Olfactory Nerve (I)

The cell of origin of the olfactory nerves serving the sense of smell is in the olfactory mucosa covering the superior nasal concha, the upper part of the vertical portion of the middle concha and the opposite part of the nasal septum. The axons, which are unmyelinated, originate as the central or deep processes of the olfactory neurones and collect in bundles that cross in various directions, forming a plexiform network in the mucosa. The bundles finally form approximately 20 branches that traverse the cribriform plate in lateral and medial groups and end in the glomeruli of the olfactory bulb. Each branch has a sheath consisting of dura mater and pia-arachnoid; the former continues into the nasal periosteum, and the latter into the connective tissue sheaths surrounding the nerve bundles (Figs 11.1, 11.2).

Fig. 11.1 Sensory innervation of the lateral wall of the nasal cavity, hard and soft palates and nasopharynx. Secretomotor fibres to mucous glands are distributed in branches from the pterygopalatine ganglion.

Fig. 11.2 Bundles of olfactory nerve fibres and nerves associated with the septum (left side).

Olfactory pathways subserving the sense of smell are described in Chapter 12.

Optic Nerve (II)

The optic nerve is the second cranial nerve (Figs 11.3, 11.4). It arises from the optic chiasma on the floor of the diencephalon and enters the orbit through the optic canal, accompanied by the ophthalmic artery. It changes shape, starting out flat at the chiasma and becoming rounded as it passes through the optic canal. In the orbit it passes forward, laterally and downward and pierces the sclera at the lamina cribrosa, slightly medial to the posterior pole. It has a somewhat tortuous course within the orbit to allow for movements of the eyeball. It is surrounded by extensions of the three layers of meninges.

Fig. 11.3 The common tendinous ring with its muscle origins superimposed, and the relative positions of the nerves entering the orbital cavity through the superior orbital fissure and optic canal. Note that the attachments of levator palpebrae superioris and superior oblique lie external to the common tendinous ring but are attached to it. The ophthalmic veins frequently pass through the ring. The recurrent meningeal artery, a branch of the ophthalmic artery, is often conducted from the orbit to the cranial cavity through its own foramen.

(Based mainly on the data of Whitnall, S.E., 1932. Anatomy of the Human Orbit, 2nd ed. Oxford University Press, London; and Koornneef, I., 1977. Spatial Aspects of Orbital Musculo-fibrous Tissue in Man. Swestsa Zeitlinger, Amsterdam. Provided by the late Gordon L. Ruskell, Department of Optometry and Visual Science, The City University, London.)

Fig. 11.4 Dissection of the left orbit, viewed from the front, to show the origins of the orbital muscles and the relative positions of the nerves of the orbit.

The optic nerve has important relationships with other orbital structures. As it leaves the optic canal, it lies superomedial to the ophthalmic artery and is separated from the lateral rectus by the oculomotor, nasociliary and abducens nerves and sometimes by the ophthalmic veins. The optic nerve is closely related to the origins of the four recti muscles, whereas more anteriorly, where the muscles diverge, it is separated from them by a substantial amount of orbital fat. Just beyond the optic canal, the ophthalmic artery and the nasociliary nerve cross the optic nerve to reach the medial wall of the orbit. The central artery of the retina enters the substance of the optic nerve about halfway along its length. Near the back of the eye, it becomes surrounded by long and short ciliary nerves and vessels.

Oculomotor Nerve (III)

The oculomotor nerve is the third cranial nerve (Figs 11.3, 11.4, 11.8). It is the main source of innervation to the extraocular muscles and also contains parasympathetic fibres that relay in the ciliary ganglion.

Fig. 11.8 Nerves of the left orbit: superior aspect.

The oculomotor nerve emerges at the midbrain, on the medial side of the crus of the cerebral peduncle. It passes along the lateral dural wall of the cavernous sinus, where it divides into superior and inferior divisions that run beneath the trochlear and ophthalmic nerves. The two divisions of the oculomotor nerve enter the orbit through the superior orbital fissure, within the common tendinous ring of the recti muscles, separated by the nasociliary branch of the ophthalmic nerve.

The superior division of the oculomotor nerve passes above the optic nerve to enter the inferior (ocular) surface of superior rectus. It supplies this muscle and gives off a branch that runs to supply levator palpebrae superioris. The inferior division of the oculomotor nerve divides into three branches: medial, central and lateral. The medial branch passes beneath the optic nerve to enter the lateral (ocular) surface of the medial rectus. The central branch runs downward and forward to enter the superior (ocular) surface of the inferior rectus. The lateral branch travels forward on the lateral side of inferior rectus to enter the orbital surface of the inferior oblique. The lateral branch also communicates with the ciliary ganglion to distribute parasympathetic fibres to sphincter pupillae and ciliaris.

CASE 3 Thyroid-Associated Ophthalmopathy (Graves’ Disease)

A 45-year-old woman is referred for evaluation of a mild action tremor. She has a history of a mixed seizure disorder in the first two decades of her life, completely controlled by sodium divalproate. She subsequently complains of double vision, and neuro-ophthalmological a examination shows impaired up-gaze of the right eye with resultant vertical diplopia. There is slight lid retraction. Thyroid function testing reveals an elevated thyroxine (T₄) level and reduced thyroid-stimulating hormone (TSH). Orbital magnetic resonance imaging (MRI) documents thickening of the extraocular muscles, particularly involving the medial and inferior recti of the right eye (Fig. 11.9).

Fig. 11.9 Thyrotoxicosis (Graves’ disease). A, Exophthalmos is evident clinically. B, MRI demonstrates thickening of the extraocular muscles in the orbits.

A diagnosis of thyroid-associated ophthalmopathy (Graves’ disease) is made on the basis of the clinical presentation, abnormalities on thyroid testing and MRI findings. Treatment of the hyperthyroidism with radioactive iodine and of the ophthalmopathy with prednisone results in resolution of the tremor and less lid retraction, although the patient continues to have slight limitation of up-gaze in the right eye.

Discussion: Thyroid-associated ophthalmopathy (Graves’ disease) is a complicated and multifaceted disorder thought to be of autoimmune origin, usually occurring in association with demonstrable hyperthyroidism. Plasma cells, lymphocytes and mast cells migrate into the orbital tissues and extraocular muscles, with deposition of hydrophilic glycosaminoglycans and collagen. In some cases, the neuro-ophthalmological features may precede the clinical and laboratory manifestations of hyperthyroidism.

Ciliary Ganglion

The ciliary ganglion is a parasympathetic ganglion concerned functionally with the motor innervation of certain intraocular muscles (Figs 11.5, 11.6). It is a small, flat, reddish grey swelling, 1 to 2 mm in diameter, connected to the nasociliary nerve and located near the apex of the orbit in loose fat approximately 1 cm in front of the medial end of the superior orbital fissure. It lies between the optic nerve and lateral rectus, usually lateral to the ophthalmic artery. Its neurones, which are multipolar, are larger than in typical autonomic ganglia; a very small number of more typical neurones are also present.

Fig. 11.5 Nerves of the left orbit and the ciliary ganglion: lateral aspect.

Fig. 11.6 Ciliary ganglion, with its roots and branches of distribution. Red, sympathetic fibres; heavy black, parasympathetic fibres; blue, sensory (cerebrospinal) fibres. Alternative pathways are given for the sympathetic fibres.

Its connections or roots enter or leave it posteriorly. Eight to 10 delicate filaments, termed the short ciliary nerves, emerge anteriorly from the ganglion arranged in two or three bundles, the lower being larger. They run forward sinuously with the ciliary arteries, above and below the optic nerve, and divide into 15 to 20 branches that pierce the sclera around the optic nerve and run in small grooves on the internal scleral surface. They convey parasympathetic, sympathetic and sensory fibres between the eyeball and the ciliary ganglion; only the parasympathetic fibres synapse in the ganglion.

The parasympathetic root, derived from the branch of the oculomotor nerve to the inferior oblique, consists of preganglionic fibres from the Edinger–Westphal nucleus, which relay in the ganglion. Postganglionic fibres travel in the short ciliary nerves to the sphincter pupillae and ciliaris. More than 95% of these fibres supply the ciliaris, which is a much larger muscle in volume.

The sympathetic root contains fibres from the plexus around the internal carotid artery within the cavernous sinus. These postganglionic fibres, derived from the superior cervical ganglion, form a fine branch that enters the orbit through the superior orbital fissure inside the common tendinous ring of recti muscles. The fibres either pass directly to the ganglion or join the nasociliary nerve and travel to the ganglion in its sensory root. Either way, they traverse the ganglion without synapsing to emerge into the short ciliary nerves. They are distributed to the blood vessels of the eyeball. Sympathetic fibres innervating dilator pupillae may sometimes travel via the short ciliary nerves (rather than the more usual route via the ophthalmic, nasociliary and long ciliary nerves).

The sensory fibres that pass through the ciliary ganglion are derived from the nasociliary nerve. They enter the short ciliary nerves and carry sensation from the cornea, the ciliary body and the iris.

CASE 2 Nutritional Amblyopia

A 58-year-old malnourished chronic alcoholic notes subacute loss of vision bilaterally. Examination demonstrates markedly reduced visual acuity in both eyes, with impairment of color vision and bilateral central scotomata on testing of the visual fields. His pupils respond sluggishly to direct light. The fundi do not appear unusual. There is no other neurological deficit.

Discussion: This man has nutritional amblyopia, sometimes referred to as tobacco-alcohol amblyopia. The clinical manifestations are due to bilateral involvement of the neural fibres constituting the macular projection system, resulting in a macular syndrome. The lesions predominate in the central portion of the optic nerves bilaterally, where the macular fibres are found, thus making it a disorder of the anterior conducting system (Fig. 11.7). Although tobacco use is sometimes implicated as a cause, this is in all likelihood a nutritional disorder due to deficiency of one or more B vitamins. It responds quickly to vitamin supplementation.

Fig. 11.7 Deficiency amblyopia. Myelin sheath stain of a cross-section through an optic nerve in a case of deficiency (nutritional, tobacco-alcohol) amblyopia demonstrates degeneration in the fibres derived from the macula (papillomacular bundle).

(From Victor, M., Mancall E.L., Dreyfus, P.M. Deficiency amblyopia in the alcoholic patient. Arch. Ophthalmol. 1960; 64, (1): 1–33. Copyright © 1960 American Medical Association. All rights reserved.)

Trochlear Nerve (IV)

The trochlear nerve is the fourth cranial nerve and is the only one that emerges from the dorsal surface of the brainstem (see Fig. 11.35). It passes from the midbrain onto the lateral surface of the crus of the cerebral peduncle and runs through the lateral dural wall of the cavernous sinus. It then crosses the oculomotor nerve and enters the orbit through the superior orbital fissure, above the common tendinous ring of the recti muscles and levator palpebrae superioris and medial to the frontal and lacrimal nerves. The trochlear nerve travels only a short distance to enter the superior (orbital) surface of superior oblique, which is its sole target (see Fig. 11.8).

CASE 4 Diabetic Third Nerve Palsy

A 48-year-old obese, hypertensive woman with a 7-year history of type 2 diabetes mellitus treated with an oral hypoglycemic agent suddenly develops left orbital pain and double vision. The double vision is sometimes vertical, sometimes horizontal. Upon awakening the next morning, she is unable to open her left eye. Evaluation later that same day shows nearly complete ptosis of the left lid; slight exodeviation of the left eye when attempting to look straight ahead, with complete ophthalmoplegia except for intact abduction, and slight anisocoria, the left pupil being 1 mm larger than the right in ambient room light with normal pupillary constriction to light and accommodation.

MRI of the brain demonstrates small, scattered areas of increased signal intensity on the T2-weighted and FLAIR images. Her fasting blood glucose level is 190 mg/dl. Magnetic resonance angiography of the intracranial vessels demonstrates mild irregularities in the distal carotid and middle cerebral arteries bilaterally, consistent with atherosclerotic disease.

Two months later the patient is entirely normal except for persistent mild left ptosis.

Discussion: Vasculopathic third nerve palsies are most commonly associated with diabetes mellitus and usually come under the rubric of diabetic ophthalmoplegia. Other conditions associated with microvascular disease, such as hypertension and dyslipidemia, may also be responsible. Most cases are thought to be the result of ischaemia affecting the central portion of the oculomotor nerve, sparing the peripherally located pupillary fibre bundle; nonetheless, a small proportion of patients exhibits minor pupillary involvement. In contrast, compressive third nerve palsies, such as those associated with an intracranial aneurysm, almost always have marked pupillary involvement and usually present with more pain. See Figure 11.10.

Fig. 11.10 Third cranial nerve palsy (diabetic ophthalmoplegia) evidenced by eyelid ptosis (A) and inability to direct the involved eye medially on request (B).

Trigeminal Nerve (V)

Innervation of the Face and Scalp

The sensory innervation of the face and scalp is primarily from the three divisions of the mandibular nerve, with smaller contributions from the cervical spinal nerves. The two muscles of mastication that relate to the face are innervated by the mandibular division of the trigeminal nerve.

Three large areas of the face can be mapped to indicate the peripheral nerve fields associated with the three divisions of the trigeminal nerve. The fields are not horizontal but curve upward (Fig. 11.11A), apparently because the facial skin moves upward with growth of the brain and skull. Embryologically, each division of the trigeminal nerve is associated with a developing facial process that gives rise to a specific area of the face in the adult. Thus the ophthalmic nerve is associated with the frontonasal process, the maxillary nerve with the maxillary process and the mandibular nerve with the mandibular process.

Fig. 11.11 A, Sensory nerves of the left side of the scalp, face and neck, and the branches of the facial nerve, which are distributed to the muscles of ‘facial expression.’ The pinna has been reflected forward. B, Cutaneous innervation of the face and neck, showing dermatomes.

Ophthalmic Nerve

The ophthalmic nerve (Fig. 11.11B), a division of the trigeminal nerve, travels through the orbit to supply targets primarily in the upper part of the face. It arises from the trigeminal ganglion in the middle cranial fossa and passes forward along the lateral dural wall of the cavernous sinus. It gives off three main branches—the lacrimal, frontal and nasociliary nerves—just before it reaches the superior orbital fissure. The cutaneous branches of the ophthalmic nerve supply the conjunctiva, skin over the forehead, upper eyelids and much of the external surface of the nose.

CASE 5 Herpes Zoster Ophthalmicus

A 32-year-old woman is referred for pain in the right side of the forehead. Pain in the right supraorbital region spreading into the right superior frontal region began several days before the initial evaluation and was unremitting. She has a prior history of migraine and is being treated for non-Hodgkin’s lymphoma, with good response to chemotherapy. Her only other complaint is mild photophobia in the right eye.

The initial neurological examination demonstrates slightly decreased pin appreciation in the territory of the ophthalmic division of the right trigeminal nerve but is otherwise normal. There is slight conjunctival injection. One day later she reports a rash, and evaluation shows several vesicles in the right supraorbital region, with intense right conjunctival injection. Visual acuity is decreased in the right eye. Ocular motility is normal.

She is diagnosed with herpes zoster ophthalmicus. Within several days, vesicles appear in the territory of the ophthalmic division of the right trigeminal nerve, along with keratitis, conjunctivitis and iritis of the right eye (Fig. 11.12).

Fig. 11.12 Herpes zoster ophthalmicus. The distribution of the ophthalmic division of the trigeminal nerve (V1) is sharply demarcated by the crusting herpetic lesions.

Treatment with acyclovir is instituted. Pain largely subsides after a week, and after several more weeks the vesicles begin to heal. Her vision returns to near normal.

Lacrimal Nerve

The lacrimal nerve enters the orbit through the superior orbital fissure, above the common tendinous ring of the recti muscles and lateral to the frontal and trochlear nerves (see Figs 11.4, 11.5, 11.8). It passes forward along the lateral wall of the orbit on the superior border of the lateral rectus and travels through the lacrimal gland and the orbital septum to supply conjunctiva and skin covering the lateral part of the upper eyelid. The lacrimal nerve communicates with the zygomatic branch of the maxillary nerve, so parasympathetic fibres associated with the pterygopalatine ganglion might be conveyed to the lacrimal gland.

Frontal Nerve

The frontal nerve is the largest branch of the ophthalmic nerve (see Figs 11.4, 11.5, 11.8). It enters the orbit through the superior orbital fissure, above the common tendinous ring of the recti muscles, and lies between the lacrimal nerve laterally and the trochlear nerve medially. It passes forward on the levator palpebrae superioris, toward the rim of the orbit; about halfway along this course it divides into the supraorbital and supratrochlear nerves.

The supraorbital nerve is the larger of the terminal branches of the frontal nerve. It continues forward along levator palpebrae superioris and leaves the orbit through the supraorbital notch or foramen to emerge onto the forehead. It ascends on the forehead with the supraorbital artery and divides into medial and lateral branches, which supply the skin of the scalp nearly as far back as the lambdoid suture. The supraorbital nerve supplies the mucous membrane that lines the frontal sinus, the skin and conjunctiva covering the upper eyelid and the skin over the forehead and scalp. The postganglionic sympathetic fibres that innervate the sweat glands of the supraorbital area probably travel in the supraorbital nerve, having entered the ophthalmic nerve through its communication with the abducens nerve within the cavernous sinus.

The supratrochlear nerve runs medially above the superior oblique pulley. It gives a descending branch to the infratrochlear nerve and ascends onto the forehead through the frontal notch to supply the skin and conjunctiva covering the upper eyelid and the skin over the forehead.

Nasociliary Nerve

The nasociliary nerve is intermediate in size between the frontal and lacrimal nerves and is more deeply placed in the orbit, which it enters through the common tendinous ring, lying between the two rami of the oculomotor nerve (see Figs 11.4, 11.5, 11.8). It crosses the optic nerve with the ophthalmic artery and runs obliquely below superior rectus and superior oblique to reach the medial orbital wall. Here, as the anterior ethmoidal nerve, it passes through the anterior ethmoidal foramen and canal and enters the cranial cavity. It runs forward in a groove on the upper surface of the cribriform plate beneath the dura mater and descends through a slit lateral to the crista galli into the nasal cavity, where it occupies a groove on the internal surface of the nasal bone and gives off two internal nasal branches. The medial internal nasal nerve supplies the anterior septal mucosa, and the lateral internal nasal nerve supplies the anterior part of the lateral nasal wall. The anterior ethmoidal nerve emerges, as the external nasal nerve, at the lower border of the nasal bone and descends under the transverse part of the nasalis to supply the skin of the nasal alae, apex and vestibule.

The nasociliary nerve has connections with the ciliary ganglion and has long ciliary, infratrochlear and posterior ethmoidal branches.

The ramus communicans to the ciliary ganglion usually branches from the nerve as it enters the orbit lateral to the optic nerve. It is sometimes joined by a filament from the internal carotid sympathetic plexus or from the superior ramus of the oculomotor nerve as it enters the posterosuperior angle of the ganglion.

Two or three long ciliary nerves branch from the nasociliary nerve as it crosses the optic nerve (see Fig. 11.5). They accompany the short ciliary nerves and pierce the sclera near the attachment of the optic nerve. Running forward between the sclera and choroid, they supply the ciliary body, iris and cornea and are thought to contain postganglionic sympathetic fibres for the dilator pupillae from neurones in the superior cervical ganglion. An alternative pathway for the supply of the dilator pupillae is via the sympathetic root associated with the ciliary ganglion.

The posterior ethmoidal nerve leaves the orbit by the posterior ethmoidal foramen and supplies the ethmoidal and sphenoidal sinuses.

Maxillary Nerve

The maxillary nerve is a sensory division of the trigeminal nerve. Most of the branches from the maxillary nerve arise in the pterygopalatine fossa. It gives rise to the zygomatic and infraorbital nerves that pass into the orbit through the inferior orbital fissure and two others that pass through the pterygopalatine ganglion without synapsing and are distributed to the nose, palate and pharynx. The maxillary nerve passes through the orbit to supply the skin of the lower eyelid, the prominence of the cheek, the alar part of the nose, part of the temple and the upper lip.

CASE 6 Trigeminal Neuralgia

A 63-year-old woman, previously well, complains of severe intermittent, paroxysmal, lancinating right facial pain of 8 months’ duration. The pain generally begins just anterior to the ear, radiating in lightning-like fashion to the lower jaw and occasionally to the upper jaw as well. Attacks of pain may be precipitated by speaking, chewing or swallowing or by minor cutaneous stimulation. These attacks last up to 15 minutes and recur at unpredictable intervals. Facial grimacing or tearing may accompany these episodes, but no other neurological changes have been observed, and the neurological examination is normal.

Discussion: This woman describes typical trigeminal neuralgia (tic douloureux). The cause is generally unknown; in younger patients, multiple sclerosis may be implicated. Degenerative changes in the Gasserian ganglion, compression of the ganglion by tumour and vascular compression of the trigeminal nerve have been described in individual patients. Pain classically radiates along the peripheral distribution of the branches of the trigeminal nerve, most commonly the mandibular and maxillary divisions. Typically, examination demonstrates no abnormality. Management, whether pharmacological or surgical, is often unsatisfactory.

Zygomatic Nerve

The zygomatic nerve is located close to the base of the lateral wall of the orbit. It soon divides into two branches—the zygomaticotemporal and zygomaticofacial nerves—which run for only a short distance in the orbit before passing onto the face through the lateral wall of the orbit. Either these two branches enter separate canals within the zygomatic bone or the zygomatic nerve itself enters the bone before dividing.

The zygomaticotemporal nerve exits the zygomatic bone on its medial surface and pierces the temporal fascia to supply the skin over the temple. It also gives a branch to the lacrimal nerve, which may carry parasympathetic fibres to the lacrimal gland (Figs 11.5, 11.13).

Fig. 11.13 A and B, Left ophthalmic, maxillary and mandibular nerves and the submandibular and pterygopalatine ganglia.

The zygomaticofacial nerve leaves the zygomatic bone on its lateral surface to supply skin overlying the prominence of the cheek.

Infraorbital Nerve

The infraorbital nerve emerges onto the face at the infraorbital foramen (see Fig. 11.5), where it lies between the levator labii superioris and levator anguli oris. It divides into three additional groups of branches. The palpebral branches ascend deep to orbicularis oculi and pierce the muscle to supply the skin of the lower eyelid and join with the facial and zygomaticofacial nerves near the lateral canthus. Nasal branches supply the skin of the side of the nose and movable part of the nasal septum and join the external nasal branch of the anterior ethmoidal nerve. Superior labial branches, large and numerous, descend behind levator labii superioris to supply the skin of the anterior part of the cheek and upper lip. They are joined by branches from the facial nerve to form the infraorbital plexus.

Orbital Branches of Pterygopalatine Ganglion

Several rami orbitales arise dorsally from the pterygopalatine ganglion and enter the orbit through the inferior orbital fissure. Branches leave the orbit through the posterior ethmoidal air sinus. There is strong experimental evidence from studies in animals, including monkeys, that postganglionic parasympathetic branches pass directly to the lacrimal gland, ophthalmic artery and choroid.

Mandibular Nerve

The mandibular nerve is the largest trigeminal division and is a mixed nerve (Figs 11.11, 11.13, 11.14). Its sensory branches supply the teeth and gums of the mandible; the skin in the temporal region; part of the auricle, including the external meatus and tympanic membrane, and the lower lip; the lower part of the face (see Fig. 11.11); and the mucosa of the anterior two-thirds (presulcal part) of the tongue and floor of the oral cavity. The motor branches innervate the muscles of mastication. The large sensory root emerges from the lateral part of the trigeminal ganglion and exits the cranial cavity through the foramen ovale. The small motor root passes under the ganglion and through the foramen ovale to unite with the sensory root just outside the skull. As it descends from the foramen ovale, the nerve is approximately 4 cm from the surface and a little anterior to the neck of the mandible. The mandibular nerve immediately passes between tensor veli palatini, which is medial, and lateral pterygoid, which is lateral, and gives off a meningeal branch and nerve to the medial pterygoid from its medial side. The nerve then divides into small anterior and large posterior trunks. The anterior division gives off branches to the four main muscles of mastication and a buccal branch that is sensory to the cheek. The posterior division gives off three main sensory branches—the auriculotemporal, lingual and inferior alveolar nerves—and motor fibres to supply the mylohyoid and anterior belly of the digastric (see Figs 11.13, 11.14).

Fig. 11.14 Right otic and pterygopalatine ganglia and their branches displayed from the medial side.

Meningeal branch

The meningeal branch (nervus spinosus) reenters the cranium through the foramen spinosum with the middle meningeal artery. It divides into anterior and posterior branches that accompany the main divisions of the middle meningeal artery and supply the dura mater in the middle cranial fossa and, to a lesser extent, in the anterior fossa and calvaria.

Nerve to medial pterygoid

The nerve to the medial pterygoid is a slender ramus that enters the deep aspect of the muscle. It supplies one or two filaments that pass through the otic ganglion without interruption to supply tensor tympani and tensor veli palatini (see Fig. 11.14).

Anterior Trunk

The anterior trunk of the mandibular nerve gives rise to the buccal nerve, which is sensory, and the masseteric, deep temporal and lateral pterygoid nerves, which are motor.

Buccal nerve

The buccal nerve (Fig. 11.15) passes between the two heads of the lateral pterygoid. It descends deep to the temporalis tendon, passes laterally in front of the masseter, and anastomoses with the buccal branches of the facial nerve. It carries the motor fibres to the lateral pterygoid, and these are given off as the buccal nerve passes through the muscle. It may also give off the anterior deep temporal nerve. The buccal nerve supplies sensation to the skin over the anterior part of the buccinator and buccal mucous membrane, together with the posterior part of the buccal gingivae adjacent to the second and third molar teeth.

Fig. 11.15 Dissection of the left pterygoid region, showing some of the branches of the mandibular nerve and maxillary artery. The temporalis and the coronoid process of the mandible have been reflected upward. The masseter has been removed (with the exception of a small inferior portion). The zygomatic arch has also been removed.

Nerve to masseter

The nerve to the masseter (see Fig. 11.15) passes laterally above the lateral pterygoid and on to the skull base, anterior to the temporomandibular joint and posterior to the temporalis tendon. It crosses the posterior part of the mandibular notch with the masseteric artery and ramifies on and enters the deep surface of the masseter. It also provides articular branches that supply the temporomandibular joint.

Deep temporal nerves

The deep temporal nerves usually consist of two branches, anterior and posterior, although there may be a middle branch. They pass above the lateral pterygoid to enter the deep surface of the temporalis. The anterior nerve frequently arises as a branch of the buccal nerve. The small posterior nerve sometimes arises in common with the nerve to the masseter.

Nerve to lateral pterygoid

The nerve to the lateral pterygoid enters the deep surface of the muscle. It may arise separately from the anterior division of the mandibular nerve or from the buccal nerve.

Posterior Trunk

The posterior trunk of the mandibular nerve is larger than the anterior and is mainly sensory, although it receives fibres from the motor root for the nerve to the mylohyoid. It divides into auriculotemporal, lingual and inferior alveolar (dental) nerves.

Auriculotemporal nerve

The auriculotemporal nerve usually has two roots that encircle the middle meningeal artery (see Figs 11.13, 11.14). It runs back under the lateral pterygoid on the surface of tensor veli palatini, passes between the sphenomandibular ligament and neck of the mandible and then runs laterally behind the temporomandibular joint related to the upper part of the parotid gland. Emerging from behind the joint, it ascends over the posterior root of the zygoma, posterior to the superficial temporal vessels, and divides into superficial temporal branches. It communicates with the facial nerve and otic ganglion. The rami to the facial nerve, usually two, pass anterolaterally behind the neck of the mandible to join the facial nerve at the posterior border of the masseter. Filaments from the otic ganglion join the roots of the auriculotemporal nerve close to their origin (Figs 11.14, 11.16). The cutaneous branches of the auriculotemporal nerve supply the tragus and part of the adjoining auricle of the ear and posterior part of the temple.

Fig. 11.16 Parasympathetic connections of the pterygopalatine, otic and submandibular ganglia. The parasympathetic fibres, both pre- and postganglionic, are shown as blue lines. The parasympathetic fibres in the palatine nerves are secretomotor to the nasal, palatine and pharyngeal glands.

Lingual nerve

The lingual nerve (see Figs 11.13–11.15) is sensory to the mucosa of the anterior two-thirds of the tongue, the floor of the mouth and the mandibular lingual gingivae. It arises from the posterior trunk of the mandibular nerve and at first runs beneath the lateral pterygoid and superficial to tensor veli palatini, where it is joined by the chorda tympani branch of the facial nerve and often by a branch of the inferior alveolar nerve. Emerging from under cover of the lateral pterygoid, the lingual nerve then runs downward and forward on the surface of the medial pterygoid and is thus carried progressively closer to the medial surface of the mandibular ramus. It becomes intimately related to the bone a few millimetres below and behind the junction of the vertical ramus and horizontal body of the mandible. Here it lies anterior to, and slightly deeper than, the inferior alveolar (dental) nerve. It next passes below the mandibular attachment of the superior pharyngeal constrictor and pterygomandibular raphe, closely applied to the periosteum of the medial surface of the mandible, until it lies opposite the posterior root of the third molar tooth, where it is covered only by the gingival mucoperiosteum. At this point it usually lies 2 to 3 mm below the alveolar crest and 0.6 mm from the bone; however, in approximately 5% of cases, it lies above the alveolar crest. It next passes medial to the mandibular origin of mylohyoid, and this carries it progressively away from the mandible and separates it from the alveolar bone covering the mesial root of the third molar tooth.

Inferior alveolar nerve

The inferior alveolar (dental) nerve descends behind the lateral pterygoid. At the lower border of the muscle the nerve passes between the sphenomandibular ligament and the mandibular ramus and enters the mandibular canal via the mandibular foramen. Below the lateral pterygoid it is accompanied by the inferior alveolar artery, a branch of the first part of the maxillary artery, which also enters the canal with associated veins. The mental nerve is the terminal branch of the inferior alveolar nerve. It enters the face through the mental foramen, where it is directed backward. It supplies the skin of the lower lip.

CASE 7 Mental Neuropathy

A 64-year-old woman with no recognized prior illness develops pain and numbness in a very restricted pattern in the left lower jaw. She has no known dental problems. On examination, pain appreciation is found to be decreased in a sharply limited zone involving the skin over the left mandible, below the lower lip and extending to but not beyond the midline. The examination is otherwise normal.

Neuroimaging demonstrates a lytic lesion in the left mandible, involving the mental foramen. The patient is found to have a mass in her left breast, which is proved on biopsy to be carcinomatous. No lesions are found elsewhere.

Discussion: This woman has a very focal neuropathy involving the left mental nerve, a branch of the inferior alveolar nerve, as it exits the left mental foramen. As in this patient, the majority of cases of so-called mental neuropathy are due to metastatic disease.

Abducens Nerve (VI)

The abducens nerve is the sixth cranial nerve, and it emerges from the brain-stem between the pons and the medulla oblongata (see Figs 11.5, 11.35). It is related to the cavernous sinus, but unlike the oculomotor, trochlear, ophthalmic and maxillary nerves, which merely invaginate the lateral dural wall, it passes through the sinus itself, lying lateral to the internal carotid artery (see Fig. 4.6). The abducens nerve enters the orbit through the superior orbital fissure, within the common tendinous ring of the recti muscles (see Fig. 11.3), at first below and then between the two divisions of the oculomotor nerve and lateral to the nasociliary nerve. It passes forward to enter the medial (ocular) surface of the lateral rectus, which is its sole target.

CASE 8 Sixth Cranial Nerve Palsy

A 61-year-old woman with diabetes and hypertension complains of double vision while looking to the left. On examination, she has incomplete paralysis of gaze past the midline bilaterally, worse in the left eye (Fig. 11.17). Upward gaze is also slightly impaired. She complains of headache and difficulty walking. Neuroimaging reveals a posterior fossa ependymoma with hydrocephalus. She undergoes a suboccipital craniectomy for resection of the ependymoma; after several months, her vision returns to normal.

Fig. 11.17 Abducens palsy in a case of obstructive hydrocephalus due to an ependymoma.

Discussion: Sixth cranial nerve (abducens) palsies may occur as a complication of hydrocephalus and may be present bilaterally or unilaterally. The precise pathophysiology is unknown but is thought to be related to the long intracranial course of the nerve. Other causes of isolated abducens nerve palsies include nerve infarction (microvascular abducens palsy), skull base tumour, otitis media, trauma or post viral infection. Myasthenia gravis may mimic an abducens palsy.

Facial Nerve (VII)

The facial nerve enters the temporal bone through the internal acoustic meatus accompanied by the vestibulocochlear nerve (Fig. 11.18; see also Fig. 10.20). At this point, the motor root, which supplies the muscles of the face, and the nervus intermedius, which contains sensory fibres concerned with the perception of taste and parasympathetic (secretomotor) fibres to various glands, are separate components. They merge within the meatus. At the end of the meatus, the facial nerve enters its own canal, the facial canal, which runs across the medial wall and down the posterior wall of the tympanic cavity to the stylomastoid foramen (Fig. 11.19). As the nerve enters the facial canal, there is a bend that contains the geniculate ganglion (Figs 11.18, 11.20). The branches that arise from the facial nerve within the temporal bone can be divided into those that come from the geniculate ganglion and those that arise within the facial canal.

Fig. 11.18 Facial nerve. A, Course of the facial nerve and its branches through the temporal bone; the vestibulocochlear nerve has been omitted. B, A plan of the intrapetrous section of the facial nerve, its branches and communications. The course of the taste fibres from the mucous membrane of the palate and from the anterior presulcal part of the tongue is represented by blue lines.

(A, Reprinted by permission from Hall-Craggs, E.C.B., 1986. Anatomy as a Basis for Clinical Medicine, 2nd ed. Baltimore, Urban and Schwarzenberg.)

Fig. 11.19 Oblique section through the left temporal bone, showing the medial wall of the middle ear. The cochlea and semicircular canals are in blue. Note the relationship of the first coil of the cochlea to the promontory and the closeness of the facial nerve canal and the lateral semicircular canal to the medial wall of the aditus.

Fig. 11.20 Left auditory apparatus, as if viewed through a semitransparent temporal bone. Compare with Figure 11.19. Note the bend (genu) in the facial nerve at the site of the geniculate ganglion.

The main branch from the geniculate ganglion is the greater (superficial) petrosal nerve. It is a branch of the nervus intermedius. The greater petrosal nerve passes anteriorly, receives a branch from the tympanic plexus and traverses a hiatus on the anterior surface of the petrous part of the temporal bone. It enters the middle cranial fossa and runs forward in a groove on the bone above the lesser petrosal nerve. It passes beneath the trigeminal ganglion to reach the foramen lacerum. There it is joined by the deep petrosal nerve from the internal carotid sympathetic plexus to become the nerve of the pterygoid canal (Vidian’s nerve). The greater petrosal nerve contains parasympathetic fibres destined for the pterygopalatine ganglion and taste fibres from the palate.

The nerve to the stapedius arises from the facial nerve in the facial nerve canal behind the pyramidal eminence of the posterior wall of the tympanic cavity. It passes forward through a small canal to reach the muscle.

The chorda tympani (Figs 11.14, 11.21) leaves the facial nerve approximately 6 mm above the stylomastoid foramen and runs anterosuperiorly in a canal to enter the tympanic cavity via the posterior canaliculus. It then curves anteriorly in the substance of the tympanic membrane between its mucous and fibrous layers (Fig. 11.22) and crosses medial to the upper part of the handle of the malleus to the anterior wall, where it enters the anterior canaliculus (Fig. 11.23). It exits the skull at the petrotympanic fissure. It contains parasympathetic fibres that supply the submandibular and sublingual salivary glands via the submandibular ganglion (see Fig. 11.16) and taste fibres from the anterior two-thirds of the tongue.

Fig. 11.21 Chorda tympani nerve crossing the tympanic membrane.

(Courtesy of Mr. Simon A. Hickey.)

Fig. 11.22 Oblique vertical section through the left temporal bone, showing the roof and lateral wall of the middle ear and the mastoid antrum.

Fig. 11.23 Lateral wall of the left tympanic cavity.

The geniculate ganglion also communicates with the lesser petrosal nerve.

The facial nerve emerges from the base of the skull at the stylomastoid foramen. At this point, the facial nerve lies approximately 9 mm from the posterior belly of the digastric muscle and 11 mm from the bony external acoustic meatus. It gains access to the face by passing through the substance of the parotid gland. Although mainly motor, there are some cutaneous fibres from the facial nerve that accompany the auricular branch of the vagus and probably innervate the skin on both auricular aspects, in the conchal depression and over its eminence.

Close to the stylomastoid foramen the facial nerve gives off the posterior auricular nerve, which supplies the occipital belly of occipitofrontalis and some of the auricular muscles, and the nerves to the posterior belly of the digastric and stylohyoid. The nerve then enters the parotid gland high up on the posteromedial surface and passes forward and downward behind the mandibular ramus. Within the substance of the gland, the facial nerve branches into the temporofacial and cervicofacial trunks, just behind (within about 5 mm of) the retromandibular vein. In approximately 90% of cases, the two trunks lie superficial to the vein, in intimate contact with it. Occasionally (temporofacial trunk, about 9% of cases; cervicofacial trunk, about 2%), the trunks pass beneath the retromandibular vein. The trunks branch farther to form a parotid plexus (pes anserinus), which exhibits variations in its branching pattern. Five main terminal branches arise from the plexus and diverge within the gland. They leave the parotid gland by its anteromedial surface, medial to its anterior margin, and supply the muscles of facial expression.

The temporal branches are generally multiple and pass across the zygomatic arch to the temple to supply intrinsic muscles on the lateral surface of the auricle and the anterior and superior auricular muscles. They join with the zygomaticotemporal branch of the maxillary nerve and the auriculotemporal branch of the mandibular nerve. The more anterior branches supply the frontal belly of occipitofrontalis, orbicularis oculi and corrugator and join the supraorbital and lacrimal branches of the ophthalmic nerve.

Zygomatic branches are generally multiple and cross the zygomatic bone to the lateral canthus of the eye, supplying orbicularis oculi, and join filaments of the lacrimal nerve and zygomaticofacial branch of the maxillary nerve. The branches may also help supply muscles associated with the buccal branch of the facial nerve.

The buccal branch has a variable origin and passes horizontally to a distribution below the orbit and around the mouth. It is usually single, but two branches occur in 15% of cases. The buccal branch has a close relationship to the parotid duct and usually lies below it. Superficial branches run deep to subcutaneous fat and the superficial musculo-aponeurotic system. Some branches pass deep to procerus and join the infratrochlear and external nasal nerves. Upper deep branches pass under the zygomaticus major and levator labii superioris, supply them and form an infraorbital plexus with the superior labial branches of the infraorbital nerve. They also supply levator anguli oris, zygomaticus minor, levator labii superioris alaequae nasi and the small nasal muscles. These branches are sometimes described as lower zygomatic branches. Lower deep branches supply the buccinator and orbicularis oris and join filaments of the buccal branch of the mandibular nerve.

The marginal mandibular branches, of which there are usually two, run forward toward the angle of the mandible under platysma, at first superficial to the upper part of the digastric triangle, then turning up and running forward across the body of the mandible to pass under depressor anguli oris. The branches supply risorius and the muscles of the lower lip and chin and join the mental nerve. The marginal mandibular branch has an important surgical relationship with the lower border of the mandible and may pass below the lower border with a reported incidence of 20% to 50%, the farthest distance being 1.2 cm.

The cervical branch issues from the lower part of the parotid gland and runs anteroinferiorly under platysma to the front of the neck, to supply platysma and communicate with the transverse cutaneous cervical nerve. In 20% of cases there are two branches.

The peripheral branches of the facial nerve just described are joined by anastomotic arcades between adjacent branches to form the parotid plexus of nerves, which shows considerable variation. In surgical terms, these anastomoses are important and presumably explain why accidental or essential division of a small branch often fails to result in the expected facial nerve weakness. Six distinctive anastomotic patterns were originally classified by Davis and colleagues (1956) and are illustrated in Figure 11.24. These observations have been confirmed by others, although some variation in the frequency has been reported.

Fig. 11.24 Patterns of branching of the facial nerve.

(Modified with permission from Berkovitz, B.K.B., Moxham, B.J., Head and Neck Anatomy. 2002. © 2002 Informa Healthcare and from Davis, R.A., Anson, B.J., Budinger, J.M., Kurth, I.E., 1956. Surgical anatomy of the facial nerve and parotid gland based upon a study of 350 cervicofacial halves. Surg. Gynecol. Obstet. 102, 385–412, with permission from the American College of Surgeons.)

Facial Nerve Lesions

Facial nerve paralysis may be due to an upper motor neurone lesion (when frontalis is partially spared due to bilateral innervation of the muscle of the upper part of the face) or a lower motor neurone lesion (when all branches may be involved). Bell’s palsy and acoustic neuromas can produce a complete lower motor neurone facial paralysis as a result of compression of the facial nerve trunk as it passes through the middle ear. More commonly, cheek lacerations or malignant parotid tumours result in weakness in part of the face, depending on which branch of the nerve is involved. Unfortunately, the presence of facial paralysis is not a reliable diagnostic sign of a malignant tumour. It is not uncommon for a facial nerve infiltrated by a malignant tumour to continue to function normally. However, when paralysis does accompany a parotid mass, it is certainly malignant.

CASE 9 Bell’s Palsy

A 36-year-old previously healthy woman presents with a 2-day history of drooling from the left corner of her mouth. Later in the day that the drooling began, she noted a tendency for food to become lodged between her left cheek and left lower gum while eating. She could not whistle or purse her lips. The next morning she awoke with left retro-auricular pain. When she attempted to brush her teeth, she noticed that the left side of her mouth did not move; when she blinked, her left eyelid did not fully close. When applying makeup, she observed that the left eyebrow was lower than the right. During breakfast she noted a disturbed sense of taste and mildly slurred speech.

Neurological evaluation the next day documents near complete paralysis of the left facial muscles, including frontalis, orbicularis oculi, orbicularis oris, platysma and buccinator. A Schirmer test reveals decreased left eye tearing. There is decreased perception of taste on the anterior two-thirds of the tongue on the involved side. Inspection of the left ear and external auditory canal demonstrates no vesicles suggestive of herpes zoster, and results of routine laboratory tests are normal. Serum Lyme titres are negative.

Discussion: This woman has typical Bell’s palsy, or idiopathic peripheral facial palsy (Fig. 11.25). The seventh cranial (facial) nerve has a complex anatomy and subserves multiple functions. Idiopathic facial palsy is most often the result of a lesion (probably inflammatory) within the confines of the fallopian canal. This often impacts the greater superficial petrosal nerve (decreased tearing), nerve to the stapedius (hyperacusis) and chorda tympani (dysgeusia, with impaired taste on the ipsilateral anterior two-thirds of the tongue), in addition to the main branch of the facial nerve (ipsilateral facial paralysis and subtle sensory disturbance in the region of the ipsilateral ear).

Fig. 11.25 Bell’s palsy. Weakness reflecting involvement of both the upper and lower divisions of the facial nerve is evident when the patient attempts to smile or elevate the eyebrow.

Vestibulocochlear Nerve (VIII)

The vestibulocochlear nerve emerges from the pontocerebellar angle (Fig. 11.35). It courses through the posterior cranial fossa to enter the petrous temporal bone via the internal acoustic meatus, where it divides into an anterior trunk, the cochlear nerve, and a posterior trunk, the vestibular nerve. Both contain the centrally directed axons of bipolar neurones, the cell bodies of which are situated close to their peripheral terminals, together with a smaller number of efferent fibres that arise from brain stem neurones and terminate on cochlear and vestibular sensory cells (Figs. 11.20, 11.29).

In audiological practice, it is important to distinguish between intratemporal and intracranial lesions. However, it is relevant to note that this surgical distinction does not correlate with the precise anatomical descriptions of peripheral and central portions of the auditory and vestibular systems. Clinically, the term ‘peripheral auditory lesion’ is used to describe lesions peripheral to the spiral ganglion, and the term ‘peripheral vestibular disturbance’ includes lesions of the vestibular ganglion and the entire vestibular nerve. Furthermore, the intratemporal portion of the vestibulocochlear nerve in humans consists of two histologically distinct portions: a central glial zone adjacent to the brain stem, and a peripheral or non-glial zone. In the glial zone, the axons are supported by central neuroglia, whereas in the non-glial zone, they are ensheathed by Schwann cells. The non-glial zone extends into the cerebellopontine angle medial to the internal acoustic meatus in more than 50% of human vestibulocochlear nerves.

Intratemporal Vestibular Nerve

The maculae and crests are innervated by dendrites of bipolar neurones in the vestibular (Scarpa’s) ganglion situated in the trunk of the nerve within the lateral end of the internal auditory meatus (Fig. 11.27).

Fig. 11.27 Portion of a human vestibular ganglion, showing neuronal perikarya, myelinated axons and small blood vessels (toluidine blue stained resin section).

(Courtesy of H. Felix, M. Gleeson and L.-G. Johnsson, ENT Department, University of Zurich and GKT School of Medicine, London.)

Fig. 11.26 Human vestibulocochlear nerve, in transverse section. On the left, the cochlear nerve (seen as a comma-shaped profile) abuts the inferior division of the vestibular nerve (right). The singular nerve is a separate fascicle between the superior and inferior divisions of the vestibular nerve.

(Courtesy of H. Felix, M. Gleeson and L.-G. Johnsson, ENT Department, University of Zurich and GKT School of Medicine, London.)

The peripheral processes of the ganglion cells are aggregated into definable nerves, each with a specific distribution (Fig. 11.28). The main nerve divides at and within the ganglion into superior and inferior divisions, which are connected by an isthmus. The superior division, the larger of the two, passes through the small holes in the superior vestibular area to supply the ampullary crests of the lateral and anterior semicircular canals via the lateral and anterior ampullary nerves, respectively. A secondary branch of the lateral ampullary nerve supplies the macula of the utricle; however, the greater part of the utricular macula is innervated by the utricular nerve, which is a separate branch of the superior division. Another branch of the superior division, Voit’s nerve, supplies part of the saccule.

Fig. 11.28 The membranous labyrinth (blue) projected onto the bony labyrinth. The arrows indicate the direction of pressure waves in the cochlea.

The inferior division of the vestibular nerve passes through small holes in the inferior vestibular area to supply the remainder of the saccule and the posterior ampullary crest via saccular and singular branches, respectively; the latter passes through the foramen singulare. Occasionally, a very small supplementary or accessory branch innervates the posterior crest; it is probably a vestigial remnant of the crista neglecta, an additional area of sensory epithelium found in some other mammals but seldom in humans.

Afferent and efferent cochlear fibres are also present in the inferior division of the vestibular nerve, but they leave at the anastomosis of Oort to join the main cochlear nerve (see review by Warr 1992). Another anastomosis, the vestibulofacial anastomosis, is situated more centrally between the facial and vestibular nerves and is the point at which fibres originating in the intermediate nerve pass from the vestibular nerve to the main trunk of the facial nerve.

There are approximately 20,000 fibres in the vestibular nerve, of which 12,000 travel in the superior division and 8000 in the inferior division. The distribution of fibre diameters is bimodal, with peaks at 4 and 6.5 µm. The smaller fibres go mainly to the Type II hair cells, and the larger fibres tend to supply the Type I hair cells. In addition to the afferents, efferent and autonomic fibres have been identified. Efferent fibres synapse exclusively with the afferent calyceal terminals around Type I cells and usually with the afferent boutons on Type II cells, although a few are in direct contact with the cell bodies of Type II cells. The autonomic fibres do not contact vestibular sensory cells but terminate beneath the sensory epithelia. Two distinct sympathetic components have been identified in the vestibular ganglion: a perivascular adrenergic system derived from the stellate ganglion, and a blood vessel–independent system derived from the superior cervical ganglion.

The cell bodies of the bipolar neurones that contribute to the vestibular nerve vary considerably in size: their circumferences range from 45 to 160 µm (Felix et al 1987). No topographically ordered distribution relating to size has been found. The cell bodies are notable for their abundant granular endoplasmic reticulum, which forms Nissl bodies in places, and their prominent Golgi complexes (see Fig. 11.27). They are covered by a thin layer of satellite cells and are often arranged in pairs, closely abutting each other so that only a thin layer of endoneurium separates the adjacent coverings of satellite cells. This arrangement has led to speculation that ganglion cells may affect one another directly by electrotonic spread (ephaptic transmission).

Intratemporal Cochlear Nerve

The cochlear nerve connects the organ of Corti to the cochlear and related nuclei of the brain stem. The cochlear nerve lies inferior to the facial nerve throughout the internal acoustic meatus. It becomes intimately associated with the superior and inferior divisions of the vestibular nerve, which are situated in the posterior compartment of the canal, and leaves the internal acoustic meatus in a common fascicle (Fig. 11.29).

Fig. 11.29 Horizontal section through the left temporal bone.

(Drawn from a section prepared at the Ferens Institute and lent by the late J. Kirk.)

There are approximately 30,000 to 40,000 nerve fibres in the human cochlear nerve (for review, see Nadol 1988). Their fibre diameter distribution is unimodal and ranges from 1 to 11 µm, with a peak at 4 to 5 µm. Functionally, the nerve contains both afferent and efferent somatic fibres, together with adrenergic postganglionic sympathetic fibres from the cervical sympathetic system.

Afferent Cochlear Innervation

The afferent fibres are myelinated axons with bipolar cell bodies that lie in the spiral ganglion in the modiolus (Fig. 11.30). There are two types of ganglion cell: most (90% to 95%) are large Type I cells, and the remainder are smaller Type II cells (see reviews by Nadol 1988, Eybalin 1993). Type I cells contain a prominent spherical nucleus, abundant ribosomes and many mitochondria; in many mammals (although possibly not in humans) they are surrounded by myelin sheaths. In contrast, Type II cells are smaller, are always unmyelinated and have a lobulated nucleus. The cytoplasm of Type II cells is enriched with neurofilaments but has fewer mitochondria and ribosomes than Type I cells.

Fig. 11.30 Transmission electron micrograph showing several Type II ganglion cells and nerve fibres in a human spiral ganglion. Note the absence of myelin from the surrounding sheaths of the ganglion cells.

(Courtesy of H. Felix, M. Gleeson and L.-G. Johnsson, ENT Department, University of Zurich and GKT School of Medicine, London.)

Each inner hair cell is in synaptic contact with the unbranched peripheral processes of approximately ten Type I ganglion cells. The processes of Type II ganglion cells diverge within the organ of Corti and innervate more than ten outer hair cells. The peripheral and central processes of Type I ganglion cells are relatively large in diameter and are myelinated, whereas those of Type II are smaller and unmyelinated. The peripheral processes of both types of cell radiate from the modiolus into the osseous spiral lamina, where the Type I axons lose their myelin sheaths before entering the organ of Corti through the habenula perforata.

Three distinct groupings of afferent fibres have been identified: inner radial, basilar and outer spiral fibres (Fig. 11.31).

Fig. 11.31 Innervation of the organ of Corti. The ganglion cells that give rise to the sensory nerve fibres include those related to the inner hair cells (dark green) and others innervating the outer hair cells (light green). Efferent fibres are depicted in purple. There is a great contrast between the convergent afferent innervation of the inner hair cells (approximately 10 fibres to each cell) and the divergent supply of the outer hair cells (one afferent fibre to approximately 10 cells). This illustration is a simplified view of the complex innervation of the organ of Corti (see the text for further details).

Inner radial fibres

The inner radial fibre group consists of the majority of afferent fibres. They run directly in a radial direction to the inner hair cells, each of which receives endings from several of these fibres.

Basilar fibres

Basilar fibres are afferent to the outer hair cells and take an independent spiral course, turning toward the cochlear apex near the bases of the inner hair cells. They run for a distance of about five pillar cells before turning radially again and crossing the floor of the tunnel of Corti, often diagonally, to form part of the outer spiral bundle.

Outer spiral bundles

The afferent fibres of the bundles of the outer spiral group course toward the basal part of the cochlea, continually branching off en route to supply several outer hair cells. The outer spiral bundles also contain efferent fibres.

Efferent Cochlear Fibres

The efferent nerve fibres in the cochlear nerve are derived from the olivocochlear system (see reviews by Warr 1992, Guinan 1996). Within the modiolus, the efferent fibres form the intraganglionic spiral bundle, which may be one or more discrete groups of fibres situated at the periphery of the spiral ganglion (see Fig. 11.31). There are two main groups of olivocochlear efferents: lateral and medial. The lateral efferents come from small neurones in and near the lateral superior olivary nucleus and arise mainly, but not exclusively, ipsilaterally. They are organized into inner spiral fibres that run in the inner spiral bundle before terminating on the afferent axons that supply the inner hair cells. The medial efferents originate from larger neurones in the vicinity of the medial superior olivary nucleus, and the majority arise contralaterally. They are myelinated and cross the tunnel of Corti to synapse with the outer hair cells mainly by direct contact with their bases, although a few synapse with the afferent terminals. The efferent innervation of the outer hair cells decreases along the organ of Corti from cochlear base to apex, and from the first (inner) row to the third. The efferents use acetylcholine, γ-aminobutyric acid (GABA) or both as their neurotransmitter. They may also contain other neurotransmitters and neuromodulators.

Activity of the medial efferents inhibits cochlear responses to sound; the strength of the activity grows slowly with increasing sound levels. They are believed to modulate the micromechanics of the cochlea by altering the mechanical responses of the outer hair cells, thus changing their contribution to frequency sensitivity and selectivity. The lateral efferents related to the inner hair cells also respond to sound. They appear to modify transmission through their postsynaptic action on inner hair cell afferents. The cholinergic fibres may excite the radial fibres, whereas those containing GABA may inhibit them, although their role is less well understood than that of the medial efferents (see review by Guinan 1996).

Autonomic Cochlear Innervation

Autonomic nerve endings appear to be entirely sympathetic. Two adrenergic systems have been described within the cochlea: a perivascular plexus derived from the stellate ganglion, and a blood vessel–independent system derived from the superior cervical ganglion. Both systems travel with the afferent and efferent cochlear fibres and seem to be restricted to regions away from the organ of Corti. The sympathetic nervous system may cause primary and secondary effects in the cochlea by remotely altering the metabolism of various cell types and by influencing the blood vessels and nerve fibres with which it makes contact.

CASE 10 Acoustic Neuroma

A 45-year-old woman complains of right-sided tinnitus and reduced hearing, present for several years. With the passage of time, she has also experienced difficulty coordinating her right hand and, more recently, bifrontal headache. Examination demonstrates impaired hearing on the right; a reduced right corneal reflex, with variable impaired sensation on the right side of the face; and mild papilloedema. Somewhat later, right-sided facial weakness appears, but involvement of other cranial nerves is lacking until very late in the disease course.

Discussion: This patient exhibits an indolent clinical course typical of an acoustic neuroma, with early symptoms attributed to eighth nerve dysfunction reflecting the site of the tumour, which is virtually always within the internal auditory meatus (Fig. 11.32). Although the tumour is immediately adjacent to the facial nerve in this location, the first clinical manifestations beyond the eighth nerve itself are generally attributed to the trigeminal nerve. As in this patient, loss of the corneal reflex is a relatively early sign. As the tumour grows, other cranial nerves are affected. Eventually, the enlarging mass impinges, directly or indirectly, on the fourth ventricle, with resultant hydrocephalus; increased intracranial pressure is unusual under these circumstances, in light of contemporary diagnostic procedures.

Fig. 11.32 MRI demonstrates a large contrast-enhancing acoustic neuroma (arrow) with compression of the underlying brain stem and secondary ventriculomegaly.

(© 2010 Thomas Jefferson University. All rights reserved. Reproduced with the permission of Thomas Jefferson University.)

CASE 11 Sudden Unilateral Hearing Loss

A 54-year-old man with a history of diabetes mellitus and a 40 pack-year smoking history awakens with loss of hearing in the left ear. He reports having had several episodes of vertigo in the previous month. He denies other neurological symptoms.

The neurological examination is remarkable primarily for profound sensorineural hearing loss on the left side. He has no nystagmus or ataxia, and with the exception of the hearing loss, cranial nerve functions are normal.

His hearing improves over the next several days, but he then develops severe hearing loss again, this time associated with tinnitus, vertigo and left-sided ataxia. MRI demonstrates an acute infarct involving the middle cerebellar peduncle.

Discussion: Sudden unilateral hearing loss may reflect ischaemic vascular disease involving the labyrinth or acoustic nerve, viral infection of the labyrinth, rupture of the cochlear membrane or immune-mediated injury to the inner ear. This man’s symptoms are due to occlusive vascular disease (i.e. ischaemia) in the territory of the left anterior inferior cerebellar artery. It is especially noteworthy that stroke or transient ischaemic attacks involving the posterior circulation may present as acute hearing loss.

Glossopharyngeal Nerve (IX)

The glossopharyngeal nerve (Figs 11.33–11.35) supplies motor fibres to stylopharyngeus; parasympathetic secretomotor fibres to the parotid gland (derived from the inferior salivatory nucleus); sensory fibres to the tympanic cavity, pharyngotympanic tube, fauces, tonsils, nasopharynx, uvula and posterior (postsulcal) third of the tongue; and gustatory fibres to the postsulcal part of the tongue.

Fig. 11.33 Communications between the last four cranial nerves of the left side viewed from the dorsolateral aspect. The hypoglossal canal has been split in its long axis, and the transverse process of the atlas has been divided close to the lateral mass. The descending branch of the hypoglossal nerve is not shown.

Fig. 11.34 Structures crossing the internal jugular vein and carotid arteries and those intervening between the external and internal carotid arteries.

Fig. 11.35 Dissection of the brain stem and upper part of the spinal cord after removal of large portions of the occipital and parietal bones, the cerebellum and the roof of the fourth ventricle. On the left side, the foramina transversaria of the atlas and the third, fourth and fifth cervical vertebrae have been opened to expose the vertebral artery. On the right side, the posterior arch of the atlas and the laminae of the succeeding cervical vertebrae have been divided and removed, together with the vertebral spines and the contralateral laminae. The tentorium cerebelli and the transverse sinuses have been divided and their posterior portions removed.

The nerve leaves the skull through the anteromedial part of the jugular foramen, anterior to the vagus and accessory nerves, and in a separate dural sheath. In the foramen it is lodged in a deep groove leading from the cochlear aqueductal depression and is separated by the inferior petrosal sinus from the vagus and accessory nerves. The groove is bridged by fibrous tissue, which is calcified in approximately 25% of skulls. After leaving the foramen, the nerve passes forward between the internal jugular vein and internal carotid artery and then descends anterior to the latter, deep to the styloid process and its attached muscles, to reach the posterior border of stylopharyngeus. It curves forward on the stylopharyngeus and either pierces the lower fibres of the superior pharyngeal constrictor or passes between it and the middle constrictor to be distributed to the tonsil, mucosa of the pharynx and postsulcal part of the tongue, vallate papillae and oral mucous glands.

Two ganglia, superior and inferior, are situated on the glossopharyngeal nerve as it traverses the jugular foramen (see Fig. 11.33). The superior ganglion is in the upper part of the groove occupied by the nerve in the jugular foramen. It is small, has no branches and is usually regarded as a detached part of the inferior ganglion. The inferior ganglion is larger and lies in a notch in the lower border of the petrous part of the temporal bone. Its cells are typical unipolar neurones, and their peripheral branches convey gustatory and tactile signals from the mucosa of the tongue (posterior third, including the sulcus terminalis and vallate papillae) and general sensation from the oropharynx, where it is responsible for initiating the gag reflex.

Vagus Nerve (X)

The vagus is a large mixed nerve. It has a more extensive course and distribution than any other cranial nerve and traverses the neck, thorax and abdomen (Figs 11.33, 11.34, 11.36; see also Figs 10.3, 10.8). Its central connections are described in Chapter 10.

Fig. 11.36 Dissection of the right side of the neck, showing the carotid and subclavian arteries and their branches. The parotid and submandibular glands have been removed, together with the lower part of the internal jugular vein, most of the sternocleidomastoid and the upper parts of the stylohyoid and posterior belly of the digastric.

The vagus exits the skull through the jugular foramen accompanied by the accessory nerve, with which it shares an arachnoid and a dural sheath. Both nerves lie anterior to a fibrous septum that separates them from the glossopharyngeal nerve. The vagus descends vertically in the neck in the carotid sheath, between the internal jugular vein and the internal carotid artery, to the upper border of the thyroid cartilage; it then passes between the vein and the common carotid artery to the root of the neck. Its relationships in this part of its course are therefore similar to those described for these structures. Its course then differs on the two sides. The right vagus descends posterior to the internal jugular vein to cross the first part of the subclavian artery and enter the thorax. The left vagus enters the thorax between the left common carotid and subclavian arteries and behind the left brachiocephalic vein.

After emerging from the jugular foramen, the vagus bears two marked enlargements: a small, round superior ganglion and a larger inferior ganglion (see Fig. 11.33).

Accessory Nerve (XI)

The accessory nerve is conventionally described as a single entity (see Figs 11.33, 11.35, 11.37), even though its two components—the cranial root and spinal root, which join for a relatively short part of its course—are of separate origin.

Fig. 11.37 Dissection showing the general distribution of the left hypoglossal and lingual nerves and the position and constitution of some parts of the cervical plexus of the left side.

Hypoglossal Nerve (XII)

The hypoglossal nerve is motor to all the muscles of the tongue, except the palatoglossus (see Figs 11.34, 11.36–11.38). The hypoglossal rootlets run laterally behind the vertebral artery, collected into two bundles that perforate the dura mater separately opposite the hypoglossal canal in the occipital bone and then unite after traversing it. The canal is sometimes divided by a spicule of bone. The nerve emerges from the canal in a plane medial to the internal jugular vein, internal carotid artery and ninth, tenth and eleventh cranial nerves and passes inferolaterally behind the internal carotid artery and glossopharyngeal and vagus nerves to the interval between the artery and the internal jugular vein. There it makes a half-spiral turn around the inferior vagal ganglion and is united with it by connective tissue. It then descends almost vertically between the vessels and anterior to the vagus to a point level with the angle of the mandible, becoming superficial below the posterior belly of the digastric and emerging between the internal jugular vein and internal carotid artery. It loops around the inferior sternocleidomastoid branch of the occipital artery and crosses lateral to both the internal and external carotid arteries and the loop of the lingual artery a little above the tip of the greater cornu of the hyoid; it is crossed itself by the facial vein.

Fig. 11.38 Plan of the cervical plexus.

Lesions of the Hypoglossal Nerve

The hypoglossal nerve may be damaged during neck dissection. Complete hypoglossal division causes unilateral lingual paralysis and eventual hemiatrophy. The protruded tongue deviates to the paralysed side; on retraction, the wasted and paralysed side rises higher than the unaffected side. The larynx may deviate toward the active side in swallowing, due to unilateral paralysis of the hyoid depressors associated with loss of the first cervical spinal nerve, which runs with the hypoglossal nerve. If paralysis is bilateral, the tongue is motionless. Taste and tactile sensibility are unaffected, but articulation is slow and swallowing is very difficult.

CASE 13 Hypoglossal Neuropathy

A 45-year-old man notes thinning of one side of his tongue. He denies any other symptoms. Examination demonstrates hemiatrophy of the tongue, with deviation to the wasted side when the tongue is protruded (Fig. 11.39). Otherwise, the examination is normal.

Fig. 11.39 Hypoglossal palsy with deviation of the tongue to the side of the lesion.

Discussion: This man has an isolated hypoglossal neuropathy. The lack of signs of other neurological dysfunction, in particular of the lower brain stem, indicates that the nerve is affected peripherally. Neuroimaging documents a focal lesion involving the hypoglossal foramen, in this case, a clivus meningioma. Other lesions of the skull base, such as chordoma or glomus jugulare tumour, must also be considered. The lack of other cranial nerve involvement argues against other potential diagnoses such as basal meningitis or meningeal carcinomatosis.