Facial nerve is a Sensorimotor nerve with the following functions
Special visceral efferent (facial muscle)
General visceral efferent (submandibular, sublingual and lacrimal glands)
Special visceral afferent (taste from anterior two-thirds of tongue)
General somatic afferent (sensation from external auditory meatus, mastoid and pinna)
The upper half of the face has a bilateral representation ,whereas the lower half of the face has unilateral representation.
Nuclei of facial nerve are 4 in number
Motor nucleus contains dorsal and ventral group of cells and is situated in ventral pons
Superior Salivatory nucleus control salivation
Nucleus of tractus solitaries carry taste sensation from anterior 2/3 rd of tongue and sensation from external auditory meatus
Lacrimal nucleus. for lacrimation
There is two types of movements voluntary and reflexmovements Volitional movement
There is supranuclear connection for volitional movement
It is unique for facial nerve
For Volitional movement the fibers start from lower part of precentral gyrus , corticonuclear fibers descend to pons and cross to opposite facial nucleus. Nucleus for the superior half of facial muscles receiving ipsilateral and contralateral supranuclear fibers. Nucleus for the inferior half of facial muscles mainly receive contralateral supranuclear fibers .This explains why the upperhalf is spared in UMN lesion and only lower half is affected.
Variation
Occasionally lower half of face also has ipsilateral supranuclear innervation but it is less than contralateral innervation. This will result in only paresis of lower half in UMN lesion, if ipsilateral supranuclear innervation is also equal to contralateral innervation both upper half and lower half may escape in UMN lesion.
Rarely upper half will have innervation predominantly from contralateral fibers. Thus in UMN lesion there is extension of weakness to upper half of face also. For reflex movement
Fibers from premotor area, extrapyramidal center, basal ganglia, through separate pathway, innervate the Nucleus from both sides but predominantly from Right cortex.
Lesion of this pathway produce Mimic facial palsy. Nervus intermedius
It is the Sensory counter part of facial nerve,it carry fibers of Superior Salivatory nucleus, lacrimal nucleus, and Nucleus of tractus solitarius.
It subserves the following
Somatic sensation of mastoid region ,part of pinnae, external auditory canal
Secretomotor fibers to lacrimal gland, salivary glands - sublingual and submandibular and
Visceral sensation – taste sensation from ant. 2/3rd of tongue.
Greater superficial Petrosal nerve -supplies secretomotor fibers to lacrimal gland
Braches of Vertical mastoid segment
Nerve to stapedius
Chorda tympani -arise 5 mm above the stylomastoid foramen, carry taste sensation from anterior 2/3rd of tongue. It supplies secretomotor fibers to submandibular and sublingual gland
Branches at the level of Stylomastoid foramen
Posterior auricular braches-supplies occipitalis and auricular muscles
Digastric – Posterior belly of digastric
Stylohyoid supplies stylohyoid muscle
Branches in the Parotid region Temporofacial branch
Key anatomical area you should remember in relation to anatomy of the facial nerve are the following
Pons
CerebelloPontine angle
Internal auditory meatus
Middle ear
Stylomastoid foramen
Intrapontine segment Pons
Facial nuclei is situated in the pons
Sensory and parasympathetic fibers are carried by nervus intermedius
It curves around the 6th nerve nucleus to form facial collicullus and form the first genu around the 6th Cranial nucleus Cerebellopontine Angle
Nerve emerges at the ventrolateral portion of pontomedullary junction with Nervus intermedius and 8th nerve and lies in the cp angle Meatal segment
Enters the internal auditory meatus with the 8th nerve with the nervus intermiedius in between Labyrinthine segment
It dip into the facial canal in the floor of meatal canal, reaches the medial part of tympanic cavity form the 2nd genu - geniculate ganglion – receives the Nervus intermedius.
It curves posteriorly at the genu giving the Greater superfical petrosal nerve at the genu
Then it travels backwards in the horizontal direction (tympanic segment is above the middle ear) Mastoid segment
It turns back vertically downwards to emerge through stylomastoid foramen, then turns vertically in the vertical (mastoid) segment
It gives nerve to stapedius and chorda tympani nerve in the vertical part Parotid region
The facial nerve emerge through the Stylomastoid Foramen and enters the parotid region
It emerges at the stylomastoid foramen
Leaves the parotid gland by dividing to temperofacial and cervicofacial branches
finally divides into five terminal motor branches
Normal size of pupil varies from 3 to 5 mm.
Pupils < 3mm size in average condition of illumination are called miotic and pupils > 5 mm are called mydriatic. Pin point pupil is said to be present when the pupillary size is less than or equal to 1 mm.
Normal—3-5 mm
Mydriasis > 6 mm
Miosis < 2 mm
Pin point pupil < I mm
A difference of 0.5 mm between the two pupils is abnormal
The 3rd cranial nerve-oculomotornerve
The oculomotor nuclear complex is located in the the middbrain at the level of superior colliculus. This has one unpaired and four paired nuclear columns.
The unpaired column constitute
Edinger-Westphal nucleus
Subnucleus for levator palpebrae superioris.
The paired nuclei constitutes
Subnuclei for superior, inferior and medial recti and inferior oblique.
4th cranial nerve-Trochlear nerve
Trochlear nerve passes posteriorly and the fibres from the right and left trochlear nuclei decussate on the dorsum of mid brain.
This is the only cranial nerve that emerges dorsally from the brainstem.
The left trochlear nucleus sends fibres to the right superior oblique muscle and vice versa.
6th cranial nerve-Abducent nerve
Abducent nerve has a very long intracranial course and supplies the lateral rectus muscle..
Because of its long intracranial course, this nerve is affected in conditions producing raised intracranial tension, hence producing a false localizing sign.
Myerson's sign also called as glabellar tap sign is a medical condition where a patient is unable to resist blinking when tapped on the glabella, the area above the nose and between the eyebrows. This is often an early symptom of Parkinson's disease, but it is can also be seen in early dementia as well as other progressive neurologic illness.It is named for Abraham Myerson,who is an American neurologist. How to elicit glebellar tap?
To perform this test, the examiner repeatedly taps the glabellar prominence of the patient (area between the eyebrow)lightly with his finger.
A normal individual will blink in response to the first two or three taps only. After that he adapts to it and blinking ceases.
This adaptation suffers some interference in those patients with Parkinsonism .Hence prolonged tapping continues to elicit the blink response.
Afferent both 5th and 7th Efferent - 7th CN. Center is pons. Clinical significance
Exaggerated glabellar tap is seen in supranuclear lesions of the corticopontine pathway and in extrapyramidal diseases
It is a sign of frontal, diffuse, or extrapyramidal disease History of Myerson sign
It was first described by Myerson (1944) who noticed that it was constantly present in post-encephalitic Parkinsonism though not universally so in the arteriosclerotic and “senile” forms. Wartrnburg (1952) regarded it as a definite sign of Parkinson’s syndrome, as did Doshay (1954) who gave it the eponym “Myerson’s Sign”. Nidson (1958) confirmed its diagnostic value, Schwab and England (1958) found it constantly in all forms of the disease. Garland (1952) went so far as to say “For all practical purposes, this physical sign is diagnostic of the Parkinsonian state”.
The glabellar tap is a primitive reflex where the eyes shut if an individual is tapped lightly between the eyebrows. This reflex may normally be overcome rapidly - i.e. the individual soon fails to blink, usually less than five taps.
In those patient with frontal release signs the reflex cannot be overcome, and they continue to blink for as long as the examiner cares to keep tapping. A similar response is also seen with late parkinsonism. Other names of glabellar tap
Orbicularis oculi reflex
Glabellar reflex
Nasopalpebral reflex
How to elicit glabellar tap?
The glabellar reflex is elicited by repeatedly tapping the patient between the eyebrows (the glabella area), causing them to blink.
Normally, the adult patient will habituates to this stimulus hence normal indivual will blink in response to the first two or three taps only. Thereafter he or she will adapts to it and blinking ceases
If blinking persists,that is considered abnormal in adults.
Stimulus
The examiner stand on the side of the patient and softly tap the glabellar area with a reflex hammer or finger from above (as it will avoid eliciting the blink reflex to threat).
Response: narrowing of the palpebral fissure by contraction of the orbicularis oculi muscle(possibly bilaterally)
Afferent is both 5th and 7th nerve Efferent - 7th CN. Center is pons. Clinical significance
There are primitive reflexes that are normal in infants, but they disappear with brain maturation allowing inhibition, and they reappear in disorders that affect the frontal lobes.Hence called as “released”reflex . Like most primitive reflexes.The glabellar reflex probably has evolutionary/adaptive advantage in infant apes, protecting the eyes from threat.
Exaggerated glabellar tap is seen in supranuclear lesions of the corticopontine pathway and in extrapyramidal diseases
It is a sign of frontal, diffuse, or extrapyramidal disease
Normally after a few contractions, the response ceases,
In bilateral UMN lesion and in Parkinsonism the response persists.
In LMN lesion the normal response is absent.
Persistent glabellar tap response is called Myersons sign.
How do you elicit corneal reflex ?
Ask the patient into the distance or at the ceiling. The cornea at its conjunctival margin is touched lightly with a cotton wool which is twisted into a one hair. Normally the reflex is present .There is simultaneous closure of both the eyes. Both the eyes should be tested one by one.
The cornea is stimulated from the side to avoid menace .(Closure of the eyes is seen, if an object is brought to the patient directly from the front). Afferent pathway — 5th cranial nerve (ophthalmic division). Efferent pathway — 7th cranial nerve. Centre—Pons. Response — Closure of both the eyes.
Never touch the central part of cornea because this will result in corneal ulceration in the presence of corneal anaesthesia In the absence of cotton, blowing a puff of air into each cornea will serve the purpose.
Strabismus or squint is tested by cover test or redglass test Red Glass Test
To identify the defective eye in diplopia
Patient is asked to look at an object placed in the direction of diplopia
Red glass is placed over one eye
A Red image is false if the eye over which the glass is placed is defective
A white image is false- the other eye is defective.
Principles
The outer image is the false image
The separation of the images is maximum in the direction of action of the paretic muscle Cover test
Instruct to fix the eyes on an object in front
Suddenly cover the apparently fixing eye, so that the other eye fixes
A deviation of the uncovered eye is called primary deviation
A deviation of the covered eye is called secondary deviation
Concomitant squint -primary deviation secondary deviation
Paralytic squint primary -deviation less than secondary deviation
1.Afferent fibers are situated in the olfactory mucosa.Central process of sensory cells of olfactory epithelium the 20 unmyelinated fibers join to form olfactory nerve which passes through the cribriform plate of ethmoid, Bipolar neurons are the I st order neurons
2.Central processes of these neurons proceed to the olfactory bulb where they synapse with the 2nd order neurons, Olfactory bulb neurons form the olfactory tract which lies in the olfactory sulcus of frontal lobe which divides into medial and lateral striae
Olfactory trigone dividing into medial and lateral striae. Some fibers of the medial join with the opposite side
Medial striae terminates in the medial surface of cerebral hemisphere – subcallosal and cingulate gyrus,
Medial striae on either side communicate with each other terminates in the medial surface of cerebral hemisphere - subcallosal and cingulate gyrus
Lateral striae ends in the primary olfactory cortex-- piriform lobe of hippocampus, amygdaloid nucleus and hypothalamus.
Secondary olfactory areas are the uncus and parahippocampal gyrus
Lateral striae ends in piriform lobe of hippocampus, amygdaloid nucleus and hypothalamus. What are the functions of olfactory nerve ?
To carry smell sensation from nasal mucosa to olfactory bulb
Olfactory nerve subserves the sense of smell. Anatomical Peculiarity of olfactory nerve ?
This is the only sensory pathway having no thalamic connection .
Important areas related to the course of facial nerve are the following.
Pons.
Cerebellopontine(CP)angle.
Internal auditory meatus.
Ear.
Stylomastoid foramen.
Pons
Nucleus of facial nerve is situated in pons.
Nervus intermedius that carry sensory and parasympathetic fibers curve around the abducent(6) nucleus in the pons to form the facial colliculus.
CP angle
Facial nerve emerge at the ponto medullary junction and lies in the cp angle.
Internal auditory meatus
Facial nerve enters the internal auditory meatus with 8 th cranial nerve with the nervus intermedius inbetween.
Ear
This part is called labyrinthine part, and is situated above the labyrinth.This curves posteriorly at the genu, and gives the greature superficial petrosal nerve at the genu.
Then the nerve travel backward in the horizontal or tympanic segment above the middle ear.
Then it turns vertically in the vertical or mastoid segment.
In the vertical part facial nerve give two branch nerve to stapedius and chordatympani.
Stylomastoid foramen
Facial nerve emerge at the stylomastoid foramen.
Distal to the stylomastoid foramen,it gives 2 branches.
1.Posterior auricular nerve.
2.Branch to Posterior belly of Digastric muscle and Stylohyoid muscle.
Then it enters the parotid gland.
It leaves the parotid gland by dividing into temperofacial and cervicofacial branches.
Finally divides into five motor branches.
Five major branches of facial nerve (in parotid gland) - from top to bottom are
1.Exposure of eye to brightlight send impulses along the optic nerve, optic chiasma and optic tract.
2.The fibers concerned with light reflex do not reach the lateral geniculate body but they pass from optic tract to the pretectal nucleus in the midbrain where they relay.
3.The new fibers pass to Edinger Wesphal nucleus of both sides, the deccusating fibers pass aroud the aqueduct of sylvius and account for consensual light reaction.
4.From the Edinger Westphal nucleus of both side preganglionic fibers pass through the oculomotor nerve to relay in ciliary ganglion.
5.The post ganglionic fibers pass by the short ciliary nerve to the constrictor pupillae muscle.
Method of testing
1. The patient is asked to look at a distant object to eliminate the contraction of pupil on accommodation.
2. The eye not being tested must be covered to eliminate the consensual reaction.
3. A direct source of bright light is focussed directly into the eye.
4. Normally there is constriction of both pupils.
The response of pupil of the eye upon which bright light falls is the directlight reflex and that of the opposite eye is called consensual reflex.
Consensual reflex occur due to decussation of fibers both in optic chiasma and Edinger Westphal nucleus.
Significance of testing light reflex
1. In lesions of the second cranial nerve direct light reflex on the same side and consensual light reflex on the opposite side is absent due to lesions in the afferent pathway.
2. In lesions of third cranial nerve direct light reflex is absent on the affected side but the consensual light reflex is present.
Accomodation reflex involve a triad of changes when a person looks at a nearby object.
1. Convergence of eye due to contraction of medial and lateral rectus muscle
2. Miosis -constriction of pupils due to constrictor pupillae muscle contraction.
3. Accomodation is associated with increased refractive power of the lens.This is due to the contraction of ciliaris muscle.
Pathway of accommodation
1. Afferent impulses from retina pass along the normal visual pathway to reach the visual areas in the occipital lobe
2. From the visual areas fibers descend to the oculomotor (3rd cranial nerve) nucleus of both side in the midbrain.
3. Efferent fibers pass along the 3 rd cranial nerve to the eye to supply the following muscle
Medial rectus muscle
Constrictor pupillae muscle
Ciliaris musle
In accommodation reflex the fibers reach the lateral geniculate body and the occipital cortex but they donot pass through the pretectal nucleus situated in midbrain.
Method of testing accommodation reflex
The patient is asked to look at a distant object and then at the examiners finger which is gradually brought within 5cm of the eyes.When the gaze is directed from a distant object to near one contraction of medial rectus brings about a convergence of of the ocular axis and along with this accommodation occurs by the contraction of ciliaris muscle and pupil constrict as a part of associated movement .
