Muscles of Facial Expression and the Facial Nerve (CN VII)

Core Review

The Facial Nerve (CN VII): Functional Components

CN VII is one of the most complex cranial nerves, carrying multiple fiber types:

1. Special visceral efferent (SVE) / Branchial motor: Innervates muscles of facial expression, plus the stylohyoid, posterior belly of the digastric, and stapedius muscles
2. General visceral efferent (GVE) / Parasympathetic: Provides secretomotor fibers to the lacrimal gland, submandibular and sublingual salivary glands, and nasal/palatal mucous glands
3. Special visceral afferent (SVA) / Special sensory: Carries taste sensation from the anterior two-thirds of the tongue via the chorda tympani
4. General somatic afferent (GSA) / General sensory: Small contribution to sensation from part of the external ear

Course of the Facial Nerve

The facial nerve emerges from the brainstem at the pontomedullary junction (cerebellopontine angle) and enters the internal acoustic meatus. It then travels through the facial canal within the temporal bone, where it gives off several important branches before exiting the skull at the stylomastoid foramen.

Branches within the facial canal (intratemporal):

• Greater petrosal nerve: Carries preganglionic parasympathetic fibers to the pterygopalatine ganglion. Postganglionic fibers supply the lacrimal gland and nasal/palatal mucous glands.
• Nerve to stapedius: Innervates the stapedius muscle, which dampens loud sounds. Damage to this nerve causes hyperacusis (sensitivity to loud sounds).
• Chorda tympani: Arises in the facial canal, traverses the middle ear cavity, and exits through the petrotympanic fissure to join the lingual nerve (V3). Carries taste from the anterior two-thirds of the tongue and preganglionic parasympathetic fibers to the submandibular ganglion.

Extratemporal branches (after exiting stylomastoid foramen): After exiting the stylomastoid foramen, the facial nerve gives off the posterior auricular nerve and branches to the posterior belly of the digastric and stylohyoid muscles, then enters the parotid gland where it divides into five terminal branches:

• Temporal: Innervates frontalis, orbicularis oculi (upper portion), corrugator supercilii
• Zygomatic: Innervates orbicularis oculi (lower portion), zygomaticus major and minor
• Buccal: Innervates buccinator, orbicularis oris (upper portion), muscles of the upper lip and nose
• Marginal mandibular: Innervates muscles of the lower lip (depressor anguli oris, depressor labii inferioris, mentalis)
• Cervical: Innervates the platysma

Mnemonic for terminal branches: "The Zebra Bit My Cheek" — or "To Zanzibar By Motor Car."

Muscles of Facial Expression: Key Players

All muscles of facial expression are innervated by CN VII and are derived from the second pharyngeal arch. Key muscles for the NBDHE:

Orbicularis oris: The sphincter of the mouth; closes and protrudes the lips. Essential for speech, eating, and maintaining oral competence. The modiolus, a fibromuscular condensation at the angle of the mouth, serves as an attachment point for multiple muscles (orbicularis oris, buccinator, zygomaticus major, risorius, depressor anguli oris, levator anguli oris).

Buccinator: The cheek muscle; compresses the cheeks against the teeth during mastication, preventing food from accumulating in the vestibule. Origin: pterygomandibular raphe and alveolar processes of maxilla and mandible. Insertion: orbicularis oris at the modiolus. The buccinator is pierced by the parotid duct (Stensen's duct) as it enters the oral cavity opposite the maxillary second molar. The buccinator is considered both a muscle of facial expression and an accessory muscle of mastication.

Zygomaticus major and minor: Elevate and laterally pull the corner of the mouth — the primary "smile" muscles.

Orbicularis oculi: Closes the eyelids. Innervated by temporal and zygomatic branches.

Frontalis (occipitofrontalis, frontal belly): Elevates the eyebrows and wrinkles the forehead. Innervated by the temporal branch.

Platysma: A broad, thin sheet of muscle extending from the upper chest to the lower border of the mandible; tenses the skin of the neck and depresses the lower lip. Innervated by the cervical branch.

Mentalis: Elevates and protrudes the lower lip; wrinkles the chin. Innervated by the marginal mandibular branch.

CN V (Trigeminal) vs CN VII (Facial): A Critical Distinction

The NBDHE loves to test this distinction:

Note: The parotid gland receives its parasympathetic innervation from CN IX (glossopharyngeal), not CN VII.

Bell's Palsy

Bell's palsy is an acute, unilateral lower motor neuron (LMN) facial nerve paralysis of unknown etiology, though herpes simplex virus reactivation is strongly implicated. It is the most common cause of unilateral facial paralysis.

Clinical presentation:

• Acute onset (hours to 1-2 days) of unilateral facial weakness
• Inability to close the ipsilateral eye (orbicularis oculi paralysis)
• Drooping of the corner of the mouth (orbicularis oris, depressor anguli oris paralysis)
• Loss of nasolabial fold on the affected side
• Inability to wrinkle the forehead (frontalis paralysis) — this distinguishes LMN from upper motor neuron (UMN) lesions
• Possible hyperacusis (stapedius paralysis)
• Possible loss of taste on anterior two-thirds of tongue (chorda tympani involvement)
• Possible decreased lacrimation (greater petrosal involvement)

Upper motor neuron vs Lower motor neuron lesions:

• UMN lesion (e.g., stroke affecting the motor cortex or corticobulbar tract): The forehead is SPARED on the affected side because the frontalis receives bilateral cortical innervation. Only the lower face is affected contralaterally.
• LMN lesion (e.g., Bell's palsy): The ENTIRE ipsilateral face is affected, including the forehead. This is a critical clinical distinction.

Management: Most patients recover spontaneously (70-80% within weeks to months). Corticosteroids (prednisone) initiated within 72 hours improve outcomes. Antiviral agents (acyclovir or valacyclovir) may be added for severe cases, though their benefit is debated.

Dental considerations: Patients with Bell's palsy may have difficulty maintaining oral hygiene on the affected side, are at risk for corneal abrasion due to inability to close the eye, and may have xerostomia if parasympathetic fibers are affected.

Clinical Application

During dental treatment, understanding CN VII anatomy helps anticipate and prevent complications. The marginal mandibular branch is vulnerable during surgical procedures near the inferior border of the mandible. Bell's palsy can affect a patient's ability to control oral secretions and maintain lip competence during dental procedures. Local anesthesia of the inferior alveolar nerve (V3) does not affect muscles of facial expression — if a patient cannot close their eye after an IA block, suspect something other than the anesthetic.

Common Traps

• Thinking CN VII provides motor innervation to muscles of mastication — it doesn't; that's V3
• Thinking all salivary glands get parasympathetic from CN VII — the parotid gets it from CN IX
• Forgetting that the chorda tympani carries TWO fiber types (taste AND parasympathetic)
• Confusing Bell's palsy (LMN, entire ipsilateral face) with stroke (UMN, forehead spared)
• Thinking the facial nerve emerges from the parotid gland — actually, the terminal branches emerge from the parotid; the nerve trunk enters and divides within the gland