Facial Nerve (VIIth Cranial Nerve)
1.
Origin
2.
Nuclei
3.
Course
4.
Distribution
5.
Branches
6.
Functional columns
7.
Ganglia involved
8.
Supranuclear connections
9.
Reflexes involving facial nerve
10.
Applied Aspect
Introduction-
Facial nerve is 7th cranial nerve involved in both sensory and motor
functions. Its major part is motor and small part is sensory. Apart from
giving motor supply to muscles of facial expressions and muscles of second
pharyngeal arches, it also gives motor supply to glands of head and face
regions namely lacrimal, sublingual and submandibular glands. Small part
also contribute for sensory supply to external ears.Special sensory
afferent fibers for taste supply anterior 2/3rd of tongue except
circumvallate papillae.
Cranial Nerve VII is the most commonly paralysed of all peripheral nerves,
owing to the great length of its canal in the temporal bone, where it is
at risk of compression when swollen.
Origin-
Facial nerve originates from lower part of Pons. Fibers originate from
nuclei of facial nerves namely;
motor nucleus of facial, lacrimatory necleus, superior salivatory
nucleus, nucleus of tractus solitarius and fibers from spinal nucleus of
trigeminal. These fibers loop around the nucleus of abducens nerve in pons at the
level of facial colliculli, elevation present in floor of fourth
ventricle. This looping is internal genu of facial nerve and
looping cause elevation termed as Facial colliculus. It comes out
from brainstem at the level of pontomedullary junction inbetween
pons and olive of medulla. Nerve fibers lie between abducens nerve
medially and vestibullocochlear nerve laterally. It has two types of
fibers. Motor fibers are present medially and sensory is laterally. Also,
motor fibers are more thicker as compared to sensory fibers. Sensory
fibers are known as Nervus intermedius.
Nuclei of Facial nerve-
1.
Special Visceral (Branchial) Efferent Nuclei
Motor Nucleus of facial
-
Situated in the lower part of the pons
-
Fibres arising from the nucleus - course dorsomedially
towards the floor of fourth ventricle
-
Loop behind the motor nucleus of the abducent nerve loop (internal genu of facial nerve)
-
Elevates the floor of fourth ventricle - forms the
facial colliculus
2.
General Visceral Efferent Nuclei
Superior salivatory and lacrimatory nuclei
-
Dorsal part of pons medial to the motor nucleus of
facial nerve
-
Preganglionic fibres from
superior salivatory nucleus pass to the submandibular ganglion through the nervus intermedius- geniculate ganglion, facial nerve -
chorda tympani branch
-
Preganglionic fibres from
lacrimatory nucleus reach the pterygopalatine ganglion through nervus intermedius and greater petrosal nerve
3.
Special Visceral Afferent Nuclei
Nucleus of solitary tract
-
Rostral portion of the nucleus is concerned with taste sensations
-
Receives the special visceral afferent fibres from facial,
glossopharyngeal and vagus nerves and is frequently referred to as
Gustatory nucleus
4.
General somatic afferent (GSA)
Cell bodies in the geniculate
ganglion
-
peripheral processes of these cells innervate part of the skin of the
external ear, while the central processes terminate in the
Spinal nucleus of trigeminal nerve
Branches of Facial Nerve
1.
Greater petrosal nerve
joined by
deep petrosal nerve to form nerve of pterygoid canal
provides
secretomotor supply to lacrimal, nasal and palatal glands.
2. Nerve to stapedius.
3. Chorda tympani nerve
joins lingual nerve & carry taste fibres from
anterior two-third of
the
tongue and provides preganglion fibres to the submandibular
ganglion
5.
Posterior auricular nerve supply occipitalis and posterior
auricular muscles
6. Nerve to posterior belly of diagastric and
stylohyoid muscles.
7. Terminal branches (temporal, zygomatic, buccal, mandibular
and
cervical)
to the muscles of facial expression.
Functional Column of Facial nerve
Course of Facial Nerve-
A)
INTRACRANIAL COURSE
1.
In Brainstem:
Facial nerve originates from lower part of Pons. Fibers originate from
nuclei of facial nerves namely;
motor nucleus of facial, lacrimatory necleus, superior salivatory
nucleus, nucleus of tractus solitarius and fibers from spinal nucleus of
trigeminal. These fibers loop around the nucleus of abducens nerve in pons at the
level of facial colliculli, elevation present in floor of fourth
ventricle. This looping is internal genu of facial nerve and
looping cause elevation termed as Facial colliculus. It comes out
from brainstem at the level of pontomedullary junction inbetween
pons and olive of medulla. Nerve fibers lie between abducens nerve
medially and vestibullocochlear nerve laterally. It has two types of
fibers. Motor fibers are present medially and sensory is laterally. Also,
motor fibers are more thicker as compared to sensory fibers. Sensory
fibers are known as Nervus intermedius.
2.
Pontomedullary junction to stylomastoid foramen (Intrapetrous
part)-
Motor and sensory root of facial nerve pass along with vestibulocochlear
nerve and labyrinthine vessels through internal acoustic meatus. At
the bottom of meatus , both roots join together to form trunk of facial
nerve to enter the bony facial canal. In facial canal, facial nerve
passes above the vestibule to reach medial wall of tympanic cavity and
bends backward. The bend is known as external genu of facial nerve,
having swelling known as Geniculate ganglion, containing cell
bodies of pseudo-unipolar neurons. It passes backward and downward above
the promontory to reach medial wall of aditus to mastoid antrum. Then, it
passes vertically downward along posterior wall of tympanic cavity and
exits through the stylomastoid foramen.
At the point of external genu, it gives greater petrosal nerve as a
branch consisting of parasympathetic fibers to supply lacrimal gland.
Before exit to stylomastoid foramen, it gives nerve to stapedius, to
supply stapedius muscle and chorda tympani nerve, having special sensory
and parasympathetic fibers to supply anterior 2/3rd of tongue
except circumvallate papilla for taste sensation and sublingual and
submandibular gland for secretions.
B)
EXTRACRANIAL COURSE
Facial nerve exits from stylomastoid foramen. It passes
superficial to styloid process of temporal bone and pierces the
postero-medial surface of parotid gland. Before that, it gives
posterior auricular branch to supply occipital belly of
occipito-frontalis and auricularis posterior and intrinsic muscles of
auricle. It also gives branches named as
nerve to posterior belly of diagastric and nerve to stylohyoid
muscle. Within the parotid gland it is related superficial to retromandibular
vein and external carotid artery. It divides into
temporo-facial and cervico-facial trunks. Temporo-facial
trunk gives out temporal and zygomatic branches.
Cervico-facial trunk gives out buccal,
marginal mandibular and cervical branches. Five terminal
branches look like goose feet, known as pes anserinus.
Facial Nerve in Nut-Shell
Motor
Muscles of
face, cheek, and scalp;
Stapedius,
muscle of middle ear;
stylohyoid;
and posterior belly of digastric
Sensory
Taste from
anterior two thirds of tongue, floor
of mouth,
and palate, general sensation from external ear
Secretomotor
(Parasympathetic)
Submandibular and sublingual salivary glands, lacrimal gland, and
glands of nose and Palate
Passes through
(Foramens)
Internal
acoustic meatus, facial canal, stylomastoid foramen
Facial Muscle Paralysis
Ø The facial muscles are innervated by the facial nerve. Damage to the
facial nerve in the internal acoustic meatus (by a tumor), in the middle
ear (by infection or operation), in the facial nerve canal (perineuritis,
Bell’s palsy), or in the parotid gland (by a tumor) or caused by
lacerations of the face will cause distortion of the face, with drooping
of the lower eyelid, and the angle of the mouth will sag on the affected
side. This is essentially a lower motor neuron lesion.
Ø An upper motor neuron lesion (involvement of the pyramidal tracts) will
leave the upper part of the face normal because the neurons supplying this
part of the face receive corticobulbar fibers from both cerebral cortices.
Testing the Integrity of the Facial Nerve
Ø If the 6th and 7th nerves are not functioning, this would suggest a
lesion within the pons of the brain.
Ø If the 8th and 7th nerves are not functioning, this would suggest a
lesion in the internal acoustic meatus.
Ø If the patient is excessively sensitive to sound in
one ear, the lesion probably involves the nerve to the stapedius.
Ø Loss of taste over the anterior two thirds of the
tongue implies that the seventh nerve is damaged proximal to the point
where it gives off the chorda tympani.
Ø To test the facial nerve, the patient is asked to show the teeth by
separating the lips with the teeth clenched, and then to close the eyes.
Taste on each half of the anterior two thirds of the tongue can be tested
with sugar, salt, vinegar, and quinine for the sweet, salt, sour, and
bitter sensations, respectively.
Ø part of the facial nerve nucleus that controls the muscles of the upper
part of the face receives corticobulbar fibers from both cerebral
cortices. Therefore, in patients with an upper motor neuron lesion, only
the muscles of the lower part of the face will be paralyzed.
Ø However, in patients with a lower motor neuron lesion, all the muscles on
the affected side of the face will be paralyzed. The lower eyelid will
droop, and the angle of the mouth will sag. Tears will flow over the lower
eyelid, and saliva will dribble from the corner of the mouth. The patient
will be unable to close the eye and cannot expose the teeth fully on the
affected side.
