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Facial Nerve

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					Facial Nerve
The facial nerve is the seventh cranial nerve. It is attached to the brainstem by two roots: a large motor root, and a smaller sensory root These roots are attached in the lateral part of the groove between the lower border of the ports and the upper border of the medulla. The motor root is medial to the sensory root. (Note: As in the case of the trigeminal nerve, M for motor and M for medial). The sensory root is attached midway between the motor root (medially) and the vestibulocochlear nerve (laterally). It is, therefore, called the nervus intermedius.

From this attachment the motor and sensory roots pass forwards and laterally and leave the posterior cranial fossa by entering the internal acoustic meatus (on the posterior aspect of the petrous temporal bone). The nerve has a complicated course through the substance of the petrous temporal bone. This part of the nerve bears the genicular ganglion (so called because it is situated on a sharp bend of the nerve). The nerve emerges on the base of the skull through the stylomastoid foramen. It immediately enters the substance of the parotid gland and runs forwards within it and ends behind the neck of the mandible by dividing into several branches. Intrapontine course: The facial nucleus lies deep in the reticular formation of the pons, medial to the spinal nucleus of the trigeminal nerve. The fibres arising in the nucleus follow an unusual course. They fiist run backwards and medially to reach the lower pole of the abducent nucleus. They then ascend on the medial side of that nucleus. Finally, the fibres turn forwards and laterally passing above the upper pole of the abducent nucleus. As they pass forwards the fibres lie between the facial nucleus, medially; and the spinal nucleus and tract of the

trigeminal nerve laterally. The abducent nucleus and the facial nerve fibres looping round it together form a surface elevation, the facial colliculus, in the floor of the fourth ventricle.

Course through the petrous temporal bone:

On reaching the bottom of the internal acoustic meatus the facial nerve enters a bony canal. Within this canal the nerve first passes laterally above the vestibule (1) to reach the anterosuperior angle of the medial wall of the middle ear. At this point (2) the nerve bends sharply backwards. This bend is thickened by the presence of the genicular ganglion. The nerve then runs horizontally backwards in a canal projecting into the medial wall of the middle ear (3), This canal lies above the promontory and the fenestra

vestibuli; and below the bulge produced by the lateral semicircular canal. Reaching the Junction of the medial and posterior walls of the middle ear (4) the nerve turns downward. Here it lies medial to the aditus into the tympanic antrum. Continuing downwards along the junction of the medial and posterior wall of the. middle ear (5) the nerve reaches the Stylornastoid foramen through which it leaves the skull. Course through parotid gland:

As the facial nerve runs forwards through the parotid gland it crosses the styloid process, the retromandibular vein and the external carotid artery . It divides into several branches while still within the gland. These branches emerge from the anteromedial surface of the gland and come into view along the anterior margin of the gland Branches of the facial nerve: They are as follows: (1) The greater petrosal nerve arises from the genicular ganglion. (2) The nerve to the stapedius arises from the facial as the latter turns downwards along the junction of the medial and posterior walls of the middle ear. It runs forwards through a short canal in the petrous temporal bone to reach the stapedius. (3) The chorda tympani arises from the intrapetrous part of the facial nerve about 6 mm above the Stylomastoid foramen. (4) The posterior auricular nerve is given off just after the facial nerve emerges from the Stylomastoid foramen. It runs upwards into the scalp passing behind the external acoustic meatus. It divides into an auricular branch, which supplies some muscles of the auricle; and an occipital branch which supplies the occipital belly of the occipitofrontalis. (5) The nerve to the posterior belly of the digastric muscle and the nerve to the stylohyoid arise near the stylomastoid foramen. They end by supplying the muscles concerned. (6) The remaining branches of the facial nerve arise within the parotid gland. (7) The temporal branches enter the scalp in the temporal region. They supply the frontal belly of the occipitofrontalis, the corrugator supercilii, and some muscles of the auricle. Some twigs are also given to the orbicuiaris oculi.

(8) (9)

The zygomatic branches supply the orbicuiaris oculi. The buccal branches are in two sets upper and lower. The upper branches (sometimes cailed the lower zygomatic branches) supply the zygomaticus major and minor, the levator labii superioris, the levator anguli oris, the levator labii superioris alaeque nasi, and some small muscles related to the nose. The lower buccal branches supply the buccinator and the orbicularis oris. (10) The marginal mandibular branch is related to the lower border of the mandible. It supplies the muscles of the lower lip and chin. (11) The cervical branch emerges from the parotid gland near its lower end. It enters the neck and supplies the platysma.

Greater Petrosal Nerve

The greater petrosal nerve arises from the genicular ganglion. It passes through a canal in the petrous temporal bone and reaches its anterior surface by passing through an aperture called the hiatus for the greater petrosal nerve. The nerve then runs forwards and medially in a groove on the anterior surface of the bone, passing deep to the trigeminal ganglion. Reaching the foramen lacerum the greater petrosai nerve ends by joining the deep petrosal nerve to form the nerve of the pterygoid canal. The deep petrosal nerve consists of sympathetic fibres derived from the plexus around the internal carotid artery. The nerve of the pterygoid canal passes forwards in the pterygoid canal to enter the pterygopalatine fossa. The nerve ends by joining the pterygopaiatine ganglion. The greater petrosal nerve and the nerve of the pterygoid canal serve as pathways for secretomotor fibres to the lacrimal gland and to the glands of the nasal and palatine mucosa.

Chorda Tympani

The chorda tympani is so called because it has an intimate relationship to the middle ear (tympanum). It arises from the facial nerve as the latter descends towards the stylomastoid foramen. The origin is about 6 mm above the foramen. The nerve enters the middle ear through the posterior canaliculus for the chorda tympani, which opens on the posterior wall of the middle ear, close to the posterior part of the margin of the tympanic membrane. The nerve now passes forwards through the substance of the tympanic membrane lying between its fibrous basis and the mucous membrane lining its internal surface. As it does so it crosses the handle of the malleus (which is embedded in the membrane). Reaching the anterior margin of the tympanic membrane the chorda tympani enters the anterior canaliculus (in the anterior wall of the middle ear), It passes through this canaliculus and emerges on the base of the skull through the medial end of the petrotympanic fissure. Passing medially forwards and downwards the nerve crosses medial to the spine of the sphenoid. The chorda tympani ends by joining the lingual nerve from behind. To do so it has to pass deep to the inferior alveolar nerve. Thejunction of the chorda tympani with the lingual nerve lies deep to the lateral pterygoid muscle. Functional components of the facial nerve Now that we have considered the branches of the facial nerve, can review its functional components. (1) We have seen that most of the branches of the nerve are motor. Many of them supply the muscles of facial expression. Motor fibres also supply the occipitofrontalis, the muscles of the auricle, the stapedius, the platysma, the stylohyoid and the posterior belly of the digastric. All these muscles are derived from the mesoderm of the second branchial arch. The nerve fibres concerned in their innervation arise in the facial nucleus which lies in the lower part of the pons, and belongs to the special visceral efferent column. The axons of neurons located in this nucleus collect to form the motor root of the facial nerve. (2) The facial nerve also contains general visceral efferent fibres which are as follows: (a) Preganglionic secretomotor fibres for the subrnanclibular and sublingual giands arise from neurons located in the superior sativatory nucleus. (This nucleus lies in the lower part of the pons). The fibres leave the pons through the nervus intermedius and run for

some distance in the intrapetrous part of the facial nerve. They then enter the chorda tvmpani to reach the lingual nerve. They leave the lingual nerve through branches to the submandibular ganglion. Postganglionic neurons are located in this ganglion. Some of the nerve fibres arising from them supply the submandibular gland. Others re-enter the lingual nerve and its distal part to reach the sublingual gland. Preganglionic secretomotor fibres for the lacrimal gland arise in the lacrimatory nucleus, which is believed to lie near the salivatory nuclei. They leave the pons through the nervus intermedius, pass into the greater petrosal nerve and through it into the nerve of the pterygoid canal to end in the pterygopalatine ganglion. Postganglionic neurons are located in this ganglion. Fibres arising from them pass successively through a ganglionic branch connecting the pterygopalatine ganglion to the maxillary nerve; the maxiliary nerve itself; its zygomatic branch; the zygomsticotemporal branch of the zygomatic nerve; the loop of communication between the zygomaticotemporal and lacrimal nerves; and finally through the lacrimal nerve to reach the lacrimal gland. (c) Preganglionic secretomotor fibres for glands in the nasal and palatine mucoss arise in neurons the location of which is uncertain. They probably lie near the salivatory nuclei. The preganglionic fibres follow the same path as for the lacrimal gland. Postganglionc fibres arising in the pterygopalatine ganglion pass through its greater and lesser palatine branches to reach glands in the palate; and through its nasal branches to reach glands in the nasal mucosa (3) The facial nerve contains special visceral afferent fibres that carry the sensations of taste from the part of the tongue in front of the sulcus terrninalis; and from the soft palate. These fibres are processes of unipolar neurons located in the genicular ganglion. Peripheral processes reach the tongue by passing successively through part- of the intrapetrous segment of the facial nerve, the chorda tympani and the lingual nerve. Those for the soft palate pass through the greater petrosal nerve, the nerve of the pterygoid canal, the pterygopalatine ganglion (without relay), and the lesser palatine nerves. The central processes leaving the geniculsr ganglion pass through the nervus intermedius to reach the brainstem. Here they terminate in relation to the upper part of the nucleus of the solitary tract.

Clinical Significance Paralysis of facial nerve – refer page 942 I.B Singh


				
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