The anatomy of the peripheral nervous system

Tensor tympani and stapedius reflex

Loud sound elicits reflex contraction of tensor tympani and stapedius, attenuating movement of the tympanic membrane and the ossicular chain. Afferent impulses travel in the cochlear nerve to the cochlear nuclei in the brainstem. Efferent fibres to tensor tympani arise in the motor nucleus of the trigeminal nerve and travel in the mandibular division of the nerve. Efferent fibres to stapedius originate in the facial nucleus and travel in the facial nerve.

Jaw jerk reflex

The jaw jerk reflex is a deep tendon reflex, elicited by tapping on the midline of the mandible. Rapid stretching of the muscles that close the jaw (masseter, temporalis, medial pterygoid) activates muscle spindle afferents that travel via the mandibular division of the trigeminal nerve to the mesencephalic trigeminal nucleus. Collaterals project monosynaptically to the motor nucleus of the trigeminal nerve in the pons, establishing a two-neurone reflex arc.

Supraorbital nerve

The supraorbital nerve is the larger of the two terminal branches of the frontal nerve. It continues forwards along levator palpebrae superioris until it leaves the orbit through the supraorbital notch or foramen to emerge onto the forehead. It supplies the mucous membrane that lines the frontal sinus, the conjunctiva and the skin covering the upper eyelid via palpebral branches. It ascends on the forehead with the supraorbital artery, and divides into medial and lateral branches that supply the skin of the scalp nearly as far back as the lambdoid suture. These branches are at first deep to the frontal belly of occipitofrontalis. The medial branch perforates the muscle to reach the skin, while the lateral branch pierces the epicranial aponeurosis. The postganglionic sympathetic fibres that innervate the sweat glands of the supraorbital area probably travel in the supraorbital nerve, having entered the ophthalmic nerve via its communication with the abducens nerve within the cavernous sinus.

Supratrochlear nerve

The supratrochlear nerve is the smaller terminal branch of the frontal nerve. It runs anteromedially in the roof of the orbit, passes above the trochlea for the tendon of superior oblique, and supplies a descending filament to the infratrochlear branch of the nasociliary nerve. The nerve emerges between the trochlea and the supraorbital foramen at the frontal notch, curves up on the forehead close to the bone with the supratrochlear artery, and supplies the conjunctiva and the skin of the upper eyelid. It then ascends beneath corrugator and the frontal belly of occipitofrontalis before dividing into branches that pierce these muscles to supply the skin of the lower forehead near the midline.

Anterior ethmoidal nerve

The anterior ethmoidal nerve passes through the anterior ethmoidal foramen and canal and enters the cranial cavity. It runs forwards in a groove on the upper surface of the cribriform plate beneath the dura mater, 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 supplies the roof of the nasal cavity. It gives off medial and lateral internal nasal branches that supply the anterior and upper parts of the septum and the anterior part of the lateral wall, respectively, before emerging at the inferior margin of the nasal bone as the external nasal nerve, which descends through the lateral wall of the nose and supplies the skin of the nose below the nasal bones to the nasal tip but excluding the alar portion around the external nares: damage following nasal trauma may result in paraesthesia of the nasal tip.

Posterior ethmoidal nerve

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

Infratrochlear nerve

The infratrochlear nerve leaves the orbit below the trochlea and supplies the skin of the eyelids, the conjunctiva, lacrimal sac, lacrimal caruncle and the side of the nose above the medial canthus.

Long ciliary nerves

Two or three long ciliary nerves branch from the nasociliary nerve as it crosses the optic nerve. They accompany the short ciliary nerves, pierce the sclera near the attachment of the optic nerve, and run forwards between the sclera and choroid. They supply the ciliary body, iris and cornea, and contain postganglionic sympathetic fibres for 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.

Ramus communicans to the ciliary ganglion

The ramus communicans to the ciliary ganglion usually branches from the nasociliary nerve as the latter 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.

Meningeal nerve

The meningeal branch of the maxillary nerve arises within the middle cranial fossa and runs with the middle meningeal vessels. It contributes to the innervation of the dura mater (see Fig. 25.3B ).

Ganglionic branches

Two ganglionic branches usually connect the maxillary nerve to the pterygopalatine ganglion.

Zygomatic nerve

The zygomatic branch of the maxillary nerve leaves the pterygopalatine fossa through the inferior orbital fissure with the maxillary nerve and lies close to the base of the lateral wall of the orbit. Soon after entering the orbit the nerve divides into two branches, the zygomaticotemporal and the zygomaticofacial nerves, which run within the orbit for only a short distance before passing onto the face through the lateral wall of the orbit. They may enter separate canals within the zygomatic bone or the zygomatic nerve itself may enter the bone before dividing.

Zygomaticotemporal nerve

The zygomaticotemporal nerve traverses a canal in the zygomatic bone and emerges into the anterior part of the temporal fossa, where it ascends between the bone and temporalis before piercing the temporal fascia about 2 cm above the zygomatic arch to supply the skin of the temple. It communicates with the facial and auriculotemporal nerves. As it pierces the deep layer of the temporal fascia, it sends a slender twig between the two layers of the fascia towards the lateral angle of the eye. It was thought that the branch carried parasympathetic postganglionic fibres from the pterygopalatine ganglion to the lacrimal gland, but it is now believed that these fibres innervate the lacrimal gland directly.

Zygomaticofacial nerve

The zygomaticofacial nerve traverses the inferolateral angle of the orbit and emerges on the face through a foramen in the zygomatic bone. It perforates orbicularis oculi to supply the skin on the prominence of the cheek and forms a plexus with zygomatic branches of the facial nerve and palpebral branches of the maxillary nerve. The nerve is absent occasionally.

Infraorbital nerve

The infraorbital nerve can be regarded as the terminal branch of the maxillary nerve. It leaves the pterygopalatine fossa to enter the orbit at the inferior orbital fissure and at first it lies in the infraorbital groove on the floor of the orbit. As it approaches the rim of the orbit it runs into the infraorbital canal through which it passes to emerge onto the face at the infraorbital foramen, where it lies between levator labii superioris and levator anguli oris. It gives off palpebral, nasal and superior labial branches. The palpebral branches ascend deep to orbicularis oculi, pierce the muscle to supply the skin in the lower eyelid, and join the facial and zygomaticofacial nerves near the lateral canthus. Nasal branches supply the skin of the side of the nose, the vestibule and the movable part of the nasal septum, and join the external nasal branch of the anterior ethmoidal nerve. Superior labial branches, which are 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

Fine orbital branches enter the orbit through the inferior orbital fissure and supply orbital periosteum. Some fibres also pass through the posterior ethmoidal foramen to supply the sphenoidal and ethmoidal sinuses. The orbital branches probably join branches of the internal carotid nerve to form a ‘retro-orbital’ plexus from which orbital structures such as the lacrimal gland and orbitalis receive an autonomic innervation.

Posterior superior nasal nerves (lateral and medial)

The posterior superior alveolar nerves enter the back of the nasal cavity through the sphenopalatine foramen. Lateral posterior superior nasal nerves innervate the mucosa covering the posterior part of the superior and middle nasal turbinates (conchae) and lining the posterior ethmoidal sinuses. Two or three medial posterior superior nasal nerves cross the nasal roof below the opening of the sphenoidal sinus to supply the mucosa of the posterior part of the roof and of the nasal septum.

Nasopalatine nerve

The nasopalatine nerve leaves the pterygopalatine fossa through the sphenopalatine foramen and enters the nasal cavity. It passes across the cavity to the back of the nasal septum, runs downwards and forwards on the septum in a groove in the vomer, and then turns down through the incisive fossa in the anterior part of the hard palate to enter the roof of the mouth. When an anterior and a posterior incisive foramen exist in this fossa, the left nasopalatine nerve typically passes through the anterior foramen, and the right nerve passes through the posterior foramen. The nasopalatine nerve supplies the inferior part of the nasal septum and the anterior part of the hard palate, where it communicates with the greater palatine nerve.

Surgical division of the nasopalatine nerve, for example during the removal of an ectopic canine tooth, causes no obvious sensory deficit in the anterior part of the palate, which suggests that either the territory of the greater palatine nerve reaches as far forwards as the gingivae lingual to the incisor teeth or that the nasopalatine nerve has significant regenerative potential.

Palatine nerves (greater and lesser)

The greater and lesser palatine nerves pass downwards from the pterygopalatine ganglion through the greater palatine canal. The greater palatine nerve descends through the greater palatine canal, emerges on the hard palate from the greater palatine foramen and runs forwards in a groove on the inferior surface of the bony palate almost to the incisor teeth. It supplies the gingivae, mucosa and glands of the hard palate and also communicates with the terminal filaments of the nasopalatine nerve. In the greater palatine canal it gives off posterior inferior nasal branches that emerge through the perpendicular plate of the palatine bone and ramify over the inferior nasal turbinate (concha) and walls of the middle and inferior meatuses. As it leaves the greater palatine canal, it gives off branches which are distributed to both surfaces of the adjacent part of the soft palate.

The lesser (middle and posterior) palatine nerves are much smaller than the greater palatine nerve. They descend through the greater palatine canal, from which they diverge low down to emerge through the lesser palatine foramina in the tubercle or pyramidal process of the palatine bone. They innervate the uvula, tonsil and soft palate.

Fibres conveying taste impulses from the palate probably pass via the palatine nerves to the pterygopalatine ganglion. They pass through the ganglion without synapsing, and leave via the greater petrosal nerve. Their cell bodies are located in the facial ganglion and their central processes pass via the sensory root of the facial nerve (nervus intermedius) to the gustatory nucleus in the nucleus of the tractus solitarius.

Pharyngeal nerve

The pharyngeal branch of the maxillary nerve leaves the pterygopalatine ganglion posteriorly. It passes through the palatovaginal canal with the pharyngeal branch of the maxillary artery and supplies the mucosa of the nasopharynx behind the pharyngotympanic tube.

Superior alveolar nerves

The teeth in the upper jaw are supplied by the three superior alveolar (dental) nerves that arise from the maxillary nerve either in the pterygopalatine fossa or in the infraorbital groove and canal.

The posterior superior alveolar (dental) nerve leaves the maxillary nerve in the pterygopalatine fossa and runs anteroinferiorly to pierce the infratemporal surface of the maxilla, at which point it is possible to perform a local anaesthetic block that will anaesthetize the pulps of the premolar and molar ipsilateral teeth; it then descends under the mucosa of the maxillary sinus. After supplying the lining of the sinus, the nerve divides into small branches that link up as the molar part of the superior alveolar plexus, supplying twigs to the molar teeth. It also supplies a branch to the upper molar gingivae and the adjoining part of the cheek.

The middle superior alveolar (dental) nerve arises from the infraorbital nerve as it runs in the infraorbital groove, and runs downwards and forwards in the lateral wall of the maxillary sinus. It ends in small branches that link up with the superior dental plexus, supplying small rami to the upper premolar teeth. This nerve is variable; it may be duplicated, triplicated, or absent.

The anterior superior alveolar (dental) nerve leaves the lateral side of the infraorbital nerve near the midpoint of its canal and traverses the canalis sinuosus in the anterior wall of the maxillary sinus. It curves first under the infraorbital foramen, then passes medially towards the nose, and finally turns downwards and divides into branches to supply the incisor and canine teeth. It assists in the formation of the superior dental plexus and gives off a nasal branch that passes through a minute canal in the lateral wall of the inferior meatus to supply the mucous membrane of the anterior area of the lateral wall as high as the opening of the maxillary sinus, and the floor of the nasal cavity near the anterior nasal spine. It communicates with the nasal branches of the pterygopalatine ganglion and finally emerges near the root of the anterior nasal spine to supply the adjoining part of the nasal septum.

Meningeal branch (nervus spinosus)

The meningeal branch re-enters the middle cranial fossa 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 (see Fig. 25.3B ).

Nerve to medial pterygoid

The nerve to 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.

Buccal nerve

The buccal nerve passes between the two heads of lateral pterygoid, giving off motor fibres to the muscle as it passes through it. The nerve then descends deep to the tendon of temporalis, passes laterally anterior to masseter, and anastomoses with the buccal branches of the facial nerve. It may give off the anterior deep temporal nerve. The buccal nerve supplies sensation to the skin over the anterior part of buccinator, the corresponding buccal mucous membrane and the posterior part of the buccal gingivae opposite the second and third molar teeth. It may also give off a branch that passes through a small retromolar foramen to supply the molar teeth.

Nerve to masseter

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

Deep temporal nerves

There are usually two deep temporal nerves, anterior and posterior, although there may be a middle branch. They pass above lateral pterygoid to enter the deep surface of temporalis. The anterior branch may arise as a branch of the buccal nerve or from the nerve to masseter; the small posterior nerve sometimes arises in common with the nerve to masseter. Branches from the buccal nerve and the nerve to masseter may supply the inferior aspect of temporalis independently from the deep temporal nerves.

Nerve to lateral pterygoid

The nerve to 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.

Auriculotemporal nerve

The auriculotemporal nerve usually has two roots that encircle the middle meningeal artery. It runs back under lateral pterygoid on the surface of tensor veli palatini, passes between the sphenomandibular ligament and the 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 masseter. Filaments from the otic ganglion join the roots of the auriculotemporal nerve close to their origin. The auriculotemporal nerve emerges onto the face behind the temporomandibular joint within the superior surface of the parotid gland, ascends posterior to the superficial temporal vessels, passes over the posterior root of the zygoma, and divides into superficial temporal branches. The cutaneous branches of the auriculotemporal nerve supply the tragus and part of the adjoining auricle of the ear and the posterior part of the temple. The nerve may be damaged during parotid gland surgery, resulting in impaired sensation over the tragus and temple. The auriculotemporal nerve communicates with the temporofacial division of the facial nerve, usually by two rami that pass anterolaterally behind the neck of the mandible. These communications anchor the facial nerve close to the lateral surface of the condylar process of the mandible and limit its mobility during surgery. Communications with the temporal and zygomatic branches of the facial nerve loop around the transverse facial and superficial temporal vessels.

Lingual nerve

The lingual nerve is sensory to the mucosa of the floor of the mouth, mandibular lingual gingivae and mucosa of the presulcal part of the tongue (excluding the circumvallate papillae, which are supplied by the glossopharyngeal nerve). It also carries postganglionic parasympathetic fibres from the submandibular ganglion to the sublingual and anterior lingual glands. It arises from the posterior trunk of the mandibular nerve and at first runs beneath 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 lateral pterygoid, the lingual nerve then runs downwards and forwards on the surface of 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 lower third molar tooth, where it is covered only by the gingival mucoperiosteum. At this point it usually lies approximately 3 mm below the alveolar crest with a mean distance from the bone of 2 mm: it may be in direct contact with the bone (25%) or lie above the alveolar crest (15%). The nerve next passes medial to the mandibular origin of mylohyoid, and this carries it progressively away from the mandible, separating it from the alveolar bone covering the mesial root of the lower third molar tooth. It then passes downwards and forwards on the deep surface of mylohyoid (i.e. the surface nearer the mucosa covering the floor of the mouth), crossing the lingual sulcus beneath the mucosa. In this position, the lingual nerve lies on the deep portion of the submandibular gland, which bulges over the top of the posterior border of mylohyoid. The nerve passes below the submandibular duct, which crosses it from medial to lateral, and curves upwards, forwards and medially to enter the tongue by medial and lateral branches. Within the tongue, the medial branch sends small branches to the medial part of the ventrolateral tongue, and the lateral branch runs along the lateral border of the tongue and sends larger branches to innervate the mucosa of the anterior tip of the tongue. The lingual nerve is connected to the submandibular ganglion by two or three branches. It forms connecting loops with twigs of the hypoglossal nerve at the anterior margin of hyoglossus ( ) and within the tongue ( ).

The lingual nerve is at risk during surgical removal of (impacted) lower third molars: patients occasionally develop lingual sensory disturbance postoperatively but this is rarely persistent ( ). The nerve is also at risk during operations to remove the submandibular salivary gland because the submandibular duct must be dissected from the lingual nerve, and also because its connection to the submandibular ganglion pulls it into the operating field.

Inferior alveolar (dental) nerve

The inferior alveolar nerve descends behind 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. Just before entering the mandibular canal, the inferior alveolar nerve gives off a small mylohyoid branch that also carries fibres that innervate the anterior belly of digastric. Below lateral pterygoid, the nerve is accompanied by the inferior alveolar artery, a branch of the first part of the maxillary artery, which enters the canal with its associated veins. In the mandibular canal, the inferior alveolar nerve runs downwards and forwards, generally below the apices of the teeth until it is below the first and second premolars, where it divides into terminal incisive and mental branches. The incisive branch continues forwards in a bony canal or in a plexiform arrangement, giving off branches to the first premolar, canine and incisor teeth, and to the associated labial gingivae. The lower central incisor teeth receive a bilateral innervation, fibres probably crossing the midline within the periosteum to re-enter the bone via numerous canals in the labial cortical plate. The mental nerve passes upwards, backwards and outwards to emerge from the mandible via the mental foramen between and just below the apices of the premolar teeth. It immediately divides into three branches, two of which pass upwards and forwards to form an incisor plexus labial to the teeth that supplies the labial gingivae (and probably the periosteum). From this plexus and from the dental branches, fibres turn downwards and then lingually to emerge on the lingual surface of the mandible on the posterior aspect of the mental symphysis or opposite the premolar teeth, probably communicating with the lingual or mylohyoid nerves. The third branch of the mental nerve passes through the intermingled fibres of depressor anguli oris and platysma to supply the skin of the lower lip and chin. Branches of the mental nerve communicate with terminal filaments of the mandibular branch of the facial nerve.

Nerve to mylohyoid

The nerve to mylohyoid is given off from the inferior alveolar nerve just before that nerve enters the mandibular canal. It pierces the sphenomandibular ligament and enters a shallow groove on the medial surface of the mandible, following a course roughly parallel to its parent nerve. It passes below the origin of mylohyoid to lie on the superficial surface of the muscle, between it and the anterior belly of digastric, both of which it supplies. Although primarily a motor nerve supplying mylohyoid and the anterior belly of digastric, the mylohyoid nerve carries some sensory fibres that may enter the mandible via one or more small retromental foramina in the vicinity of the genial tubercles to supply some sensation to the anterior teeth, and it may also give a few filaments to supply the skin over the point of the chin.

Dental extractions

Variations in the fascicular organization of the inferior alveolar nerve are clinically important when extracting impacted third molars. The nerve may appear as a single bundle lying a few millimetres below the roots of the teeth, or it may lie much lower and almost reach the lower border of the bone, so that it gives off a variable number of large rami that pass anterosuperiorly towards the roots before dividing to supply the teeth and interdental septa. Only rarely is it plexiform. The nerve may lie on the lingual or buccal side of the mandible (slightly more commonly on the buccal side). Even when the third molar tooth is in a normal position, the nerve may be so intimately related to it that it grooves its root. Exceptionally, the nerve may be similarly related to the second molar tooth.

Failure to achieve complete anaesthesia to the mandibular teeth following an inferior alveolar nerve block (which should anaesthetize the nerve before it enters the mandibular canal) suggests that there are alternative pathways for sensory nerves to innervate the teeth. Knowledge of these pathways is important in helping clinicians to obtain complete dental anaesthesia. There are anatomical variations in the distribution of the inferior alveolar nerve, the buccal branch of the mandibular nerve, the mylohyoid nerve, the lingual nerve, the deep temporal nerves and the cervical plexus. The inferior alveolar nerve may give multiple branches before entering the mandibular canal that may re-enter the mandible via small accessory foramina in the retromolar region to supply sensation to the molar teeth: one or more of these branches may escape anaesthesia following inferior alveolar nerve block. If a successful inferior alveolar nerve block does not completely anaesthetize teeth in a jaw quadrant, additional local injections of anaesthetic solution around the tooth in question may be necessary to block alternative pathways. There is also the possibility of overlap across the midline involving the incisive and mylohyoid nerves. In children, there is evidence to suggest that the lingula is variable in position and its position from the occlusion plane increases with age: it is therefore not considered an appropriate landmark for inferior alveolar nerve block.

Neural connections between branches of trigeminal and other nerves

Communications exist between the inferior alveolar and lingual nerves. If the lingual nerve is not anaesthetized during an inferior alveolar nerve block, this pathway could account for any remaining sensation in the teeth. Although the deep temporal nerves are primarily motor, it has been suggested that terminal branches may pass from the substance of temporalis to enter the mandible through foramina in the retromolar region to supply some sensation to molar teeth. In the cervical plexus, the great auricular nerve, formed by the anterior primary rami of the second and third cervical spinal nerves, supplies skin over the angle of the mandible: it is possible that this nerve may provide a branch that penetrates the mandible to supply one or more of the cheek teeth.

Greater petrosal nerve

The greater (superficial) petrosal nerve, a branch of the nervus intermedius, is the main branch from the geniculate ganglion. It 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, runs forwards in a groove on the bone above the lesser petrosal nerve, and then passes beneath the trigeminal ganglion to reach the foramen lacerum. Here it is joined by the deep petrosal nerve from the internal carotid sympathetic plexus, to become the nerve of the pterygoid canal (Vidian nerve). The greater petrosal nerve contains parasympathetic fibres destined for the pterygopalatine ganglion, and taste fibres from the palate. The geniculate ganglion also communicates with the lesser petrosal nerve.

Nerve to stapedius

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

Chorda tympani

The chorda tympani (see Figure 42.11, Figure 42.12 ) leaves the facial nerve some 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, and crosses medial to the upper part of the handle of the malleus to the anterior wall of the tympanic cavity, where it enters the anterior canaliculus. It exits the skull and enters the infratemporal fossa by passing through the medial end of the petrotympanic fissure posterior to the capsule of the temporomandibular joint. It next descends medial to the spine of the sphenoid bone, which it sometimes grooves, lying posterolateral to tensor veli palatini. It is crossed medially by the middle meningeal artery, the roots of the auriculotemporal nerve and the inferior alveolar nerve (see Fig. 38.23 ) and joins the posterior aspect of the lingual nerve at an acute angle. The chorda tympani carries taste fibres from the anterior two-thirds of the tongue (but not the from the circumvallate papillae) and efferent preganglionic parasympathetic secretomotor fibres destined for the submandibular ganglion in the floor of the mouth.

Dehiscences of the facial nerve canal

The facial nerve may be somewhat variable in its anatomical course through the temporal bone ( , ). It may split into two or three strands, starting at the geniculate ganglion, and then make its way across the promontory to the stylomastoid foramen, or pass a few millimetres posteriorly to its second genu, before it turns inferiorly posterior to the fossa incudis, a position where it is particularly vulnerable during surgical exploration of the mastoid antrum. The more proximal the division into strands, the more bizarre is the subsequent course. More distal bifurcations pass either side of the fenestra vestibuli. It may be dehiscent, particularly in its second part, when it occasionally overhangs the stapes, or run inferior to the stapedial superstructure, a position that renders it vulnerable during surgery to the stapes ( ). The motor fibres to the face may be carried through the chorda tympani, which is then enlarged. When this is the case, the distal facial nerve dwindles to a fibrous strand in a narrowed stylomastoid foramen. In chronic bone disease in the tympanic cavity, the facial nerve may be exposed in its canal. Inflammation may lead to facial paralysis of the infranuclear or lower motor neurone type.

Zygomatic branches

Zygomatic branches are generally multiple. They cross the zygomatic bone to the lateral canthus of the eye and supply orbicularis oculi and may also supply muscles innervated by the buccal branch. Twigs communicate with filaments of the lacrimal nerve and the zygomaticofacial branch of the maxillary nerve.

Buccal nerve

The buccal branch of the facial nerve is usually single. It has a close relationship to the parotid duct for about 2.5 cm after emerging from the parotid gland and typically lies below the duct. Superficial branches run beneath the subcutaneous fat and the superficial musculo-aponeurotic system (SMAS). Some branches pass deep to procerus and join the infratrochlear and external nasal nerves. Upper deep branches supply zygomaticus major and levator labii superioris 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 alaeque nasi and the small nasal muscles (these branches are sometimes described as lower zygomatic branches). Lower deep branches supply buccinator and orbicularis oris and communicate with filaments of the buccal branch of the mandibular nerve.

Marginal mandibular nerve

There are usually two marginal mandibular nerves. They run forwards towards the angle of the mandible under platysma, and then turn upwards 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. Filaments communicate with the mental nerve. The marginal mandibular branch has an important surgical relationship with the inferior border of the mandible.

Cutaneous branches

Cutaneous branches of the facial nerve accompany the auricular branch of the vagus and probably supply small areas on both aspects of the auricle, in the depression of the concha and over its eminence.

The facial nerve has intra- or extracranial connections with the cutaneous branches of all three divisions of the trigeminal nerve (including branches of the auriculotemporal, buccal, mental, lingual, infraorbital, zygomatic and ophthalmic nerves); with branches of the vestibulocochlear, glossopharyngeal and vagus nerves; and with branches of the cervical plexus (including the great auricular, greater and lesser occipital and transverse cervical nerves). These cutaneous connections are significant in facilitating the perineural spread of tumours that arise either within the parotid or on the face and explains why some patients with perineural spread from cutaneous or parotid malignancies may present with vocal cord palsy.

Cervical branch

The cervical branch of the facial nerve emerges from the lower part of the parotid gland and runs anteroinferiorly under platysma to the anterior neck. Typically single, it supplies platysma and communicates with the transverse cutaneous cervical nerve.

Bell’s palsy

Bell’s palsy is an idiopathic lower motor neurone facial palsy. It may be complete or partial, and is characterized by a flaccid paralysis of the ipsilateral mimetic facial muscles (muscles of facial expression); decreased lacrimation in the ipsilateral eye (which is controlled by neurones in the greater petrosal nerve); and hyperacusis or decreased tolerance of loud noises in the ipsilateral ear as a result of paralysis of stapedius. Its cause remains the subject of speculation but is most probably a herpes virus infection ( ). Magnetic resonance imaging studies suggest that there are inflammatory changes in the labyrinthine and perigeniculate segments of the facial nerve. In the vast majority of cases, spontaneous full recovery takes place after a few weeks.

Communicating branches

The glossopharyngeal nerve communicates with the sympathetic trunk and vagus and facial nerves. Its inferior ganglion is connected with the superior cervical sympathetic ganglion. Two filaments from the inferior ganglion pass to the vagus, one to its auricular branch and the other to its superior ganglion. A branch to the facial nerve arises from the glossopharyngeal nerve below the inferior ganglion and perforates the posterior belly of digastric to join the facial nerve near the stylomastoid foramen.

Tympanic nerve

The tympanic nerve leaves the inferior ganglion, ascends to the tympanic cavity through the tympanic canaliculus and divides into branches that contribute to the tympanic plexus on the promontory. The lesser petrosal nerve is derived from the tympanic plexus.

Carotid branch

The carotid branch is often double. It arises just below the jugular foramen and descends on the internal carotid artery to the wall of the carotid sinus and to the carotid body. The nerve contains primary afferent fibres from chemoreceptors in the carotid body and from the baroreceptors lying in the carotid sinus wall. It may communicate with the inferior ganglion of the vagus, or with one of its branches, and with a sympathetic branch from the superior cervical ganglion (see Fig. 35.20 ).

Pharyngeal branches

The pharyngeal branches are three or four filaments that unite with the pharyngeal branch of the vagus and the laryngopharyngeal branches of the sympathetic trunk to form the pharyngeal plexus near the middle pharyngeal constrictor. They constitute the route by which the glossopharyngeal nerve supplies sensory fibres to the mucosa of the pharynx.

Muscular branch

The muscular branch supplies stylopharyngeus.

Tonsillar, lingual and lesser petrosal branches and the tympanic plexus

The tympanic plexus supplies branches to the mucosa of the tympanic cavity, pharyngotympanic tube and mastoid air cells. The nerves that constitute the tympanic plexus ramify on the surface of the promontory on the medial wall of the tympanic cavity. They are derived from the tympanic branch of the glossopharyngeal nerve and the caroticotympanic nerves. The former arises from the inferior ganglion of the glossopharyngeal nerve and reaches the tympanic cavity via the tympanic canaliculus for the tympanic nerve. The superior and inferior caroticotympanic nerves are postganglionic sympathetic fibres derived from the carotid sympathetic plexus that traverse the wall of the carotid canal to join the plexus.

The lesser petrosal nerve, which may be regarded as the continuation of the tympanic branch of the glossopharyngeal nerve, traverses the tympanic plexus. It occupies a small canal below that for tensor tympani. It runs past, and receives a connecting branch from, the geniculate ganglion of the facial nerve. The lesser petrosal nerve emerges from the anterior surface of the temporal bone via a small opening lateral to the hiatus for the greater petrosal nerve and then traverses either the foramen ovale or the sphenopetrosal fissure or the innominate canal (of Arnold) to join the otic ganglion. Postganglionic secretomotor fibres leave this ganglion in the auriculotemporal nerve to supply the parotid gland. The tympanic plexus sends a branch to the greater petrosal nerve via an opening anterior to the fenestra vestibuli.

The tonsillar region is innervated by tonsillar branches of the maxillary and glossopharyngeal nerves. The fibres from the maxillary nerve pass through, but do not synapse in, the pterygopalatine ganglion; they are distributed through the lesser palatine nerves and form a plexus (the circulus tonsillaris) around the tonsil together with the tonsillar branches of the glossopharyngeal nerve. Nerve fibres from this plexus are also distributed to the soft palate and the region of the oropharyngeal isthmus. The tympanic branch of the glossopharyngeal nerve supplies the mucous membrane lining the tympanic cavity. Infection, malignancy and postoperative inflammation of the tonsil and tonsillar fossa may therefore be accompanied by pain referred to the ear.

Meningeal branches

Meningeal branches appear to start from the superior vagal ganglion and pass through the jugular foramen to be distributed to the dura mater in the posterior cranial fossa (see Fig. 25.3B ).

Auricular branch

The auricular branch (Arnold’s nerve) arises from the superior vagal ganglion and is joined by a branch from the inferior ganglion of the glossopharyngeal nerve. It passes behind the internal jugular vein and enters the mastoid canaliculus on the lateral wall of the jugular fossa. Traversing the temporal bone, it crosses the facial canal approximately 4 mm above the stylomastoid foramen and here supplies an ascending branch to the facial nerve. Fibres of the nervus intermedius may pass to the auricular branch at this point, which may explain the cutaneous vesiculation in the auricle that sometimes accompanies geniculate herpes. The auricular branch then traverses the tympanomastoid fissure and divides into two rami. One ramus joins the posterior auricular nerve and the other is distributed to the skin of part of the ear and to the external acoustic meatus.

Pharyngeal branch

The pharyngeal branch of the vagus is the main motor nerve of the pharynx and consists of axons arising from neuronal cell bodies in the nucleus ambiguus. It supplies all the muscles of the soft palate (excluding tensor veli palatini, which is supplied by the mandibular division of the trigeminal nerve via the nerve to medial pterygoid) and all the muscles of the pharynx (excluding stylopharyngeus, which is supplied by the glossopharyngeal nerve). It emerges from the upper part of the inferior vagal ganglion, passes between the external and internal carotid arteries to reach the upper border of the middle pharyngeal constrictor, and divides into numerous filaments that join rami of the sympathetic trunk and glossopharyngeal nerve to form the pharyngeal plexus. It also gives off a minute filament, the ramus lingualis vagi, which joins the hypoglossal nerve as it curves round the occipital artery.

Branches to the carotid body

Branches to the carotid body are variable in number. They may arise from the inferior ganglion or travel in the pharyngeal branch, or in the superior laryngeal nerve. They form a plexus with the glossopharyngeal rami and branches of the cervical sympathetic trunk.

Superior laryngeal nerve

The superior laryngeal nerve is larger than the pharyngeal branch and issues from the middle of the inferior vagal ganglion. It receives one or more communicating branches from the superior cervical sympathetic ganglion; most frequently, the connection is with the external laryngeal nerve. The sympathetic communication contributes to the innervation of the carotid body ( ) and the thyroid gland ( ). The superior laryngeal nerve descends alongside the pharynx, at first posterior, then medial, to the internal carotid artery: it may be found on the lateral side of the artery. It divides into a smaller, external, and a larger, internal, branch, approximately 1.5 cm below the ganglion: rarely, both branches may arise from the ganglion (see Figure 35.8, Figure 35.12 ).

Internal laryngeal nerve

The internal laryngeal nerve passes forwards approximately 7 mm before piercing the thyrohyoid membrane, usually at a higher level than the superior thyroid artery ( ). It splits into superior, middle and inferior branches on entering the larynx. The superior branch supplies the mucosa of the piriform fossa. The large middle branch is distributed to the mucosa of the ventricle, specifically the quadrangular membrane, and therefore probably conveys the afferent component of the cough reflex. The inferior ramus is mainly distributed to the mucosa of the ventricle and subglottic cavity. On the medial wall of the piriform fossa, descending branches give twigs to the interarytenoid muscle and share a number of communicating branches with the recurrent laryngeal nerve ( ).

The internal laryngeal nerve is sensory to the laryngeal mucosa down to the level of the vocal folds. It also carries afferent fibres from the laryngeal neuromuscular spindles and other stretch receptors. It descends to the thyrohyoid membrane, pierces it above the superior laryngeal artery and divides into an upper and lower branch. The upper branch is horizontal and supplies the mucosa of the pharynx, the epiglottis, vallecula and laryngeal vestibule. The lower branch descends in the medial wall of the piriform recess, supplies the aryepiglottic fold, the mucosa on the back of the arytenoid cartilage and one or two branches to transverse arytenoid (the latter unite with twigs from the recurrent laryngeal to supply the same muscle). The internal laryngeal nerve ends by piercing the inferior pharyngeal constrictor to unite with an ascending branch from the recurrent laryngeal nerve. As it ascends in the neck, it supplies branches, more numerous on the left, to the mucosa and tunica muscularis of the oesophagus and trachea, and to the inferior constrictor.

External laryngeal nerve

The external laryngeal nerve, smaller than the internal, descends behind sternohyoid with the superior thyroid artery, but on a deeper plane. It lies first on (occasionally within) the inferior pharyngeal constrictor, to which it contributes some small branches, and then pierces it to curve round the inferior thyroid tubercle. It passes beneath the attachment of sternothyroid to the oblique line of the thyroid cartilage to reach cricothyroid, which it supplies. A communicating nerve continues from the posterior surface of cricothyroid, crosses the piriform fossa and enters thyroarytenoid, where it anastomoses with branches from the recurrent laryngeal nerve. It has been suggested that these communicating branches may provide both additional motor components to thyroarytenoid and sensory fibres to the mucosa in the region of the subglottis. An anastomosis between the external and internal laryngeal nerves has also been described ( ). The external laryngeal nerve also gives branches to the pharyngeal plexus and communicates with the superior cardiac nerve and superior cervical sympathetic ganglion.

The close relationship of the external laryngeal nerve to the superior thyroid artery puts the nerve at potential risk when the artery is clamped during thyroid lobectomy, particularly when it is close to the artery (in approximately 20% of cases), or where, instead of crossing the superior thyroid vessels approximately 1 cm or more above the superior pole of the gland, it actually passes below this point (in some 20% of cases) ( , ). The external laryngeal nerve is also at risk in parathyroidectomy, carotid endarterectomy and anterior cervical spine procedures.

Recurrent laryngeal nerve

The recurrent laryngeal nerve supplies all the laryngeal muscles except cricothyroid, communicates with the internal laryngeal nerve and carries sensory filaments from the laryngeal mucosa below the vocal folds and from laryngeal stretch receptors. The nerve differs in origin and course on the two sides. On the right, it almost always arises from the vagus anterior to the first part of the subclavian artery, and curves backwards below and then behind it to ascend obliquely to the side of the trachea behind the common carotid artery. Near the lower pole of the lateral lobe of the thyroid gland, it is closely related to the inferior thyroid artery and crosses either in front of, behind or between its branches. On the left, the recurrent laryngeal nerve arises from the vagus on the left of the aortic arch, curves below it immediately behind the attachment of the ligamentum arteriosum to the concavity of the aortic arch and ascends to the side of the trachea (see Fig. 57.61 ). As the recurrent laryngeal nerve curves round either the subclavian artery or the aortic arch, it gives cardiac filaments to the deep cardiac plexus.

On both sides, the recurrent laryngeal nerve ascends in or near the tracheo-oesophageal groove. It is closely related to the medial surface of the thyroid gland before it passes under the lower border of the inferior constrictor, and it enters the larynx behind the articulation of the inferior thyroid cornu with the cricoid cartilage, passing either deep to (usually) or between (sometimes) the fibres of cricopharyngeus at its attachment to the lateral aspect of the cricoid cartilage. The nerve supplies cricopharyngeus as it passes. At this point, the nerve is in intimate proximity to the posteromedial aspect of the thyroid gland. The main trunk divides into two or more branches, usually below the lower border of the inferior constrictor, although branching may take place higher up. The anterior branch is mainly motor and is sometimes called the inferior laryngeal nerve (a term that is best avoided to prevent any confusion with a former synonym for the recurrent laryngeal nerve). It ascends posterior to the cricothyroid joint and its ligament, usually covered by fibres of posterior cricoarytenoid, then bends over the joint and continues forwards over lateral cricoarytenoid before terminating within thyroarytenoid. It innervates posterior cricoarytenoid by one or more branches, and interarytenoid, lateral cricoarytenoid and thyroarytenoid (Maranillo et al 2005). The posterior branch of the recurrent laryngeal nerve is mainly sensory and ascends deep to posterior cricoarytenoid to join the descending branch of the internal laryngeal nerve.

The recurrent laryngeal nerve does not always lie in a protected position in the tracheo-oesophageal groove but may be slightly anterior to it (more often on the right), and it may be markedly lateral to the trachea at the level of the lower part of the thyroid gland. The upper part of the nerve has a close but variable relationship to the inferior thyroid artery. On the right, it is as often anterior to, or posterior to, or intermingled with, the terminal branches of the artery; on the left, the nerve is usually posterior to the artery, though occasionally lies anterior to it. The stated incidences of these relationships vary between different authors. The recurrent laryngeal nerve has a highly variable branching pattern (see Fig. 41.13 ) ( ).

A very rare anomaly of relevance to laryngeal pathology and surgery is the ‘non-recurrent’ laryngeal nerve, where the right recurrent laryngeal nerve arises directly from the vagus nerve trunk high up in the neck and enters the larynx close to the inferior pole of the thyroid gland. Only the right side is affected, and it is always associated with an abnormal origin of the right subclavian artery from the aortic arch on the left side. If unrecognized, a non-recurrent laryngeal nerve may be susceptible to injury during surgery. It may also potentially be compressed by small tumours of the thyroid gland ( ).

Pharyngeal plexus

Almost all of the nerve supply to the pharynx, whether motor or sensory, is derived from the pharyngeal plexus, which is formed by the pharyngeal branches of the glossopharyngeal and vagus nerves with contributions from the superior cervical sympathetic ganglion. The plexus lies on the external surface of the pharynx, especially on the middle constrictor. Filaments from the plexus ascend or descend external to the superior and inferior constrictors before branching within the muscular layer and mucosa of the pharynx. Whether a ‘cranial root’ of the accessory nerve serves as the conduit for the major motor drive to the pharyngeal plexus, albeit via the vagus nerve, is a matter of debate. The plexiform connections that may be demonstrated between the accessory nerve and the vagus nerve within the posterior cranial fossa are too variable and insubstantial to support this function.

Laryngeal nerve anastomoses

Anastomotic communications between the internal, external and recurrent laryngeal nerves probably explain why normal or near normal voice function can be present following neck surgery when one or more branches of these nerves have been divided ( , , ). Numerous communications have been described ( , ). The ansa Galeni, lying on the interarytenoid muscles, forms a direct connection between the recurrent and internal laryngeal nerves: it may be a single or double trunk or a plexus. The cricoid communication connects branches that originate bilaterally from the recurrent laryngeal nerve with the superior branch of the deep portion of the arytenoid plexus. The thyroarytenoid communication links the ascending branch of the recurrent laryngeal nerve and the descending branch from the anterior branch of the internal laryngeal nerve. There are also communications between the external laryngeal and recurrent laryngeal nerves, the internal and external laryngeal nerves and the recurrent laryngeal and superior laryngeal nerves. The anastomosis between the recurrent laryngeal nerve and the sympathetic trunk probably explains why ptosis is a rare complication following thyroidectomy.

Right vagus nerve

The right vagus nerve descends posterior to the right internal jugular vein in the neck and crosses the first part of the right subclavian artery to enter the thorax; the ansa subclavia is lateral, and the phrenic nerve is further lateral. After giving off the right recurrent laryngeal nerve, it descends through the superior mediastinum, at first posterior to the right brachiocephalic vein, and then lateral to the trachea and posteromedial to the superior vena cava. Superiorly, the right mediastinal part of the parietal pleura and superior lobe of the lung are lateral; inferiorly, the azygos vein and its arching terminal portion separate the right vagus nerve from the parietal pleura (see Fig. 56.7A ). The nerve next passes posterior to the right main bronchus and lies on the posterior aspect of the right lung hilum. Here, it gives off posterior bronchial branches that unite with rami from the second to fifth/sixth thoracic sympathetic ganglia to form the right posterior pulmonary plexus. Two or three branches descend from the inferior part of this plexus on the posterior aspect of the oesophagus to join a left vagal branch and form the posterior oesophageal plexus. The anterior vagal trunk, containing nerve fibres from both vagus nerves, leaves the plexus and passes inferiorly on the posterior surface of the oesophagus, entering the abdomen by passing through the oesophageal hiatus in the respiratory diaphragm.

Left vagus nerve

The left vagus nerve enters the thorax posterior to the left brachiocephalic vein, between the left common carotid artery (anterior) and left subclavian artery (posterior). It descends through the superior mediastinum, crosses the left side of the aortic arch, gives off the left recurrent laryngeal nerve and then passes posterior to the left lung hilum (see Fig. 56.7B ). Superior to the aortic arch, it is crossed anterolaterally by the left phrenic nerve, and on the arch, it is crossed laterally by the left superior intercostal vein and sometimes by a continuation of the accessory hemiazygos vein. Posterior to the hilum of the left lung, the left vagus nerve gives off posterior bronchial branches that unite with rami of the second to fourth thoracic sympathetic ganglia to form the left posterior pulmonary plexus. Two or three branches descend anteriorly on the oesophagus and join with a ramus from the right posterior pulmonary plexus to form the anterior oesophageal plexus. The posterior vagal trunk, containing nerve fibres from both vagus nerves, passes inferiorly on the anterior surface of the oesophagus and enters the abdomen through the oesophageal hiatus. Numerous branches from the anterior vagal and left sympathetic trunks are distributed to the descending thoracic aorta. This pathway transmits the typical intense, excruciating pain of an intramural aortic haematoma from aortic dissection or laceration.