FACIAL NERVE

Introduction

It is the seventh cranial nerve. It is the nerve of second branchial arch.

Surface Marking

Marked by a short horizontal line which joins the following two points

A point at the middle of the anterior border of the mastoid process. The stylomastoid foramen lies 2 cm deep to this point.

A second point behind the neck of the mandible. Here the nerve divides into its 5 branches for the facial muscles.

Functional Components Special visceral or branchial efferent, to

muscles responsible for facial expression and for elevation of the hyoid bone.

General visceral efferent or parasympathetic. These fibers are secretomotor to the submandibular and sublingual salivary gland, lacrimal gland, and glands of the nose, the palate and the pharynx.

General visceral afferent component carries afferent impulses from the previously mentioned glands.

Special visceral afferent fibers carry taste sensations from the anterior two-thirds of the tongue except from valate papillae and from palate.

General somatic afferent fibers probably innervate a part of the skin of the ear. The nerve

does not give any direct branches to the ear, but some fibers may reach it through communications with the vagus nerve. Proprioceptive impulses from muscles of face travel through branches of trigeminal nerve to reach the mesencephalic nucleus of the nerve.

Nuclei

The fibers of the nerve arise from 4 nuclei situated in the lower pons.

1. Motor nucleus of branchiomotor2. Superior salivatory nucleus or

parasympathetic3. Lacrimatory nucleus is also parasympathetic4. Nucleus of tractus solitarius which is

gustatory and also receives afferent fibers from the glands

The motor nucleus lies deep in the reticular

formation of the lower pons. The part of the nucleus that supplies

muscles of the upper part of the face receives corticonuclear fibers from the motor cortex of both the right and left sides.

The part of the nucleus that supplies muscles of the lower part of the face receive corticonuclear fibers only from the opposite cerebral hemisphere.

Course and Relations

The facial nerve is attached to the brainstem by 2 roots, motor and sensory (nervus intermedius).

The 2 roots of the facial nerve are attached to the lower border of the pons just medial to the 8th cranial nerve.

The 2 roots run laterally and forwards, with the 8th nerve to reach the internal acoustic meatus.

Inside the meatus, the motor root lies in a

groove on the 8th nerve, with the sensory root intervening.

Here the 7th and 8th nerves are accompanied by labyrinthine vessels.

At the bottom or fundus of the meatus, the 2 roots. Sensory and motor use to form a single trunk, which lies in the petrous part of temporal bone.

Within the canal, the course of the nerve can be divided into 3 parts by 2 bends.

The first part is directed laterally above the vestibule

The second part runs backward in relation to the medial wall of the middle ear, above the promontry.

The third part is directed vertically downwards behind the promontry.

The 1st bend at the junction of the 1st and 2nd parts is sharp. It lies over the anterosuperior part of the promontry called the genu.

The 2nd bend is gradual, and lies b/w promontry and aditus to mastoid antrum

The facial nerve leaves the skull by passing through the stylomastoid foramen.

In its extracranial course, the facial nerve crosses the lateral side of the base of styloid process.

It enters the posteromedial surface of parotid gland, runs forwards though the gland crossing the retromandibular vein and ECA.

Behind the neck of the mandible it divides into its 5 terminal branches which emerge along the anterior border of the parotid gland.

BRANCHES AND DISTRIBUTION

Within the facial canal

Greater petrosal nerve Nerve to stapedius : arises opposite the

pyramid of middle ear and supplies the stapedius muscle. The muscle damps excessive vibrations of the stapes caused by high – pitched sounds. In paralysis of the muscle even normal sounds appear too loud and is known as hyperacusis.

Chords tympani : arises in the vertical part of facial canal about 6 mm above the stylomastoid foramen. It carries preganglionic secretmotor fibers to the submandibular and sublingual salivary glands and taste fibers from the anterior two-thirds of the tongue.

At its exit from stylomastoid foramen Posterior auricular : arises just below the

stylomastoid foamen and supplies auricularis posterior, occipitalis and intrinsic muscles on the back of auricle.

Digastric : arises close to the posterior auricular. It is short and supplies the posterior belly of the digastric.

Stylohyoid : arise with the digastric branch, is long and supplies stylohyoid muscle.

Terminal branches within parotid gland Temporal : cross zygomatic arch and

supply auricularis anterior, auricularis superior, intrinsic muscles on the lateral side of ear, frontalis, orbicularis oculi, corrugator supercili.

Zygomatic : run across the zygomatic bone and supply the orbicularis oculi.

Buccal : 2 in number. Upper branch runs above the parotid gland and lower below it

and supplies the muscle in that vicinity. Marginal mandibular : runs below the angle

of mandible deep to platysma. It crosses the body of mandible and supplies muscles of the lower lip and chin.

Cervical : emerges from apex of parotid gland, and runs downwards and forwards in the neck to supply the platysma.

Communicating branches with adjacent cranial & spinal nerves

For effective coordination b/w the movements of the muscles of the 1st, 2nd and 3rd branchial arches, the motor nerves of the 3 arches communicate with each other.

The facial nerve also communicates with the sensory nerves distributed over its motor territory.

GANGLIA

Geniculate ganglion

Located on the 1st bend of facial nerve, in relation to the medial wall of the middle ear.

It is a sensory ganglion. The taste fibers present in the nerve are

peripheral processes of pseudounipolar neurons present in the geniculate ganglion.

Submandibular ganglion

Parasympathetic ganglion. For relay of secretomotor fibres to the

submandibular and sublingual glands.

Pterygopalatine ganglion

Parasympathetic ganglion. Secretomotor fibers meant for the

lacrimal gland relay in this ganglion.

Clinical anatomy Supranulear and infranuclear lesions. In supranulear lesions; usually a part of

hemiplegia, only lower part of the opposite side of the face is paralyzed. The upper part with the frontalis and orbicularis oculi escapes due to bilateral representation in the cerebra cortex.

In infranuclear lesions, known as Bell’s palsy, the whole of the face of the same side gets paralyzed. The face becomes asymmetrical and is drawn up to the normal side. The affected side is motionless. Wrinkles disappear from the forehead. Eye cannot be closed. Food accumulates b/w cheek and teeth during mastigation.

The symptoms according to the level of injury of facial nerve.

At internal auditory meatus; loss of lacrimation, stapedial reflex, taste from most of anterior two-third of tongue, lack of salivation and paralysis of muscles of facial expression.

Below geniculate ganglion; loss of stapedial reflex, taste from anterior two-

third of tongue, lack of salivation and paralayis of facial expression muscles.

Region b/w nerve to stapedius and chorda tympani : loss of taste from anterior two-third of tongue, lack of salivation and paralysis of facial expression muscles.

Region below stylomastoid foramen : paralysis of facial expression muscles.