Root Origin
- Classification: Cranial nerve VII
- Brainstem origin: Pontomedullary junction (motor nucleus in the pons)
- Type: Mixed (motor to facial muscles, sensory taste to anterior two-thirds of tongue, parasympathetic to lacrimal, submandibular, and sublingual glands)
Course
- Brainstem. The nerve exits the brainstem at the pontomedullary junction, lateral to the abducens nerve (CN VI).
- Internal acoustic meatus. The nerve enters the internal acoustic meatus of the temporal bone alongside the vestibulocochlear nerve (CN VIII). It then turns posteriorly at the geniculate ganglion (the sensory ganglion for taste fibers).
- Facial canal. The nerve passes through the facial canal within the temporal bone — a narrow bony canal that follows a Z-shaped path through the middle ear region. Within the canal, three important branches exit:
- Greater petrosal nerve — parasympathetic to the lacrimal gland and nasal mucosa
- Nerve to stapedius — motor to the stapedius muscle (dampens vibration of the stapes bone). Loss = hyperacusis (painful sensitivity to sound).
- Chorda tympani — taste fibers from the anterior two-thirds of the tongue and parasympathetic to the submandibular and sublingual glands. This branch travels across the middle ear cavity, making it vulnerable during ear surgery.
- Stylomastoid foramen. The nerve exits the skull through the stylomastoid foramen, located between the styloid process and the mastoid process. It is now a purely motor nerve (the sensory and parasympathetic branches exited within the canal).
- Parotid gland. After exiting the stylomastoid foramen, the nerve enters the parotid gland (but does NOT innervate it — the parotid is innervated by CN IX). Within the gland, it divides into its five terminal motor branches.
- Five terminal branches. The terminal branches radiate across the face like a "goose foot" (pes anserinus):
- Temporal branch — frontalis, upper orbicularis oculi
- Zygomatic branch — lower orbicularis oculi, zygomaticus major and minor
- Buccal branch — buccinator, orbicularis oris, upper lip elevators
- Marginal mandibular branch — lower lip depressors, mentalis
- Cervical branch — platysma
Motor Distribution
| Branch | Muscles Supplied | Clinical Action | Key Test |
|---|---|---|---|
| Temporal | Frontalis, corrugator supercilii, upper orbicularis oculi | Raises eyebrows, wrinkles forehead | "Raise your eyebrows" — inability = upper motor neuron (stroke) if bilateral, lower motor neuron (Bell's) if unilateral |
| Zygomatic | Lower orbicularis oculi, zygomaticus major/minor | Eye closure, smiling | "Close your eyes tightly" — inability to close the eye is the most concerning finding in Bell's palsy (corneal exposure risk) |
| Buccal | anatomy/muscles/buccinator, orbicularis oris, levator labii superioris, levator anguli oris, nasalis | Blowing, whistling, puffing cheeks, upper lip movement | "Puff out your cheeks" — air leaks from the affected side |
| Marginal mandibular | Depressor anguli oris, depressor labii inferioris, mentalis | Lower lip depression, chin wrinkling | "Show me your bottom teeth" — asymmetry |
| Cervical | anatomy/muscles/platysma | Tenses anterior neck skin, depresses mandible/lower lip | "Grimace" — platysma contraction visible on the intact side only |
Sensory Distribution
- Taste — anterior two-thirds of the tongue (via chorda tympani). Sweet, salty, sour, and bitter taste from the front of the tongue. Loss of taste on one side is a finding in Bell's palsy, though patients often do not notice it because the opposite side compensates.
- General sensation — small area of the external ear (concha). A tiny patch of skin in the ear receives sensory fibers from CN VII. This is clinically relevant in Ramsay Hunt syndrome (herpes zoster of the geniculate ganglion), which produces vesicles in the ear canal alongside facial paralysis.
- No major facial skin sensation. Facial skin sensation is entirely trigeminal (CN V). The facial nerve supplies motor to the face, not sensation. This distinction is critical: Bell's palsy produces facial weakness without facial numbness. If the patient has both facial weakness AND facial numbness, the lesion involves both CN V and CN VII — think brainstem pathology or extensive skull base lesion.
Entrapment Sites
1. Facial Canal (Bell's Palsy)
- Location: Within the narrow bony facial canal in the temporal bone
- Structure: The facial canal is the narrowest bony passage any cranial nerve traverses. Inflammatory swelling of the nerve within this rigid canal compresses the nerve against the bone.
- Condition: Bell's palsy (idiopathic facial paralysis) — thought to be caused by viral reactivation (HSV-1, VZV) producing nerve inflammation and edema within the canal
- Presentation: Acute onset unilateral facial paralysis — the patient wakes up or notices over hours that one side of the face is weak. ALL muscles on the affected side are involved: cannot raise the eyebrow, cannot close the eye, mouth droops, food collects in the cheek, drooling. May be accompanied by: hyperacusis (stapedius paralysis), loss of taste on the anterior tongue (chorda tympani involvement), and decreased tearing (greater petrosal nerve involvement). Pain behind the ear often precedes the paralysis by 1-2 days.
- Key differentiator from stroke: Bell's palsy affects ALL facial muscles on one side including the forehead. Stroke (upper motor neuron lesion) spares the forehead because the upper face receives bilateral cortical innervation — the forehead muscles receive input from BOTH hemispheres, so a unilateral stroke leaves forehead function intact while the lower face is paralyzed. "Can the patient raise their eyebrow?" is the single question that differentiates Bell's palsy from stroke in the vast majority of cases.
- MT relevance: Bell's palsy is a medical condition (prednisone within 72 hours is first-line treatment). The MT role is supportive: gentle facial massage to maintain muscle tone, eye protection education (the inability to close the eye risks corneal drying and ulceration — tape the eye shut at night, use artificial tears), and reassurance (85% of Bell's palsy cases recover fully within 3-6 months).
2. Parotid Gland Region
- Location: Where the nerve passes through the parotid gland
- Structure: Parotid tumors, parotid surgery, or facial trauma can damage the nerve within or just after the gland
- Presentation: Selective branch injury — only one or two terminal branches may be affected, producing partial facial weakness (e.g., inability to close the eye with preserved mouth movement, or vice versa). This contrasts with Bell's palsy, which affects all branches equally.
- MT relevance: Post-surgical facial weakness following parotid surgery is an iatrogenic complication. If a patient has partial facial weakness after parotid surgery, one or more terminal branches were damaged. This is usually permanent.
Clinical Tests
| Test | Procedure | Positive Finding | What It Tells You |
|---|---|---|---|
| Forehead test ("raise your eyebrows") | Ask the patient to raise both eyebrows as high as possible. | Inability to raise the eyebrow on the affected side, with loss of forehead wrinkles. | THE critical differentiating test. Forehead involvement = lower motor neuron (Bell's palsy, parotid lesion). Forehead sparing = upper motor neuron (stroke). |
| Eye closure | Ask the patient to close both eyes tightly. | Inability to close the eye on the affected side. The eye rolls upward when closure is attempted (Bell's phenomenon) — this is normal and actually protective. | Orbicularis oculi weakness. The most concerning functional finding — inability to close the eye risks corneal damage. |
| Smile/show teeth | Ask the patient to smile or show their teeth. | The mouth pulls to the unaffected side. The nasolabial fold flattens on the affected side. | Lower facial motor function. Asymmetric smile is often the first finding the patient or their family notices. |
| Puff cheeks | Ask the patient to puff out both cheeks and hold air. | Air escapes from the affected side due to buccinator and orbicularis oris weakness. | Buccal branch function. Also tests the oral competence needed for eating and drinking — leaking from the corner of the mouth is a functional problem. |
Clinical Notes
- Bell's palsy versus stroke — the forehead question is everything. When a patient presents with sudden facial drooping, two possibilities dominate: Bell's palsy and stroke. The forehead test resolves this in seconds. Bell's palsy (lower motor neuron): the entire half of the face is weak INCLUDING the forehead — the patient cannot wrinkle the forehead or raise the eyebrow on the affected side. Stroke (upper motor neuron): the lower face is weak but the forehead is SPARED — the patient can wrinkle the forehead because the frontalis receives bilateral cortical input. If the forehead is involved, it is almost certainly Bell's palsy. If the forehead is spared, assume stroke until proven otherwise and refer emergently.
- 85% recovery rate — but timing matters. The majority of Bell's palsy cases recover fully within 3-6 months. Prednisone started within 72 hours of onset improves outcomes. If you suspect Bell's palsy, refer urgently for medical treatment — the window for steroids is narrow. Do not reassure without referral.
- Eye protection is the immediate priority. The inability to close the eye allows the cornea to dry, leading to corneal abrasion and potentially permanent vision loss. Artificial tears during the day, taping the eye shut at night, and wearing protective glasses in wind are essential. Educate the patient on this at the first visit — it is more urgent than the facial weakness itself.
- Synkinesis is the troublesome late complication. In some patients who recover from Bell's palsy, the nerve fibers regrow to the wrong muscles — this is aberrant regeneration or synkinesis. The patient may close the eye when they smile, or cry when they chew (crocodile tears). Synkinesis indicates that the nerve was significantly damaged (axonotmesis) and the regenerating fibers took wrong paths. Facial retraining exercises (biofeedback, mirror therapy) can improve synkinesis.
- Massage therapy in Bell's palsy recovery. Gentle facial massage maintains muscle tone and circulation during the paralysis phase. Avoid aggressive or deep facial work — the muscles are denervated and cannot respond normally. Facial exercises (active movements in front of a mirror) begin when voluntary movement starts returning. The MT can guide these exercises and provide the facial massage as part of a rehabilitation plan.
Related Nerves
- anatomy/nerves/trigeminal-nerve — CN V. Supplies facial SENSATION (the facial nerve supplies facial MOVEMENT). Bell's palsy: weak face, normal sensation. Trigeminal lesion: numb face, normal movement. Combined weakness and numbness = both nerves involved, suggesting a larger lesion.
- anatomy/nerves/spinal-accessory-nerve — CN XI. Both are tested in the cranial nerve examination. The accessory nerve (SCM and trapezius) is the other motor cranial nerve MTs encounter frequently.
Key Takeaways
- Motor nerve to all muscles of facial expression — Bell's palsy (acute unilateral paralysis) is the most common cranial nerve disorder, affecting the entire ipsilateral face including the forehead.
- The forehead test differentiates Bell's palsy from stroke: forehead involved = Bell's palsy (lower motor neuron); forehead spared = stroke (upper motor neuron) — refer emergently if the forehead is spared.
- Eye protection is the immediate priority in Bell's palsy — inability to close the eye risks corneal damage, so artificial tears and eye taping take precedence over all other treatment.
- 85% of Bell's palsy cases recover fully in 3-6 months — refer for prednisone within 72 hours for best outcomes.