of sensory or motor neurons (e.g., the facial nerve possesses sensory and motor neurons).
• Ganglion. A ganglion is a collection of nerve cell bodies in the peripheral nervous system.
• Nucleus. A nucleus is a collection of nerve cell bodies in the central nervous system (CNS).

Cranial Nerve Modalities

The 12 pairs of cranial nerves may possess one or a combination of the following sensory and motor modalities (Figure 17-1; Table 17-1):

• Sensory (afferent) neurons. Conduct information from the body tissues to the CNS.
  • General sensory (general somatic afferent). Transmit sensory information (e.g., touch, pain, and temperature), conducted mainly by CN V but also by CNN VII, IX, and X.
  • Special sensory (special visceral afferent). Include special sensory neurons (e.g., smell, vision, taste, hearing, and equilibrium), mainly conducted by the olfactory, optic, and vestibulocochlear nerves (CNN I, II, and VIII, respectively) as well as by CN VII and CN X.
  • Visceral sensory (general visceral afferent). Convey sensory information from the viscera, including the gastrointestinal tract, trachea, bronchi, lungs, and heart, as well as the carotid body and sinus. Visceral sensory neurons course within CN IX and CN X.
• Motor (efferent) neurons. Conduct information from the CNS to body tissues.
  • Somatic motor (general somatic efferent) neurons. Innervate skeletal muscles derived from somites, including the extraocular and tongue muscles. Innervation is accomplished via the oculomotor, trochlear, abducens, and hypoglossal nerves (CNN III, IV, VI, and XII, respectively).
  • Branchial motor (special visceral efferent) neurons. Innervate skeletal muscles derived from the branchial arches, including the muscles of mastication and facial expression and the palatal, pharyngeal, laryngeal, trapezius and sternocleidomastoid muscles. Innervation is accomplished via the trigeminal, facial, glossopharyngeal, vagus, and spinal accessory nerves (CNN V, VII, IX, X, and XI, respectively).
  • Visceral motor (general visceral efferent) neurons. Innervate involuntary (smooth) muscles or glands, including visceral motor neurons that constitute the cranial outflow of the parasympathetic division of the autonomic nervous system. The preganglionic neurons originate in the brainstem and synapse outside the brain in parasympathetic ganglia. The postganglionic neurons innervate smooth muscles and glands via CNN III, VII, IX, and X.

The nuclei of the cranial nerves (where motor neurons originate or sensory neurons terminate) are located in the brainstem, with the exception of CN I and CN II, which are extensions of the forebrain.

Cranial Nerve Targets

The specific functions of cranial nerves depend on the nature of the anatomic targets of the cranial nerves (Table 17-2).

• CNN V, VII, IX, and X. Innervate almost all of the structures of the head and neck, such as the skin, mucous membranes, muscle, and glands derived from the pharyngeal arches. Of these four nerves, CN V and CN VII innervate most of these structures, whereas CN IX innervates only a few structures of the head, oral cavity, pharynx, and neck. Almost all targets of CN X are in the trunk.
• CNN III, IV, and VI. Innervate only structures in the orbit.
• CNN I, II, and VIII. Possess only special sensory neurons for smell, sight, balance, and hearing.
• CN XII. Innervates only the tongue muscles.
• CNN III, VII, IX, and X. The cranial nerves that carry parasympathetic neurons.

Big Picture

The olfactory nerves contain only special sensory neurons concerned with smell (Figure 17-2A).

Pathways

The olfactory neurons originate in the olfactory epithelium in the superior part of the lateral and septal walls of the nasal cavity. The nerves ascend through the cribriform foramina of the ethmoid bone to reach the olfactory bulbs. The olfactory neurons synapse with neurons in the bulbs, which course to the primary and association areas of the cerebral cortex.

Big Picture

The optic nerve contains only special sensory neurons concerned with vision (Figure 17-2B).

Pathways

Optic nerve fibers arise from the retina and all converge at the optic disc. CN II exits the orbit via the optic canals. Both optic nerves form the optic chiasm, the site where neurons from the nasal side of either retina cross over to the contralateral side of the brain. The neurons then pass via the optic tracts to the thalamus, where they synapse with neurons that course to the primary visual cortex of the occipital lobe.

• Visual fields. The visual field is the part of the world seen by the eyes. The entire visual field is divided into right and left and upper and lower regions, defined when the patient is looking straight ahead. Each eye has its own visual field or, in other words, the part of the world seen by each eye alone. The lateral visual field of an eye is called the temporal field, whereas the medial visual field of the same eye is called the nasal field. Although the fields of vision of the two eyes overlap greatly, the right eye sees things far to the right that the left eye cannot see, and vice versa.
• Optic chiasm. Each optic nerve carries axons from the entire retina of the eye. However, after coursing through their respective optic canals, the right and left optic nerves engage in a redistribution of axons at the optic chiasma, located just anterior to the pituitary stalk. The optic chiasma is created by neurons from the nasal half of each retina crossing over to the opposite side.
• Optic tracts. The two optic tracts emerge from the optic chiasma. The right optic tract contains axons from the temporal half of the right retina and the nasal half of the left retina and carries information about the entire left visual field. The left optic tract contains neurons from the temporal half of the left retina and the nasal half of the right retina and carries information about the entire right visual field. An optic tract is named according to the side of the body on which it lies, but it is concerned with the contralateral visual field.

Big Picture

The oculomotor nerve innervates the levator palpebrae superioris muscle, four of the six extraocular muscles as well as the pupillary constrictor and ciliary muscles.

Pathways

Upon exiting the midbrain, the oculomotor nerve courses between the posterior cerebral and superior cerebellar arteries and runs along the lateral wall of the cavernous sinus superior to CN IV (Figure 17-3A). CN III enters the orbit through the superior orbital fissure, where it divides into superior and inferior divisions. CN III has the following modalities:

• Somatic motor neurons. CN III innervates four of the extraocular muscles (superior rectus, medial rectus, inferior rectus, and inferior oblique) and the levator palpebrae superioris muscle. The somatic motor component of CN III plays a major role in controlling the muscles responsible for the precise movements of the eyes.
• Visceral motor neurons. Preganglionic parasympathetic neurons of CN III originate in the Edinger–Westphal nucleus and synapse in the ciliary ganglion, providing innervation to the ciliary body (lens accommodation) and sphincter pupillae muscle (pupil constriction) (Figure 17-3B).

Big Picture

The trochlear nerve innervates the superior oblique muscle.

Pathways

The trochlear nerve is the only cranial nerve that originates from the posterior aspect of the brainstem (Figure 17-3A). CN IV courses around the brainstem in the free edge of the tentorium cerebelli, through the lateral wall of the cavernous sinus, and enters the orbit via the superior orbital fissure. The trochlear nerve supplies somatic motor innervation to the superior oblique muscle, which causes the eyeball to move down and out.

Big Picture

The abducens nerve innervates the lateral rectus muscle.

Pathways

The abducens nerve originates from the pons and courses through the cavernous sinus, entering the orbit via the superior orbital fissure (Figure 17-3A). The abducens nerve supplies somatic motor innervation to the lateral rectus muscle, which abducts the eye.

Big Picture

The trigeminal nerve is the principal general sensory supply to the head (Figure 17-4A).

Pathways

CN V originates from the lateral surface of the pons as a large sensory root and a smaller motor root. These roots enter the trigeminal (Meckel's) cave of the dura, lateral to the body of the sphenoid bone and the cavernous sinus. The sensory root leads to the trigeminal (semilunar) ganglion, which houses the cell bodies for the general sensory neurons. The motor root runs parallel to the sensory root, bypassing the ganglion and becoming part of the mandibular nerve (CN V-3). As well as being the primary general sensory distribution to the head, CN V also aids in distributing postganglionic parasympathetic neurons of the head to their destinations for CNN III, VII, and IX. The trigeminal ganglion gives rise to three divisions, named for the cranial location to the eyes (ophthalmic), the maxilla (maxillary), and the mandible (mandibular).

CN V-1: Ophthalmic Division

CN V-1 courses along the lateral wall of the cavernous sinus and enters the orbit via the superior orbital fissure (Figure 17-4A and B). CN V-1 provides general sensory innervation to the orbit, cornea, and the skin of the bridge of the nose, scalp and forehead (above the lateral corners of the eye).

CN V-2: Maxillary Division

CN V-2 passes through the lateral wall of the cavernous sinus and through the foramen rotundum into the pterygopalatine fossa (Figure 17-4A and B). This nerve provides general sensory innervation to the maxillary face (between the lateral corners of the eye and the corners of the mouth), including the palate, nasal cavity, paranasal sinuses, and the maxillary teeth.

CN V-3: Mandibular Division

CN V-3 courses through the foramen ovale into the infratemporal fossa (Figure 17-4A–C). CN V-3 provides general sensory innervation to the lower part of the face (below the lateral corners of the mouth), including the anterior two-thirds of the tongue, the mandibular teeth, the mandibular face, and even part of the scalp. CN V-3 also has branchial motor neurons, which innervate muscles derived from the first branchial arch which include the muscles of mastication (temporalis, masseter, and medial and lateral pterygoid muscles), mylohyoid, anterior belly of the digastricus, tensor tympani, and tensor veli palatine muscles.

Big Picture

The facial nerve provides motor innervation to the muscles of facial expression, lacrimal gland, and submandibular and sublingual salivary glands, as well as taste to the anterior two-thirds of the tongue.

CN VII Modalities

The facial nerve traverses the internal acoustic meatus carrying four modalities (Figure 17-5A).

• Branchial motor neurons. Supply muscles derived from the second branchial arch including the muscles of facial expression as well as the stapedius, posterior belly of the digastricus, and stylohyoid muscles.
• Visceral motor neurons. Provide parasympathetic innervation to almost all glands of the head (e.g., lacrimal, submandibular, sublingual, nasal, and palatal). The only exception is the parotid gland, which receives its visceral motor innervation from CN IX.
• Special sensory neurons. Transmit taste sensation from the anterior two-thirds of the tongue.
• General sensory neurons. Transmit general sensation from a portion of the external acoustic meatus and auricle.

CN VII Branches

Two distinct fascial sheaths package the four modalities carried by CN VII, with branchial motor neurons in one sheath and visceral motor, special sensory, and general sensory neurons in another sheath called the nervus intermedius.

Branchial Motor Nerve Trunk

The branchial motor components constitute the largest portion of CN VII. After entering the temporal bone via the internal acoustic meatus, a small branch of CN VII courses to the stapedius muscle, where branchial motor neurons course through the facial canal to exit the skull via the stylomastoid foramen. In the parotid gland, five terminal branches (i.e., temporal, zygomatic, buccal, mandibular, and cervical) provide voluntary control of the muscles of facial expression, including the buccinator, occipitalis, platysma, posterior digastricus, and stylohyoid muscles.

Nervus Intermedius

The nervus intermedius gives rise to the following nerves:

• Greater petrosal nerve. Contains preganglionic parasympathetic neurons that synapse in the pterygopalatine ganglion en route to the lacrimal, nasal, and palatine glands.
• Chorda tympani. Arises in the descending part of the facial canal and crosses the medial aspect of the tympanic membrane, passing between the malleus and incus. The chorda tympani exits the skull through the petrotympanic fissure and joins the lingual nerve from CN V-3 in the infratemporal fossa. The chorda tympani contains preganglionic parasympathetic neurons that synapse in the submandibular ganglion en route to innervate the submandibular and the sublingual salivary glands. The chorda tympani also contains special sensory neurons (taste) from the anterior two-thirds of the tongue, with cell bodies located in the geniculate ganglion.
• Auricular branches. Arise from the external acoustic meatus and auricle and carry general sensory neurons through the geniculate ganglion to the brainstem. The geniculate ganglion is a knee-shaped bend in CN VII, located within the temporal bone and housing sensory cell bodies for the special sensory neurons for taste and general sensory neurons from the ear.

Big Picture

The vestibulocochlear nerve provides sensory innervation for hearing and equilibrium.

Pathways

The vestibulocochlear nerve traverses the internal acoustic meatus with CN VII and has the following modalities (Figure 17-5B):

• Special sensory neurons. CN VIII originates from the grooves between the pons and the medulla oblongata. CN VIII divides into the cochlear branch to the cochlea (hearing) and the vestibular branch to the semicircular canal (equilibrium).

Big Picture

The glossopharyngeal nerve provides motor innervation to the stylopharyngeus muscle and parotid gland and sensory innervation from the carotid body and sinus, posterior one-third of the tongue, and the auditory tube.

Brainstem Origin

The glossopharyngeal nerve emerges from the lateral aspect of the medulla oblongata and traverses the jugular foramen, where its superior and inferior sensory ganglia are located (Figure 17-6).

Modalities

CN IX consists of the five modalities described as follows.

• Branchial motor neurons. Course in a pharyngeal branch to innervate the stylopharyngeus muscle the only skeletal muscle derived from the third branchial arch. CN IX then courses between the superior and middle pharyngeal constrictor muscles en route to the oropharynx and the tongue.
• Visceral motor neurons. Parasympathetic neurons from CN IX innervate the parotid gland via the following pathway:
  • Preganglionic parasympathetic neurons originate in the inferior salivatory nucleus and enter the petrous part of the temporal bone via the tympanic canaliculus to the tympanic cavity.
  • Within the tympanic cavity, the tympanic nerve forms the tympanic plexus on the promontory of the middle ear to provide general sensation to the tympanic membrane and nearby structures. The visceral motor preganglionic parasympathetic neurons course through the plexus and merge to become the lesser petrosal nerve.
  • The lesser petrosal nerve reenters and travels through the temporal bone to emerge in the middle cranial fossa, lateral to the greater petrosal nerve. The lesser petrosal nerve then proceeds anteriorly to exit the skull via the foramen ovale with CN V-3.
  • Upon exiting the skull, preganglionic parasympathetic neurons within the lesser petrosal nerve synapse in the otic ganglion. Postganglionic parasympathetic neurons exit the otic ganglion and travel with the auriculotemporal branch of CN V-3 to innervate the parotid gland.
• General sensory neurons. Course within the tympanic plexus, providing innervation to the internal surface of the tympanic membrane, the middle ear, and the auditory tube. General sensory neurons also course in the pharyngeal branch from the posterior third of the tongue and the oropharynx.
• Special sensory neurons. Course from the posterior third of the tongue within the pharyngeal branch of CN IX, providing taste sensation to the brain.
• Visceral sensory neurons. Course from the carotid sinus and carotid body and ascend in the sinus nerve, joining CN IX. The cell bodies of the visceral sensory neurons reside in the inferior ganglion.

Big Picture

The vagus nerve innervates muscles of the larynx, pharynx, palate in addition to the gut tube, heart and lungs.

Brainstem Origin

The vagus nerve emerges from the lateral aspect of the medulla oblongata and traverses the jugular foramen, where the superior and inferior sensory ganglia are located.

CN X Modalities

The vagus nerve exits the medulla oblongata and travels with CNN IX and XI into the jugular foramen. CN X consists primarily of the following four modalities:

• Visceral sensory neurons. Provide visceral sensory information from the larynx (below the vocal folds), trachea, and esophagus, and the thoracic and abdominal viscera as well as the stretch receptors of the aortic arch and the chemoreceptors of the aortic and carotid bodies (Figure 17-7A and B).
• General sensory neurons. Provide general sensory information from part of the external acoustic meatus, the pinna, and the laryngopharynx (Figure 17-7A).
• Branchial motor neurons. Supply the palatoglossus, laryngeal, and pharyngeal muscles (except for the stylopharyngeus muscle, which is supplied by CN IX) and the palatal muscles (except for the tensor veli palatini, which is supplied by CN V-3) (Figure 17-7A).
• Visceral motor neurons. Provide parasympathetic innervation to the smooth muscle and the glands of the respiratory system and gastrointestinal tract to the transverse colon (Figure 17-7B). In general, CN X increases secretion from glands and smooth muscle contraction. CN X slows the heart rate, stimulates bronchiolar secretions, bronchoconstriction, and peristalsis, and increases secretions.

Upon traversing the jugular foramen, CN X travels between the internal jugular vein and the internal carotid artery within the carotid sheath.

Branches of CN X

The branchial motor fibers exit the vagus nerve as the following branches:

• Pharyngeal branch. The pharyngeal branch is the principal motor innervation of the pharyngeal muscles. It branches just below the inferior vagal ganglion and courses between the internal and external carotid arteries. The pharyngeal branch then enters the middle pharyngeal constrictor muscle, where it forms the pharyngeal plexus. The pharyngeal plexus innervates all of the pharyngeal muscles (i.e., superior, middle, and inferior pharyngeal constrictors; salpingopharyngeus, palatopharyngeus, palatoglossus, and levator veli palatini), with the exception of the stylopharyngeus muscle by CN IX and the tensor veli palatini by CN V-3.
• Superior laryngeal branch. The superior laryngeal nerve branches immediately below the pharyngeal nerve. The nerve descends in the neck adjacent to the pharynx and splits to form the following nerves:
  • External laryngeal nerve. Provides branchial motor innervation to part of the inferior pharyngeal constrictor muscle and the cricothyroideus muscle.
  • Internal laryngeal nerve. Pierces the thyrohyoid membrane and provides general sensory innervation of the larynx above the vocal folds.
• Recurrent laryngeal branch. The path of the recurrent laryngeal nerve differs on the right and left sides of the body.
  • Left recurrent laryngeal nerve. Branches from the vagus nerve at the level of the aortic arch. The nerve loops posteriorly around the aortic arch by the ligamentum arteriosus and ascends through the superior mediastinum to enter the groove between the esophagus and the trachea.
  • Right recurrent laryngeal nerve. Branches from the vagus nerve before entering the superior mediastinum at the level of the right subclavian artery. The nerve hooks posteriorly around the subclavian artery and also ascends in the groove between the esophagus and trachea.
  • Both recurrent laryngeal nerves pass deep to the lower margin of the inferior constrictor muscle to innervate the intrinsic laryngeal muscles and visceral sensory innervation below the vocal folds.

Big Picture

The spinal accessory nerve innervates the trapezius and sternocleidomastoid muscles.

Pathways

The spinal accessory nerve originates from the medulla oblongata, exits the jugular foramen, and provides branchial motor innervation to the trapezius and sternocleidomastoid muscles (Figure 17-8).

Big Picture

The hypoglossal nerve innervates the tongue muscles.

Pathways

The hypoglossal nerve exits the medulla oblongata in the groove between the pyramid and the olive (Figure 17-8). Upon exiting the hypoglossal canal, CN XII courses between the internal carotid artery and the internal jugular vein, deep to the posterior digastricus muscle. CN XII then courses along the lateral surface of the hyoglossus muscle deep to the mylohyoid muscle. CN XII provides somatic motor innervation to all intrinsic and extrinsic tongue muscles (with the exception of the palatoglossus, supplied by CN X).

Big Picture

All preganglionic sympathetic neurons destined for the head originate at the T1 level of the spinal cord and synapse in the superior cervical ganglion. Postganglionic sympathetic neurons course along cranial arteries to the end organs such as the sweat glands, the superior tarsal muscle, and the dilator pupillae muscle.

Preganglionic parasympathetic neurons originate in the brainstem, course in CNN III, VII, IX, or X, and synapse in one of four ganglia (i.e., ciliary, pterygopalatine, submandibular, and otic). Postganglionic parasympathetic neurons then course along nerves to their end organs (e.g., salivary glands and pupillary sphincter muscle).

Sympathetic Innervation

Remember that all preganglionic sympathetic neurons in the body originate at spinal cord levels T1–L2 (Figure 17-9). Therefore, all visceral motor sympathetic innervation to the head originates in the upper levels of the thoracic spinal cord, with most originating at the T1 level.

• Preganglionic sympathetic fibers. Originate from T1 of the spinal cord, ascend in the sympathetic trunk, and synapse with postganglionic fibers in the superior cervical ganglion.
• Postganglionic sympathetic fibers. Originate in the superior cervical ganglion and follow the arteries throughout the head to innervate the blood vessels, sweat glands, superior tarsal muscle (to elevate the upper eyelid), and the dilatator pupillae muscle (to dilate the pupil).

Parasympathetic Innervation

There are four parasympathetic ganglia associated with the cranial nerves (Figure 17-9; Table 17-3).

• Ciliary ganglion. Located posterior to the eyeball between the optic nerve and the lateral rectus muscle. Preganglionic parasympathetic neurons from the inferior division of CN III synapse in the ciliary ganglion and send postganglionic parasympathetic neurons to the sphincter pupillae and the ciliary muscles via the short ciliary nerves.
• Pterygopalatine ganglion. Located in the pterygopalatine fossa, just inferior to CN V-2 and lateral to the sphenopalatine foramen. Preganglionic parasympathetic neurons from CN VII synapse in the pterygopalatine ganglion and send postganglionic parasympathetic neurons to the lacrimal, nasal, and palatal glands.
• Submandibular ganglion. Suspended from the lingual nerve of CN V-3, deep to the mylohyoid muscle. Preganglionic parasympathetic neurons from CN VII synapse in the submandibular ganglion and send postganglionic parasympathetic neurons to supply the submandibular and sublingual salivary glands.
• Otic ganglion. Located in the infratemporal fossa inferior to the foramen ovale between CN V-3 and the tensor veli palatini muscle. Preganglionic parasympathetic neurons from CN IX synapse in the otic ganglion and send postganglionic neurons in the auriculotemporal nerve of CN V-3 to supply the parotid gland.

There are four cranial nerves that carry visceral motor parasympathetic innervation to the head: CNN III, VII, IX, and X.

• CN III (oculomotor nerve). Preganglionic parasympathetic neurons synapse in the ciliary ganglion, with postganglionic parasympathetic neurons serving the ciliary muscles and sphincter pupillae for light accommodation and constriction of the pupil.
• CN VII (facial nerve). Preganglionic neurons traveling in the greater petrosal nerve synapse in the pterygopalatine ganglion, with postganglionic parasympathetic neurons coursing to serve the lacrimal, nasal, and palatal glands. Preganglionic neurons from CN VII also travel within the chorda tympani nerve to synapse in the submandibular ganglion, with postganglionic neurons serving the submandibular and sublingual salivary glands.
• CN IX (glossopharyngeal nerve). Preganglionic parasympathetic neurons synapse in the otic ganglion, with postganglionic parasympathetic neurons serving the parotid gland.
• CN X (vagus nerve). Preganglionic parasympathetic neurons synapse at or near the target organ, with postganglionic parasympathetic neurons serving smooth muscle and glands of the gastrointestinal tract to the transverse colon.