Facial nerve palsy: why the forehead tells you the lesion
Two droops can look alike. The forehead separates an upper motor neuron lesion from a lower motor neuron one, and the nerve's branches show where along the nerve the lesion sits.
Chapters (11)
Explain why upper motor neuron facial lesions spare the forehead while lower motor neuron (facial nerve) lesions do not.
The short version
- The forehead has input from both hemispheres, so an upper motor neuron lesion spares it.
- An upper motor neuron lesion weakens the opposite lower face; a lower motor neuron lesion weakens the whole half of the face on the same side.
- A whole-face palsy with loss of taste, a dry eye or sensitivity to loud sound places the lesion proximal to the stylomastoid foramen.
- Bell palsy is the most common cause of lower motor neuron facial palsy.
Facial nerve palsy: two droops, one clue
A 34-year-old woman wakes up unable to smile on the left. Her left eye will not close, and her forehead is smooth on that side. A second patient, after a stroke, cannot smile on the right, but he raises both eyebrows normally.
The droop looks similar, but the forehead is different. This lesson explains why an upper motor neuron facial lesion spares the forehead, and a lower motor neuron lesion does not.
The facial nucleus, the facial nerve and the corticobulbar tract
The facial nucleus lies in the pons. Its lower motor neurons form the facial nerve, which supplies the muscles of facial expression.[1]
Its upper motor neurons come from the face area of the motor cortex and travel in the corticobulbar tract.[1][4]
The key: the forehead has input from both hemispheres
The part of the facial nucleus that serves the forehead and upper face receives input from both hemispheres. The part that serves the lower face receives input almost entirely from the opposite hemisphere.[1][2]
| Part of the face | Cortical input |
|---|---|
| Forehead and upper face | Both hemispheres |
| Lower face | Almost entirely the opposite hemisphere |
Upper motor neuron facial palsy: the forehead is spared
Take a stroke in the left motor cortex or the left internal capsule. The right lower face loses almost all of its cortical input, so the right side of the mouth droops.[2]
The upper face still receives input from the intact right hemisphere, so the forehead wrinkles and the eye closes. Classically, then, an upper motor neuron lesion gives weakness of the opposite lower face, with the forehead spared.[2]
Lower motor neuron facial palsy: the whole half of the face
Now take a lesion of the facial nucleus or of the nerve itself. Every muscle of that half of the face has lost its only supply. The forehead is smooth, the eye will not close and the mouth droops, all on the same side as the lesion.[1][2]
A lower motor neuron lesion therefore gives weakness of the whole half of the face, on the same side. That is the first patient. The most common cause is Bell palsy, an idiopathic facial nerve palsy.[3]
| Feature | Upper motor neuron | Lower motor neuron |
|---|---|---|
| Side of face weak | Opposite the lesion | Same as the lesion |
| Forehead | Spared | Weak |
| Extent | Lower face | Whole half of the face |
Branches of the facial nerve that place the lesion
The facial nerve carries more than movement. Three branches help to localise a lower motor neuron lesion.[1][3]
- The chorda tympani carries taste from the front two-thirds of the tongue.
- The nerve to the stapedius dampens loud sound.
- The greater petrosal nerve drives tears.
A lesion inside the facial canal, before those branches leave, can add loss of taste, sounds that seem too loud and a dry eye. A lesion past the stylomastoid foramen leaves only the muscles of the face affected.[1]
Worked case: a parotid tumour
A 61-year-old man with a parotid tumour has weakness of the whole right face, with normal taste, tears and hearing.
- Whole faceWeakness of the whole face means a lower motor neuron lesion.
- Normal taste, tears and hearingThese put the lesion beyond those branches, most likely at or beyond the stylomastoid foramen, outside the skull.
A lesion of the facial nucleus in the pons gives the same whole-face weakness, usually with other brainstem signs.[1]
Practice question
Original · USMLE styleA 72-year-old man has sudden drooping of the left side of his mouth and a mild left arm drift. On examination, he wrinkles both sides of his forehead and closes both eyes tightly. Taste, tearing and hearing are normal.
Common questions
Why does an upper motor neuron lesion spare the forehead?
The part of the facial nucleus serving the forehead and upper face receives input from both hemispheres, so the intact hemisphere still supplies it.
What is the difference between upper and lower motor neuron facial palsy?
An upper motor neuron lesion weakens the opposite lower face and spares the forehead. A lower motor neuron lesion weakens the whole half of the face on the same side.
What is the most common cause of lower motor neuron facial palsy?
Bell palsy, an idiopathic facial nerve palsy.
How do taste, tears and loud sounds localise a facial nerve lesion?
The chorda tympani carries taste, the greater petrosal nerve drives tears and the nerve to the stapedius dampens loud sound. Loss of these with a whole-face palsy places the lesion proximal to the stylomastoid foramen. Normal function places it beyond those branches, most likely at or beyond the stylomastoid foramen, unless brainstem signs point to the facial nucleus in the pons.
Transcript
Read the full narration, by chapter713 words
0:00Same droop, different lesion
A 34-year-old woman wakes up unable to smile on the left. Her left eye will not close, and her forehead is smooth on that side. A second patient, after a stroke, cannot smile on the right either, but he raises both eyebrows normally. A similar droop with a different forehead. Why?
0:19The facial nucleus, the facial nerve and the corticobulbar tract
Today: why an upper motor neuron facial lesion spares the forehead, and a lower motor neuron lesion does not. The facial nucleus lies in the pons, and its lower motor neurons form the facial nerve, which supplies the muscles of facial expression. Its upper motor neurons come from the face area of the motor cortex, and travel in the corticobulbar tract.
0:43The key: forehead input from both hemispheres
Here is the key. The part of the facial nucleus that serves the forehead and upper face receives input from both hemispheres, but the part that serves the lower face receives input almost entirely from the opposite hemisphere.
0:59Upper motor neuron lesion: forehead spared
Upper motor neuron lesion first: a stroke in the left motor cortex or the left internal capsule. The right lower face loses its only input, so the right side of the mouth droops. The upper face still receives input from the intact right hemisphere, so the forehead wrinkles and the eye closes. So, classically, an upper motor neuron lesion gives weakness of the opposite lower face, with the forehead spared.
1:27Lower motor neuron lesion: the whole half of the face, and Bell palsy
Now a lower motor neuron lesion: the facial nucleus, or the nerve itself. Every muscle of that half of the face has lost its only supply. So the forehead is smooth, the eye will not close, and the mouth droops, all on the same side as the lesion. A lower motor neuron lesion gives weakness of the whole half of the face, on the same side. That is our first patient. The most common cause of lower motor neuron facial palsy is Bell palsy, an idiopathic facial nerve palsy.
2:01Branches that place the lesion: taste, loud sound, tears
The facial nerve carries more than movement. The chorda tympani carries taste from the front two-thirds of the tongue. The nerve to the stapedius dampens loud sound. The greater petrosal nerve drives tears. A lesion inside the facial canal, before those branches leave, can add loss of taste, sounds that seem too loud, and a dry eye. A lesion past the stylomastoid foramen leaves only the muscles of the face affected.
2:31Worked case: a parotid tumour
Try one. A 61-year-old man with a parotid tumour has weakness of the whole right face, with normal taste, tears and hearing. Whole face means lower motor neuron. Normal taste, tears and hearing put the lesion beyond those branches, outside the skull. A lesion of the facial nucleus in the pons gives the same whole-face weakness, usually with other brainstem signs.
2:58Practice question
Your turn. A 72-year-old man has a sudden droop of the left side of his mouth, and a mild left arm drift. He wrinkles both sides of his forehead and closes both eyes tightly. Taste, tearing and hearing are normal. Where is the lesion? A, the left facial nerve in the parotid gland. B, the left facial nucleus in the pons. C, the left geniculate ganglion. D, the right internal capsule. E, the right stylomastoid foramen. Pause here and decide.
3:38The options explained
A is out: a lesion in the nerve itself would weaken the whole left face, forehead included. B is out: the facial nucleus is lower motor neuron territory, so the forehead would be weak too. C is out: a lesion at the geniculate ganglion is also lower motor neuron, and it would affect the forehead, and usually taste and tears as well. E is out: that is the wrong side, and a lesion there would again paralyse the whole face.
4:07The answer
So the answer is D. A lower face droop with the forehead spared is upper motor neuron. It is on the left, so the lesion is on the right, in the internal capsule, which also explains the arm drift.
4:21Recap and next lesson
Three things to keep. One: the forehead has input from both hemispheres, so an upper motor neuron lesion spares it. Two: an upper motor neuron lesion weakens the opposite lower face. A lower motor neuron lesion weakens the whole same side. Three: a whole-face palsy with loss of taste, a dry eye or loud-sound sensitivity places the lesion proximal to the stylomastoid foramen. Next, stroke syndromes by artery: anterior, middle and posterior cerebral.
Sources
Numbers in the text, e.g. [1], point here- TeachMeAnatomy. The Facial Nerve; The Pons; Descending Tracts (Motor). teachmeanatomy.info/head/cranial-nerves/facial-nerve/ · teachmeanatomy.info/neuroanatomy/brainstem/pons/ · teachmeanatomy.info/neuroanatomy/pathways/descending-tracts-motor/
- Neurology International 2025 (open access, PMC11767383) and J Clin Med 2026 (open access, PMC12786528): central versus peripheral facial palsy, bilateral cortical input to the upper face.
- StatPearls. Bell Palsy (2024), abstract via Europe PMC. ebi.ac.uk/europepmc/webservices/rest/search?query=EXT_ID:29493915&resultType=core&format=json
- Neuroscience Online (UTHealth), Section 3, Chapter 3. nba.uth.tmc.edu/neuroscience/m/s3/chapter03.html
- OpenStax. Anatomy and Physiology 2e, 14.3 Motor Responses. openstax.org/books/anatomy-and-physiology-2e/pages/14-3-motor-responses
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Plates: Gray's Anatomy (1918), plates 788 (plan of the facial and intermediate nerves) and 764 (the motor tract), Henry Vandyke Carter, public domain, via Wikimedia Commons; recoloured to paper with an ink outline, labelled and highlighted by us. Original diagrams: the faces, the facial nucleus input diagrams, the options map, the pocket card. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-NEURO-004.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Upper vs lower motor neuron lesions: signs and localisation5:08
- EP 02Dorsal column vs spinothalamic tract: where each pathway crosses5:01
- EP 03Spinal cord syndromes compared: rebuild each from three tracts5:06
- EP 04Cranial nerve nuclei: a brainstem map by level and position5:10
- EP 05Brainstem crossed findings: the localisation rule4:50
- EP 06Medial vs lateral medullary syndrome: artery, structures and signs5:30
- EP 07Facial nerve palsy: why the forehead tells you the lesion4:59
- EP 08Stroke syndromes by artery: ACA, MCA and PCA4:41
- EP 09Lacunar stroke syndromes: pure motor, pure sensory and more4:54
- EP 10Visual pathway lesions and field defects4:42
- EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement5:19
- EP 12Cerebellar lesion localisation: vermis vs hemisphere4:42
- EP 13Brain herniation syndromes: what is squeezed, and where4:56
- EP 14CSF flow and hydrocephalus: where the flow fails5:12
- EP 15Corticospinal tract: from cortex to anterior horn4:59
- EP 16Hypothalamic nuclei and their functions5:24