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Monkey Intel
Facial Sensation & Movement — CN V and CN VII
• Lecture 16 •
Overview
Face sensation routes through trigeminal pathways; face movement through facial pathways. Learn the map once and localize fast. Forehead sparing means a central lesion. Eye protection is as important as steroids in LMN facial palsy.
CN V — Trigeminal
sensory + mastication
Core job
Somatic sensation to face, oral cavity, sinuses, supratentorial dura and tentorium; motor to mastication muscles (masseter, temporalis, pterygoids), mylohyoid, anterior digastric, tensor tympani, tensor veli palatini.
Divisions & exits
V1 ophthalmic → superior orbital fissure. V2 maxillary → foramen rotundum. V3 mandibular → foramen ovale. V1/V2 traverse cavernous sinus; V3 does not.
Ganglion
Gasserian ganglion in Meckel’s cave; entry to brainstem at pons.
Sensory pathways
Light touch → main sensory nucleus (pons) → decussation → VPM → somatosensory cortex. Pain/temp → descend as spinal tract of V to medulla → cross → VPM. Jaw proprioception → mesencephalic nucleus (midbrain).
Reflex pearls
Corneal reflex afferent = V1; efferent = VII → bilateral blink. Jaw jerk uses mesencephalic V (afferent) + motor V (efferent); a brisk jaw jerk suggests a supramedullary UMN process.
Numb-chin sign
Red flag for malignancy along V3/mental nerve; image mandible and skull base; check cancer history and B-symptoms.
Trigeminal neuralgia
Lightning-like V2/V3 pain, often touch-triggered, with normal sensation between attacks. First-line carbamazepine or oxcarbazepine; refractory → microvascular decompression or ablative procedures. Young/bilateral → evaluate for MS (pontine entry zone).
CN VII — Facial
expression + taste + parasympathetics
Course
Nucleus in dorsal pons; loops around CN VI (facial colliculus) → internal auditory canal with CN VIII → facial canal → exits stylomastoid foramen → branches across parotid (no parotid innervation).
Key branches
Geniculate ganglion (sensory/taste). Greater petrosal (lacrimal/nasal glands). Nerve to stapedius. Chorda tympani (taste anterior 2/3 + submandibular/sublingual). Motor branches: temporal, zygomatic, buccal, marginal mandibular, cervical.
UMN vs LMN
LMN lesion → entire ipsilateral face weak (can’t raise brow, close eye, or smile). UMN lesion → contralateral lower face weakness with forehead sparing via bilateral cortical input.
Bell’s palsy
Idiopathic LMN VII palsy; onset hours; hyperacusis from stapedius weakness; possible taste/lacrimation issues. Treat with oral steroids ± antivirals; protect cornea with artificial tears and taping. Most improve by one month; may develop synkinesis/crocodile tears.
Ramsay-Hunt
VZV in geniculate ganglion; look for ear vesicles, facial paralysis, hyperacusis ± hearing loss/vertigo. Treat with steroids + antivirals.
Lesion by level
Brainstem (nuclear/fascicle) may pair with ipsilateral gaze palsy (CN VI) or contralateral body weakness. CPA angle often combines VII + VIII signs. Geniculate segment impacts taste and lacrimation. Tympanomastoid segment affects taste/salivation. Extracranial branch lesions spare taste/saliva and vary by branch.
Rapid check
Corneal reflex
Afferent V1. Efferent VII. Bilateral blink.
Rapid check
Forehead sparing
Only in UMN lesions due to bilateral input to upper face.
Rapid check
V3 red flag
Numb-chin sign → evaluate for malignancy.
Mini-Cases
Case 1 — “Electric face”
Brief stabbing pain along right cheek/jaw triggered by brushing teeth; normal exam between attacks → trigeminal neuralgia (V2/V3). Start carbamazepine; MRI if young/bilateral.
Case 2 — “Smile but can’t blink?”
Left lower-face droop with preserved brow and eye closure → UMN pattern from right hemisphere lesion.
Case 3 — “Watery eye, loud sounds”
Acute LMN VII palsy with hyperacusis and impaired eye closure → Bell’s palsy. Treat with steroids ± antivirals and corneal protection.
Case 4 — “Siren in one ear”
Progressive unilateral hearing loss + facial weakness + imbalance → CPA mass (vestibular schwannoma). Order MRI IAC with contrast.
Case 5 — “Crossed findings”
Ipsilateral facial weakness with impaired horizontal gaze and contralateral arm/leg weakness → pontine lesion affecting CN VI/VII fascicles and corticospinal tract.
Case 6 — “Red vesicles in ear”
Ear vesicles + otalgia + facial paralysis ± vertigo → Ramsay-Hunt; treat promptly with steroids + antivirals.
Case 7 — “Half face numb, hand numb”
Contralateral face + hand numbness pattern → small VPM/VPL or cortical lesion (cheiro-oral). Localize and image.
Case 8 — “Chin feels dead”
New mental nerve anesthesia without dental cause → numb-chin sign; evaluate for metastatic disease.
Self-Test (tap to reveal)
expand your quiz bank
Which nucleus carries jaw proprioception?
Mesencephalic nucleus of V.
Name the afferent and efferent limbs of the corneal reflex.
Afferent V1; efferent VII.
Why is the forehead spared in a stroke?
Upper face receives bilateral cortical input to the CN VII nucleus.
Which CN V division does not pass through the cavernous sinus?
V3.
First-line medication for trigeminal neuralgia?
Carbamazepine or oxcarbazepine.
What muscle causes hyperacusis when weak, and which nerve innervates it?
Stapedius; CN VII.
List the five terminal motor branches of the facial nerve.
Temporal, zygomatic, buccal, marginal mandibular, cervical.
Where does chorda tympani carry taste from and where does it synapse centrally?
Anterior two-thirds of tongue; nucleus solitarius.
Schirmer test screens what and helps localize which pathway?
Tear production; greater petrosal/parasympathetic pathway of CN VII.
Describe the hallmark sensory pattern of lateral medullary syndrome.
Ipsilateral facial pain/temp loss with contralateral body pain/temp loss.
UMN facial weakness localizes above which brainstem level?
Above the mid-pons (before corticobulbar fibers reach CN VII nucleus).
Which lesion level pairs facial palsy with horizontal gaze palsy?
Pontine nuclear/fascicular region (CN VI nucleus area with CN VII loop).
High-yield. Zero fluff. — Monkey Intel