Types of Fibers
The facial nerve comprises four types of fibers:
- Motor Fibers: Arise from the motor nucleus in the pons.
- Sensory Fibers: Supply a small area of skin in the posterior wall of the external auditory meatus.
- Special Sensory (Taste) Fibers: Provide taste to the anterior two-thirds of the tongue via the chorda tympani nerve, which joins the facial nerve approximately 6mm above the stylomastoid foramen.
- Parasympathetic Secretomotor Fibers:
- Lacrimal Gland: Through the greater superficial petrosal (G.S.P.) nerve.
- Submandibular & Sublingual Salivary Glands: Through the chorda tympani nerve
Course of the Facial Nerve
The facial nerve enters the internal auditory meatus alongside the 8th nerve and travels laterally to enter the bony facial canal (Fallopian canal), which is approximately 30mm in length. It progresses to the Labyrinthine segment (4mm), runs above the inner ear to reach the geniculate ganglion.
At the geniculate ganglion, the nerve makes a sharp bend backward (1st genu) and becomes the tympanic segment (horizontal part), approximately 11mm long, running above the promontory. It then bends downward in the medial wall of the aditus (2nd genu) to become the Vertical or Mastoid segment.
The Vertical or Mastoid segment is about 13mm in length, descending in the posterior wall of the middle ear to reach the stylomastoid foramen, where it emerges to enter the parotid gland and divides into superficial and deep lobes.
Within the parotid gland, the nerve divides into its five terminal branches: frontal, zygomatic, buccal, mandibular, and cervical. It provides motor supply to all facial and scalp muscles, as well as the posterior belly of the digastric and stylohyoid muscles.
Branches in the Temporal Bone
- Greater Superficial Petrosal Nerve: Arises from the geniculate ganglion and carries secretomotor fibers to the lacrimal gland.
- Nerve to Stapedius Muscle: Originates from the upper portion of the vertical part.
- Chorda Tympani Nerve