26/03/2026
The Facial Nerve (CN VII)
The facial nerve is the primary motor supply to all muscles of facial expression. After emerging from the stylomastoid foramen, it enters the parotid gland, where it typically divides into two main trunks and subsequently into five terminal branches that spread across the face like a fan.
1. Posterior Auricular Branch
Before the nerve enters the parotid gland, it gives off the posterior auricular nerve.
Innervation: Supplies the occipitalis muscle (posterior belly of the epicranius) and some of the auricular muscles.
2. The Five Terminal Branches
These branches emerge from the anterior border of the parotid gland to supply the remaining muscles:
Temporal Branch: Ascends across the zygomatic arch.
Innervation: Supplies the frontalis (forehead wrinkling), corrugator supercilii, and superior auricular muscles.
Zygomatic Branch: Directed towards the lateral canthus of the eye.
Innervation: Supplies the orbicularis oculi (eye closure) and muscles above and below the palpebral fissure.
Buccal Branch: Runs horizontally toward the cheek.
Innervation: Supplies the buccinator and the muscles of the upper lip (e.g., levator labii superioris).
Mandibular Branch: Dips below the body of the mandible before curving back up.
Innervation: Supplies the muscles of the lower lip (e.g., depressor anguli oris).
Cervical Branch: Descends into the neck.
Innervation: Supplies the platysma muscle.
3. Clinical Significance: Facial Nerve Palsy
Damage to the facial nerve (such as in Bell’s Palsy) results in characteristic deficits depending on which branches are affected:
Temporal/Zygomatic: Inability to wrinkle the forehead or close the eye tightly (lagophthalmos).
Buccal: Drooping of the corner of the mouth and food collecting in the cheek (due to buccinator weakness).
Mandibular: Asymmetrical smile and difficulty controlling the lower lip.
Clinical Pearl:
"When testing the facial nerve, always ask the patient to raise their eyebrows. In a lower motor neuron lesion (like Bell's Palsy), the forehead remains smooth and immobile. In an upper motor neuron lesion (like a stroke), the forehead is often spared due to bilateral cortical innervation, which is a vital distinction in the ER!"
