Cranial Nerves

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Chapter 11 Cranial Nerves

Cranial nerves are the means by which the brain receives information from, and controls the activities of, the head and neck and, to a lesser extent, the thoracic and abdominal viscera. Briefly, there are 12 pairs of cranial nerves that are individually named and numbered (using roman numerals) in a rostrocaudal sequence (see Table 1.1). Unlike spinal nerves, only some are mixed in function and thus carry both sensory and motor fibres. Others are purely sensory or purely motor. The first cranial nerve (I; olfactory) has an ancient lineage and is derived from the forerunner of the cerebral hemisphere. It retains this unique position through the connections of the olfactory bulb and is the only sensory cranial nerve that projects directly to the cerebral cortex rather than via the thalamus, as do all other sensory modalities. The areas of cerebral cortex involved have a primitive cellular organization and are an integral part of the limbic system, which is concerned with the emotional aspects of behaviour. The second cranial nerve (II; optic) consists of the axons of second-order visual neurones and terminates in the thalamus. The other 10 pairs of cranial nerves attach to the brain stem. Most of the component fibres originate from, or terminate in, named cranial nerve nuclei (Ch. 10).

The sensory fibres in individual spinal and cranial nerves have characteristic, but often overlapping, peripheral distributions. As far as the innervation of the body surface is concerned, the area supplied by a particular spinal or cranial nerve is referred to as a dermatome. Detailed dermatome maps are described on a regional basis. The motor axons of individual spinal and cranial nerves tend to innervate anatomically and functionally related groups of skeletal muscles, which are referred to as myotomes.

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.

CASE 1 Anosmia

A 52-year-old man falls approximately 10 feet from a ladder, striking his head on the pavement below. He loses consciousness for a short time. He is found to have a left frontal skull fracture involving the base of the anterior fossa, responsible for a right hemiparesis and mild language dysfunction. He also experiences a leak of spinal fluid through his nose for several months thereafter. Although he recovers normal neurological function with the passage of time, it becomes clear that he has lost the sense of smell on the side of the skull fracture.

Discussion: This man has suffered a basal skull fracture involving the floor of the anterior fossa. The fracture line crosses the olfactory groove, resulting in laceration of the olfactory nerve and thus anosmia. Tumours and other mass lesions on the floor of the anterior fossa may similarly result in ipsilateral loss of the sense of smell; bilateral anosmia, with or without impairment of taste, may be due to the use of a variety of drugs, such as calcium channel blockers, some antibiotics, anticonvulsants and opiates, among others. (See also Ch. 9, Case 1.)

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.

Retina

The retina is described in Chapter 12.

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).

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.

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.