The Cranial Nerves

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The Cranial Nerves

• There are 12 pairs of cranial nerves and they are continuous with the brain and are numbered from anterior to posterior, according to the their attachment to the brain.

• Note: details on the cranial nerves inside the brain is not needed (this is done in neuroanatomy in Medicine II).

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• The cranial nerves provide motor (or efferent) and sensory (or afferent) innervation for the head, neck, thorax and abdomen. They are called cranial nerves because they emerge through foramina in the cranium and are covered with tubular sheaths formed from the cranial meninges.

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The Olfactory Nerve (CN I) (p. 853)

• Type: Special Sensory
• This is the nerve of smell. Its fibres are the unmyelinated axons of olfactory neurosensory cells that are located in the olfactory neuroepithelium covering the superior concha of the nasal cavity and the superior part of the nasal cavity.
• The nerve bundles that form the olfactory nerves pass through foramina in the cribriform plate of the ethmoid bone and enter the olfactory bulb in the anterior cranial fossa.

Optic Nerve (CN II) (pp. 854, 857)

• This is the nerve of sight.

• Its fibres arise from ganglion cells in the neuroepithelium of the optic retina and converge towards the optic disc at the posterior pole of the eye.
• These fibres unite to form the large optic nerve that passes posteromedially through the posterior half of the orbit.
• It passes through the optic canal into the middle cranial fossa to joint its partner at the optic chiasma.
• The midregion of the optic chiasma is composed of crossed fibres from the medial or temporal half of each optic nerve.

• Within the optic canal, the ophthalmic artery lies inferolateral to the optic nerve. From the optic chiasma, the optic tracts continue dorsolaterally around the midbrain.
• Most fibres in the optic tracts terminate in the lateral geniculate bodies of the thalamus.

• CN II is enclosed by three sheaths that are continuous with the three layers of cranial meninges (dura, arachnoid, and pia). The central artery and vein of the retina pass through these meningeal sheaths and are included in the distal part of the optic nerve.

The Oculomotor Nerve (CN III) (p. 859)

• This nerve moves the eye.
• It is somatic motor to striated ocular muscles and visceromotor to smooth muscles in the eye.
• The CN III is attached to the base of the midbrain near the median plane.

• The oculomotor nerve supplies somatic efferent fibres to all the extraocular muscles of the eye except the superior oblique and lateral rectus.
• It also supplies the levator palpebrae superioris muscle and, through its ciliary parasympathetic ganglion, it supplies general visceral efferent fibres to smooth muscles in the sphincter pupillae and ciliary muscles.

• CN III enters the orbit through the superior orbital fissure within the common tendinous ring. It divides into superior and inferior branches between the two heads of the lateral rectus muscle.
• The superior branch of CN III supplies the superior rectus and levator palpebrae superioris muscles.
• The inferior branch of CN III supplies the medial and inferior rectus muscles and terminates in the inferior oblique muscle.
• It also supplies a motor parasympathetic root to the ciliary ganglion. Postganglionic fibres pass in the short ciliary nerves to the sphincter pupillae and ciliaris muscles.

The Trochlear Nerve (CN IV) (p. 861)

• This is the smallest cranial nerve. It supplies only one eye muscle, the superior oblique.

• The fibres of the right and left trochlear nerves decussate dorsal to the cerebral aqueduct before emerging from the midbrain.
• CN IV is the only cranial nerve to emerge dorsally from the brain stem, and it has the longest intracranial course of all the cranial nerves.
• CN IV emerges at the medial border of the superior cerebellar peduncle.
• It leaves the cranium through the superior orbital fissure, superior to the common tendinous ring.
• CN IV runs medially, superior to the levator palpebrae superioris muscle, and supplies the superior oblique muscle from its superior aspect.

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The Trigeminal Nerve (CN V) (pp. 861, 863, 865)

Click here to go to the innervation of the skin of the face.

• This is the motor nerve for the muscles of mastication and several small muscles and the principal general sensory nerve for the head.

• CN V is a triple nerve (L. trigemius, triplet) that consists of three large sensory nerves from the face: ophthalmic (CN V¹), maxillary (CN V²), and mandibular (CN V³).
• These nerves form the large sensory root of the trigeminal nerve. There is also a much smaller motor root of the trigeminal nerve. Together, these roots make CN V the largest cranial nerve.

• Fibres in the sensory root are mainly axons of neurons in the trigeminal ganglion (semilunar ganglion), which occupies a recess in the dura of the middle cranial fossa called the trigeminal cave.
• The peripheral processes of cells in this ganglion constitute the ophthalmic and maxillary nerves (CN V¹ and CN V²) and the sensory component of the mandibular nerve (CN V³).
• CN V is attached to the lateral part of the pons at the junction between the pons and the middle cerebral peduncle.
• Its two roots run anteriorly in the posterior cranial fossa, inferior to the tentorium cerebelli.

The Ophthalmic Nerve (CN V¹) (p. 863)

• This is the smallest of the three divisions of the trigeminal nerve.
• This nerve is purely sensory and supplies the superficial and deep parts of the superior region of the face, including the eyeball, lacrimal gland, conjunctiva, nasal mucosa, and the skin of the scalp, forehead, upper eyelid, and nose.
• It forms from the dendritic processes of the ganglion cells in the trigeminal ganglion.

Branches of the Ophthalmic Nerve

• CN V¹ divides into three branches: nasociliary, frontal and lacrimal nerves. These branches enter the orbit through the superior orbital fissure.

• The frontal nerve is the largest branch. It passes superior to the tendinous ring and travels anteriorly, just inferior to the roof of the orbit.
• It forms a small supratrochlear nerve and a larger supraorbital nerve.

• The nasociliary nerve, which is intermediate in size between the frontal and lacrimal nerves, enters the orbit through the central tendinous ring.
• After entering the orbit, the nasociliary nerve curves towards the medial wall and gives off several branches; the long ciliary nerves, the ganglionic branches to the ciliary ganglion, the ethmoid and infratrochlear nerves.

• The lacrimal nerve is the smallest of the main branches of the ophthalmic nerve, enters the orbit through the superior orbital fissure.
• It runs along the superior border of the lateral rectus muscles with the lacrimal artery and supplies the lacrimal gland and adjoining conjunctiva.
• It ends in the upper eyelid, where it joins filaments of the facial nerve.

The Maxillary Nerve (CN V²) (p. 863)

Click here for a diagram on the distribution of the maxillary nerve.

• This is the intermediate division of the trigeminal nerve and is purely sensory.
• It arises from the trigeminal ganglion and runs anteriorly in the inferior part of the cavernous sinus, inferior to the ophthalmic nerve.
• This nerve leaves the middle cranial fossa through the foramen rotundum.

Branches of the Maxillary Nerve

• The meningeal branches arise from this nerve which it is still in the middle cranial fossa. These nerves supply the dura.
• Ganglionic branches arise within the pterygopalatine fossa and enter the pterygopalatine ganglion as its sensory root.
• The zygomatic nerve supplies the skin of the temple and the prominence of the cheek; and brings parasympathetic fibres to the lacrimal gland.
• Superior alveolar nerves (posterior, middle, anterior); the nasal branches; greater and lesser palatine nerves.

The Mandibular Nerve (CN V³) (pp. 863, 865)

• This is a mixed nerve (sensory and motor) and contains all the motor fibres of the trigeminal nerve.
• This nerve descends to the foramen ovale and then passes through this opening in the middle cranial fossa.
• Just outside the foramen, the motor and sensory roots of the mandibular nerve unite and then CN V³ divides into anterior and posterior divisions.

Branches of the Mandibular Nerve

• From the nerve; nerve to medial pterygoid, tensor tympani and tensor veli palatini.
• From anterior trunk; buccal nerve, sensory, supplies cheek and mandibular buccal gingiva; and the motor branches to the muscles of mastication.
• From the posterior trunk; inferior alveolar nerve, lingual nerve (sensory and taste of anterior 2/3 of the tongue-taste fibres from chorda tympani) and auriculotemporal nerve (encircles the middle meningeal artery and with postsynaptic fibres of CN IX pass to the parotid gland).

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The Abducent Nerve (CN VI) (p. 865)

• The abducent nerve supplies the lateral rectus muscle that abducts the eye. CN VI supplies only this muscle.
• It leaves the ventral aspect of the brainstem between the caudal border of the pons and the superior end of the pyramid of the medulla.
• It travels through the cavernous sinus, inferior to the internal carotid artery.
• It leaves the skull through the medial end of the superior orbital fissure.

The Facial Nerve (CN VII) (p. 866)

• This nerve is composed of two distinct roots; the facial nerve proper (the motor root and larger trunk), and the nervus intermedius (the sensory root).

• The roots emerge from the brainstem at the caudal border of the pons, lateral to the superior end of the olive of the medulla.
• Both roots enter the internal acoustic meatus. The facial nerve runs laterally in this canal and exits the skull through the stylomastoid foramen. CN VII enters the parotid gland and breaks up into its five terminal motor branches.

• The motor roots of CN VII supplies the muscles of facial expression, muscles of the scalp and auricle, the buccinator, platysma, stapedius, and stylohyoid muscles, and the posterior belly of the digastric muscle.
• The sensory root of CN VII conveys taste fibres from the chorda tympani nerve, which comes from the anterior 2/3 of the tongue; and taste fibres from the soft palate through the palatine and greater petrosal nerves; and preganglionic parasympathetic (secretomotor) innervation of the submandibular, sublingual, and lacrimal glands of the nasal and palatine mucosae.

Branches of the Facial Nerve (p. 866)

• The greater petrosal nerve, a slender branch containing taste and parasympathetic fibres. A branch of the tympanic plexus joins it.
• It arises in the facial canal from the genu (bend) of the facial nerve and emerges from the anterior part of the petrous part of the temporal bone.
• It passes toward the foramen lacerum and enters the pterygoid canal. Here it joins the deep petrosal nerve to form the nerve of the pterygoid canal that passes through this canal and enters the pterygopalatine ganglion.
• From here, taste fibres pass to the palate, and postsynaptic parasympathetic fibres go to the lacrimal gland and the mucosa of the palate, nasopharynx, and nasal cavity.

• The nerve to the stapedius muscle arises within the facial canal.
• The chorda tympani nerve (parasympathetic and taste) arises from the facial nerve within the facial canal. Taste fibres are distributed to the anterior 2/3 of the tongue though the lingual nerve.
• Parasympathetic fibres, after synapsing in the submandibular ganglion, supply the submandibular and sublingual salivary glands.

• Below the stylomastoid foramen, the nerves to stylohyoid and digastric arise.

• Facial branches (motor) arise from CN VII within the parotid gland.
• There are five main groups: temporal, zygomatic, buccal, mandibular, and cervical nerves to the muscles of facial expression and the platysma.

Click here to go to the facial nerve under the innervation of the muscles of facial expression.

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The Vestibulocochlear Nerve (CN VIII) (pp. 866, 869)

• This nerve is composed of two parts: a vestibular part, called the vestibular nerve, is concerned with the maintenance of equilibrium, and a cochlear part, called the cochlear nerve, and is concerned with hearing.

• Both parts emerge from the brainstem separately in the groove between the pons and the medulla.

The Vestibular Nerve (p. 869)

• This nerve is the means whereby movements of the eyes and head are linked with the part of the membranous labyrinth that governs the appreciation of the body in space.
• It arises from the vestibular ganglion that is located at the bottom of the internal acoustic meatus.

The Cochlear Nerve (p. 869)

• This nerve arises from cells in the spiral ganglion (cochlear ganglion).

The Glossopharyngeal Nerve (CN IX) (p. 869)

• This cranial nerve supplies one muscle, the stylopharyngeus, the parasympathetic secretomotor fibres to the parotid gland, and it carries the sensory fibres from the pharynx, tonsil, and posterior part of the tongue.

• CN IX arises from as three or four rootlets and these emerge from the groove between the olive and the inferior cerebellar peduncle and are closely related to the rootlets of the vagus nerve.
• The CN IX rootlets merge and leave the skull through the jugular foramen.

Branches of the Glossopharyngeal Nerve (p. 869)

• Tympanic nerve --> tympanic plexus --> lesser petrosal nerve --> otic ganglion --> back to V³ through its auriculotemporal nerve (encircling middle meningeal artery) --> parotid gland (parasymapathetic fibres).

• There are branches (sensory) to the carotid sinus.
• The nerve to the stylopharyngeus is the only motor branch of the glossopharyngeal nerve.
• The pharyngeal branches form the pharyngeal plexus with branches of the vagus nerve.
• The lingual branches (sensory) supply the mucosa of the posterior third of the tongue with general sensory and special sensory (taste) fibres.

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The Vagus Nerve (CN X) (p. 871)

• This is a mixed nerve containing both motor and sensory fibres.
• This nerve has a more extensive course and distribution than any of the other cranial nerves. It traverses and supplies structures in the neck, thorax and abdomen.

• The CN X arises as eight to ten rootlets that emerge from the medulla in the groove between the olive and inferior cerebellar peduncle.
• These rootlets converge and pass through the jugular foramen. Just before it exits the cranium, CN X exhibits a swelling known as the rostal or superior vagal ganglion. It is joined to the cranial root of CN XI by one or two filaments.
• It is also joined to the inferior glossopharyngeal ganglion and to the sympathetic trunk by a filament from the superior cervical ganglion.

• Just after it leaves the jugular foramen, the vagus nerve exhibits another swelling called the caudal or inferior vagal ganglion. The hypoglossal nerve (CN XII) connects with it just superior to this ganglion.
• In the thorax, the vagus nerve joins the opposite vagus nerve to form the oesophageal plexus.

Branches of the Vagus Nerve (p. 871)

• The meningeal branches arise from the superior vagal ganglion and supply the dura in the posterior cranial fossa.
• Auricular branches (sensory) also arise from the superior vagal ganglion and pass to the ear, where they supply part of the auricle and the external acoustic meatus.
• The pharyngeal branch (motor) arises from the inferior vagal ganglion and pass to the pharynx. It supplies all the striated muscles of the pharynx and soft palate, except the stylopharyngeus (supplied by CN IX) and the tensor veli palantini (supplied by CN V³).
• The carotid branches (sensory) arise from the inferior vagal ganglion and pass to the walls of the carotid sinus. These fibres are auxiliary to those supplied by the glossopharyngeal nerve.
• The superior laryngeal nerve (sensory and motor) arises form the inferior vagal ganglion and descends in the neck. Here it divides into two branches: (1) the internal laryngeal nerve that pierces the thyrohyoid membrane and supplies the larynx superior to the vocal folds (cords) and (2) the external laryngeal nerve that is motor to the cricothyroid muscle of the larynx.
• The recurrent laryngeal nerve (sensory and motor) turns superiorly around the subclavian artery on the right side and around the arch of the aorta on the left side. This nerve is sensory to the larynx inferior to the vocal folds, and motor to the intrinsic muscles of the larynx.
• Cardiac branches (parasympathetic) leave the vagus nerve in the neck and pass to the cardiac plexus of nerves. Here, the fibres synapse and postsynaptic fibres pass to the heart and act to slow it and to constrict the coronary arteries.
• Pulmonary branches (parasympathetic) are given off in the thorax that contributes to the anterior and posterior pulmonary plexuses.
• Abdominal branches arise from the anterior and posterior vagal trunks and synapses with ganglion cells in the walls of the viscera.

The Accessory Nerve (CN XI) (p. 873)

• This is a motor nerve.
• This nerve has cranial and spinal roots. These roots are united for only a short distance.

• The spinal root of CN XI (spinal accessory nerve) arises from the lateral part of the grey matter of the cervical region of the spinal cord.
• The filaments arise from segments C6 to C1 and merge to form a trunk, which ascends through the foramen magnum. Here it joins briefly with the cranial root and exits then through the jugular foramen.

• The cranial root of CN XI arises from the nucleus ambiguus. It really belongs to the vagus nerve and gives all its fibres to it to supply the skeletal muscles in the pharynx and palate.

• This root emerges from the lateral part of the medulla as four or five rootlets. These rootlets merge and pass through the jugular foramen where they unite with the spinal root of CN XI. It is also connected with the superior vagal ganglion.

The Hypoglossal Nerve (CN XII) (p. 873)

• This is the motor nerve of the tongue.
• It supplies all the tongue muscles except the palatoglossus that is innervated by the vagus through the pharyngeal plexus.
• A series of 10 to 15 rootlets emerge between the olive and pyramid of the medulla.
• The hypoglossal nerve passes through the hypoglossal canal.

Branches of the Hypoglossal Nerve (pp. 873-875)

• There are meningeal, descending, thyrohyoid, and muscular branches. The branches of CN XII that occur before it reaches the tongue are all derived from C1 fibres. They join CN XII as it exits from the skull.
• The fibres of CN XII have no supply outside of the tongue.
• The superior root of ansa cervicalis (decendens hypoglossi) brings fibres from C1 to geniohyoid and thyrohyoid.
• The terminal lingual branches go to all intrinsic muscles of the tongue and all extrinsic muscles of the tongue (styloglossus, hyoglossus, and genioglossus), except palatoglossus.

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