FACIAL NERVE

Dr Shermil sayd

Introduction

Seventh cranial nerve Nerve of the second branchial arch Motor nerve supply of the face

Surface marking

It is 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 2cm deep to this point

A second point behind the neck of the mandible. Here the nerve divides into its five branches

Functional Components

1. Special visceral or branchial efferent-for muscles responsible for facial expression and for elevation of the hyoid bone

2. General visceral efferent or parasympathetic- they are secretomotor to the submandibular and sublingual salivary glands, lacrimal glands & glands of the nose.

3. General visceral afferent- carries afferent impulses from the above mentioned glands

4. Special visceral afferent fibres- carry taste sensations from the anterior two third of the tongue, except from vallate papillae & from the palate

5. General somatic afferent- probably innervate a part of the skin of the ear. This nerve doesn’t give any direct branches to the ear. But may reach it through the communication with the vagus nerve.

Nuclei

The four nuclei are presented in the lower pons

1. motor nucleus or the branchiomotor

2. superior salivary nucleus or parasympathetic

3. lacrimatory nucleus is also parasympathetic

4. nucleus of the tractus solitarius which is gustatory and receives afferent fibres from the glands

Motor nucleus lies deep in the reticular formation of the lower pons

The part of the nucleus that supplies the muscles of the upper part of the face receives corticonuclear fibres from the motor cortex of the both left and right sides.

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

Course and relations(intracranial)

Attaches to the brain stem by two roots, motor & sensory (nervus intermedius)

Attached to the lateral part of the lower border of the pons just medial to the eighth cranial nerve

Two roots run laterally forward to reach the internal acoustic meatus

In the meatus, motor root lies in a groove on the 8th nerve, with sensory root intervening & accompanied by the labyrinthine vessels

At bottom or fundus of the root , sensory and motor join to form a single nerve trunk which lies in the petrous temporal bone

Within the canal the nerve course is divided in 3 parts by two bends

1. Directed laterally above the vestibule

2. Runs backwards and in relation to the medial wall of the middle ear, above the promontory

3. Vertically downwards behind the promontory

1st bend at the junction of 1st and 2nd part is sharp, lies over the anterosuperior part of the promontory, Also called as the genu.

it is called so because it lies on the genu 2nd bend is gradual and lies between the promontory and

the aditum to the mastoid antrum Leaves the skull through the stylomastoid foramen

Course and relations(extracranial)

Facial nerve crosses the lateral side of the base of the styloid process

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

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

Branches and distribution

Within the facial canal

1. Greater petrosal nerve

2. Nerve to the stapedius

3. Chorda tympani

As it exits from the stylomastoid foramen

1. Posterior auricular

2. Digastric

3. Stylohyoid

Terminal branches within the parotid gland

1. Temporal

2. Zygomatic

3. Buccal

4. Marginal Mandibular

5. Cervical Communicating branches with the adjacent cranial and

spinal nerves

Greater petrosal nerve

Carries gustatory and parasympathetic fibres Arises from the geniculate ganglion of the facial nerve,

enters the middle cranial fossa through the hiatus for the greater petrosal nerve on the anterior surface of the petrous temporal bone

It proceeds towards the foramen lacerum Where it joins the deep petrosal nerve which carries

sympathetic fibres to form the nerve of the pterygoid canal

Nerve to the stapedius

Arises opposite the pyramid of the middle ear, and supplies the stapedius muscle

Damps excessive vibrations of the stapes caused by high pitched sounds.

In paralysis, it causes hyperacusis

Chorda tympani nerves

Arises in the vertical part of the facial canal about 6mm above the stylomastoid foramen

Runs upwards and forwards in a bony canal Enters the middle ear and runs forwards in close relation

to the tympanic membrane Leaves the middle ear by passing through the

petrotympanic fissure It then passes medial to the spine of the sphenoid and

enters the infratemporal fossa. Joins the lingual nerve through which it is distributed

Carries

1. Preganglionic secretomotor fibres to the submandibular ganglion for supply of the submandibular and sublingual salivary glands

2. Taste fibres from the anterior two thirds of the tongue

Posterior auricular nerve

Arises just below the stylomastoid foramen Ascends between the mastoid process and the external

acoustic meatus and supplies1. The auricularis posterior

2. The occipitalis

3. The intrinsic muscles on the back of the auricle

Digastric branch

Arises close to the previous nerve It is short and supplies the posterior belly of the digastric

Stylohyoid branch

Arise with the digastric branch Its long and supplies the stylohyoid muscle

Temporal branches

Crosses the zygomatic branch– auricularis anterior– Auricularis superior– Intrinsic muscles on the lateral side of the ear– Frontalis– The orbicularis occuli– Corrugator supercili

Zygomatic branch

Runs across the zygomatic bone and supply the orbicularis occuli

Buccal branches

Two branches1. Upper- runs above the parotid duct

2. Lower- runs below the duct

They supply the muscles in the vicinity, i.e. muscles of the cheek and upper lip

Marginal mandibular branch

Runs below the angle of the mandible deep into the platysma

Crosses the body of the mandible and supplies muscles of the lower lip and the chin

Cervical branch

Emerges from the apex of the parotid gland Runs downwards and forwards in the neck to supply the

platysma

Communicating branches

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

Also communicates with the sensory nerves distributed over its motor territory

Ganglia

Three ganglions1. The geniculate ganglion is situated on the 1st bend of the

facial nerve, in relation to the medial wall of the middle ear. A sensory ganglion. Taste fibers present are peripheral processes of pseudounipolar neurons present in the geniculate ganglion

2. Submandibular ganglion -parasympathetic ganglion for relay of secretomotor fibres to the submandibular and sublingual glands

3. Pterygopalatine ganglion is also a parasympathetic ganglion

Clinical anatomy

Facial nerve paralysis

Facial nerve paralysis is the most common complication in dental practice

Paralysis of some of its branches occur whenever an infraorbital block/max. canine infiltration given

Muscle droop is observed when the LA solution is deposited in the deep lobe of the parotid gland, through which terminal portions of the facial nerve extends, which is a transient condition

Duration depends upon the duration of action of the LA solution injected

Patient has unilateral facial muscle paralysis & be unable to use these muscles

Face appears lopsided No treatment other than waiting until the action of the

drug resolves Patient is unable to voluntarily close one eye Protective lid reflex of one eye is abolished, but the

corneal reflex is normal

Bell’s palsy

Facial weakness Evidence for herpes simplex type 1 infection causing

infranuclear lesions Paralysis: Progresses to maximal deficit over 3 to 72

hours Pain (50%): Near mastoid process Hyperacusis Facial weakness Sensory loss is Mild or None

Food accumulates between the teeth and cheek Labial articulation is impaired

Supra nuclear lesion

Its usually a part of the hemiplegia Only the lower part of the opposite side of the face is

paralysed The upper part with the frontalis and orbicularis occuli

escapes due to its bilateral representation in the cerebral cortex

VII disorders

Unilateral nerve paralysis– Leprosy– Lyme disease– Neoplasm and masses– Trauma– Cardiofacial syndrome

VII disorders

Bilateral nerve paralysis

1. Melkersson syndrome

2. Möbius syndrome & Congenital facial paresis

3. Guillain barre disease

4. Leprosy

5. HIV infection

6. Myasthenia gravis

Parotid gland relation

During the removal of parotid gland, the facial nerve is preserved by removing the glands in two parts, superficial and deep separately.

The plane of cleavage is defined by tracing the nerve from behind, forwards

Mixed parotid tumour is a slowly growing parotid tumour which doesn’t involve the facial nerve, but when it turns malignant, it then involve the facial nerve

TMJ relation

Temporal branches of the facial nerve is related to the lateral aspect of the TMJ

This leads to invariable damage to the facial nerve during surgical correction of TMJ ankylosis

This can mostly avoided by taking strict care during the preocedure

Indian Journal of Dental Research. 2013 Jul-Aug;

Conclusion

Facial nerve is a nerve which is mostly motor in function, but also plays a small role in taste sensation. Its motor function is for the muscles of facial expression, which is important for a good quality of life. So every care should be taken to preserve these nerves, whatever the case may be.

References

Oral and maxillofacial surgery-Daniel M laskin Local anesthesia- malamed Differential diagnosis of oral and maxillofacial lesions-wood goaz Contemporary oral and maxillofacial surgery-peterson Human anatomy-chaurasia Indian Journal of Dental Research. 2013 Jul-Aug; Melkersson-Rosenthal syndrome and orofacial granulomatosis-

Dermatol Clin. 1996 Apr;14(2):371-9. Bell palsy in lyme disease-endemic regions of canada: a cautionary

case of occult bilateral peripheral facial nerve palsy due to Lyme disease-CJEM. 2012 Sep;14(5):321-4.

Clinical spectrum of peripheral facial paralysis in HIV-infected patients according to HIV status-int J STD AIDS. 2013 Mar 6.

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