Facial Nerve (Cranial Nerve VII) - Detailed Anatomy

Overview

Origin (Central / Nuclear)

1. Facial Motor Nucleus - Located in the lateral portion of the anterior pons (branchial motor column), more caudally than the trigeminal motor nucleus. It is composed of four cell groups. The fascicles from this nucleus loop dorsally around the abducens (CN VI) nucleus, forming the facial colliculus on the floor of the fourth ventricle - this is why a pontine lesion involving CN VII almost always also involves CN VI.
2. Superior Salivatory Nucleus - Origin of preganglionic parasympathetic (GVE) fibers carried in the nervus intermedius. Supplies the lacrimal, submandibular and sublingual glands.
3. Nucleus of the Tractus Solitarius - Receives special visceral afferent (taste) fibers from the anterior 2/3 of the tongue via the chorda tympani.

• • The facial nerve is 1.8 mm in diameter vs 3 mm for CN VIII
• • It exits 1.5 mm anterior, medial, and superior to CN VIII
• • It crosses the CPA a distance of 15-17 mm together with CN VIII and the nervus intermedius

Course and Segments

1. Intracranial Segment (Cerebellopontine Angle)

2. Intratemporal Segment (within the Fallopian Canal)

• First, shortest, and narrowest segment
• Enters through the meatal foramen (~0.68 mm diameter) - the narrowest portion of the entire canal
• A dense arachnoid band at the lateral IAC and the narrow meatal foramen together create an anatomical "bottleneck" - critical in Bell's palsy, where nerve edema is constricted here
• Courses posterocephalad to the cochlea, anteromedial to the ampulla of the superior semicircular canal, cephalad to the vestibule
• Opens into the geniculate fossa (just deep to the squamous temporal region)

• Begins at the geniculate ganglion where the nerve makes its first (internal) genu - a sharp 75-degree posterior turn
• The geniculate ganglion contains bipolar ganglion cells for sensory functions (taste and somatic sensation). It has a separate developmental origin from the facial nerve itself (well defined by the 7th embryonic week). It is separated from the middle fossa by only a thin layer of bone, which is dehiscent in ~25% of ears
• From the geniculate ganglion, the Greater Superficial Petrosal Nerve (GSPN) arises and runs anteriorly along the middle fossa floor
• The horizontal segment occupies the medial wall of the anterior attic, passes over the cochleariform process, and forms the superior wall of the oval window niche. The bony wall of this segment is dehiscent in 35-55% of the population, especially above the oval window

• At the pyramidal eminence (where the stapedius tendon emerges), the nerve makes a second (external) genu into the vertical mastoid segment
• This turn is anteroinferior to the lateral semicircular canal
• The nerve to stapedius arises at or near the second genu
• Approximately midway, the chorda tympani nerve arises (though both branches can arise anywhere between the second genu and the stylomastoid foramen)
• The vertical segment exits the temporal bone through the stylomastoid foramen

3. Extratemporal Segment

1. Immediately gives off the posterior auricular branch - motor to the occipitalis and posterior auricular muscles
2. Enters the posterior aspect of the parotid gland - the entry point is located at the midpoint of a line drawn from the superior border of the tragus to the angle of the mandible
3. Divides into upper (temporofacial) and lower (cervicofacial) trunks
4. These pass through the parotid gland with further branching and anastomosis
5. Five terminal groups emerge from the borders of the parotid gland

Five Terminal Branches - Nerve Supply, Muscles (Insertion), and Actions

1. Temporal Branch

• Origin: Upper division (temporofacial trunk)
• Course: Usually 3-5 rami that originate over the middle third of the zygomatic arch; courses between the SMAS and deep fascia, then penetrates the undersurface of muscles
• Muscles supplied (insertion sites):
  • Frontalis - inserted into the skin and subcutaneous tissue of the forehead
  • Upper orbicularis oculi - encircles the orbital aperture
  • Corrugator supercilii - inserts into the skin of the medial eyebrow
  • Anterior and superior auricular muscles - insert into the auricle
• Actions:
  • Raises the eyebrows (frontalis)
  • Closes the eyelids (orbicularis oculi - upper portion)
  • Draws the eyebrows medially and inferiorly, creating frowning wrinkles (corrugator supercilii)
  • Moves the auricle anteriorly/superiorly
• Clinical note: This is the most at-risk branch during temple surgery. Injury causes brow ptosis, forehead flattening, and inability to raise the eyebrow - with long-term upper visual field compromise

2. Zygomatic Branch

• Origin: Upper division (temporofacial trunk)
• Course: Runs horizontally and upward after emerging from the parotid gland
• Muscles supplied (insertion sites):
  • Lower orbicularis oculi - lower eyelid and cheek
  • Procerus - inserts into the skin between the eyebrows
  • Mouth elevator muscles (zygomaticus major and minor)
  • Nasal muscles (nasalis, compressor narium)
• Actions:
  • Closes the lower eyelid
  • Draws the skin of the forehead downward (procerus - produces horizontal wrinkles at the root of the nose)
  • Elevates the corner of the mouth and upper lip (smiling)
  • Compresses/dilates the nostril

3. Buccal Branch

• Origin: Both upper and lower divisions (most anastomotic branch)
• Course: Runs horizontally across the cheek, deep in the parotid tissue - least at risk branch due to depth and abundant anastomoses
• Muscles supplied (insertion sites):
  • Orbicularis oris - encircles the mouth opening
  • Buccinator - inserts into the orbicularis oris at the angle of the mouth
  • Risorius - inserts into the skin at the angle of the mouth
  • Levator labii superioris and accessory elevator muscles
  • Mentalis (partially, with marginal mandibular)
• Actions:
  • Closes and protrudes the lips (orbicularis oris - e.g., kissing, whistling)
  • Compresses the cheeks against the teeth during mastication, preventing food accumulating in the vestibule (buccinator)
  • Retracts the angle of the mouth laterally - "grinning" (risorius)

4. Marginal Mandibular Branch

• Origin: Lower division (cervicofacial trunk)
• Course: Exits the inferior portion of the parotid gland, courses over the mandible anterior to the facial artery, located 1-2 cm above the inferior border of the mandible
• Muscles supplied (insertion sites):
  • Orbicularis oris (lower portion) - encircles the mouth
  • Risorius
  • Depressor anguli oris - inserts into the angle of the mouth
  • Depressor labii inferioris - inserts into the lower lip skin
  • Mentalis - inserts into the skin of the chin
• Actions:
  • Depresses the lower lip and corner of the mouth (frowning)
  • Protrudes and wrinkles the chin skin (mentalis)
  • Closes the lower mouth
• Clinical note: Injury causes ipsilateral crooked smile, drooling, and inability to fully depress the lower lip

5. Cervical Branch

• Origin: Lower division (cervicofacial trunk)
• Course: Passes inferiorly to the neck, deep to platysma
• Muscles supplied (insertion sites):
  • Platysma - broad sheet inserted into skin of lower face, mandible, and clavicle region
• Actions:
  • Draws the angle of the mouth downward and laterally
  • Tenses the skin of the neck
  • Assists in depressing the mandible

Additional Motor Branches (arise before parotid division)

Autonomic (Parasympathetic) Branches - Nerve Supply

Greater Superficial Petrosal Nerve (GSPN)

• Arises from the geniculate ganglion
• Carries preganglionic parasympathetic fibers - synapses in the pterygopalatine ganglion
• Postganglionic fibers innervate:
  • Lacrimal gland (lacrimation)
  • Seromucinous glands of the nasal cavity and palate
• Also carries taste fibers from the soft palate

Chorda Tympani Nerve

• Arises in the mastoid segment of the fallopian canal (~midway)
• Traverses the middle ear (between malleus and incus), exits the skull via the petrotympanic fissure
• Joins the lingual nerve (CN V3) in the infratemporal fossa
• Carries:
  • Preganglionic parasympathetics - synapse in the submandibular ganglion - postganglionic fibers supply the submandibular and sublingual glands (salivation)
  • Taste (SVA) fibers from the anterior 2/3 of the tongue

Summary Table: Fiber Types and Their Origins/Targets

Upper vs Lower Motor Neuron Lesions

• The upper facial muscles (forehead/brow) receive bilateral cortical input (from both hemispheres)
• The lower facial muscles receive predominantly contralateral cortical input