Facial nerve
Nuclei associated with the facial nerve
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Facial motor nucleus (in the pons): Loops around the abducent motor nucleus
Provides branchial motor (special visceral efferent - SVE) that controllers the following:
Muscles of facial expression
Stapedius : Hyperacusis (increased sensitivity to sound).
Posterior belly of the digastric : Depresses the mandible (helps open the mouth). & Elevates the hyoid bone (important for swallowing and speaking).
Stylohyoid : Elevates and retracts the hyoid bone, helping with swallowing. Assists in widening the pharynx during swallowing
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Superior salivatory nucleus (in the pons):
Sends parasympathetic (general visceral efferent - GVE) fibers via the chorda tympani to the submandibular and sublingual salivary glands, and via the greater petrosal nerve to the lacrimal gland and nasal/palatal mucosa.
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Solitary nucleus (nucleus tractus solitarius, NTS) (in the medulla):
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Receives special visceral afferent (SVA) taste sensations from the anterior two-thirds of the tongue via the chorda tympani.
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Also receives some general visceral afferent (GVA) input from the soft palate and pharynx.
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Spinal trigeminal nucleus (in the medulla and lower pons):
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Receives general somatic afferent (GSA) fibers that carry sensory input from the external ear, external auditory canal, and a small part of the external tympanic membrane.
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Muscles of facial expression innervated by the facial nerve
Course of the Facial Nerve
1. Intracranial Course (Brainstem)
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The facial nerve arises from the pontomedullary junction of the brainstem. (supplied by Anterior Inferior Cerebellar Artery (AICA).
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It consists of two roots:
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Motor root (larger): Carries motor fibers.
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Nervus intermedius (smaller): Carries parasympathetic and sensory fibers.
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These roots travel anterolaterally and enter the internal acoustic meatus (IAM) of the temporal bone alongside the vestibulocochlear nerve (CN VIII).
2. Intratemporal Course (Within the Facial Canal)
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Meatal Segment: Porus Acusticus ==> IAM Fundus 8-10 mm in length Supplied by Labyrinthine Artery.
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Labyrinthine Segment: IAM Fundus ==> Geniculate Ganglion 4 mm in LengthSupplied by Labyrinthine Artery.
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Runs horizontally forward from the internal acoustic meatus.
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Ends at the geniculate ganglion, which contains sensory neurons.
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Gives off the greater petrosal nerve (carries parasympathetic fibers to the lacrimal gland and nasal mucosa).
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Tympanic (Horizontal) Segment: Geniculate Ganglion ==> Second Genu. 11.0 mm in length Supplied by the Superficial petrosal Artery.
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Turns posteriorly at the geniculate ganglion.
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Passes above the oval window in the middle ear.
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Mastoid (Vertical) Segment: Second Genu ==> Stylomastoid Foramen. 13.0 mm in length. Supplied by Stylomastoid Artery.
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Turns inferiorly near the posterior wall of the middle ear.
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Gives off the nerve to stapedius and chorda tympani (carries taste fibers from the anterior 2/3 of the tongue and parasympathetic fibers to the submandibular and sublingual glands)
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3. Extracranial Course (Outside the Skull) (Supplied by Posterior Auricular Artery).
Once outside the skull, the facial nerve gives off several branches:
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Posterior Auricular Nerve
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Innervates the auricular muscles and occipitalis.
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Branches to the Digastric and Stylohyoid Muscles
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Innervates the posterior belly of digastric and stylohyoid muscles.
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Parotid Plexus (Within the Parotid Gland)
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The facial nerve enters the parotid gland, where it divides into two main trunks:
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Temporofacial division (upper)
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Cervicofacial division (lower)
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These divisions form the parotid plexus, which gives rise to five terminal branches:
"To Zanzibar By Motor Car" (mnemonic for the branches)
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Temporal branch – Forehead and upper eyelid muscles (e.g., frontalis, orbicularis oculi).
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Zygomatic branch – Muscles around the eye.
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Buccal branch – Muscles of the upper cheek and lips.
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Marginal Mandibular branch – Muscles of the lower lip.
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Cervical branch – Platysma muscle.
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Surgical landmarks
In-Ear and Mastoid Surgery:
1. Processus Cochleariformis:
o Geniculate ganglion (1st Genu) lies Anterior to it.
o Tympanic segment of CN-VII starts at this level.
2. Horizontal SCC:
o Tympanic segment of CN-VII runs below Horizontal SCC.
o 2nd Genu of Facial nerve runs Infero-lateral to Horizontal SCC.
3. Oval Window:
o Tympanic segment of CN-VII runs above Oval window (Stapes).
4. Short Process of Incus:
o Tympanic segment of CN-VII lies medial to Short process of Incus at level of Aditus.
5. Pyramid:
o Mastoid segment of CN-VII runs behind Pyramid and Posterior Tympanic sulcus.
6. Facial Recess:
o Long process of Incus points toward Facial recess.
o Chorda tympani nerve serves at Lateral margin of Triangularfacial recess.
o Chorda tympani nerve can be exposed along its length and can be followed inferiorly and medially to its takeoff from the maintrunk of Facial nerve.
7. TympanoMastoid suture:
o Mastoid segment of CN-VII runs behind this suture.
8. Digastric Ridge:
o Mastoid segment of CN-VII leaves Mastoid at Anterior end of Digastric Ridge.
In-Parotid surgery:
1. Cartilaginous Pointer:
o Sharp triangular piece of cartilage of Pinna points to Facial Nerve.
o Extra-Temporal part of Facial Nerve lies 1 cm inferior and medial to the pointer.
2. Tympanomastoid Suture:
o BetweenMastoidandTympanicpartofTemporalbone.
o Facial Nerve lies 6-8 mm deep to this suture in Stylomastoidforamen.
3. Styloid Process.
o Facial Nerve crosses lateral to Styloid process.
4. Posterior belly of Digastric.
o Facial Nerve lies between attachment of Posterior belly of digastric muscle to Digastric groove and the Styloid process.
