Facial Nerve (CN VII) - Complete Pathway: Cortex to End Organ

1. CORTICAL ORIGIN (Upper Motor Neuron)

• Upper face (forehead, orbicularis oculi): Receives bilateral cortical input - both crossed and uncrossed corticobulbar fibers. This is why forehead wrinkling is spared in upper motor neuron (UMN) lesions (e.g., internal capsule stroke).
• Lower face (mouth, lips, cheeks): Receives predominantly contralateral (crossed) input only.

2. BRAINSTEM NUCLEI (Three Nuclei)

3. INTRACRANIAL SEGMENT (~24 mm)

• Exits the pons at the cerebellopontine angle (CPA) - the pontomedullary junction, lateral aspect
• Travels with the nervus intermedius (carrying parasympathetic + taste fibers) and CN VIII
• Enters the internal auditory meatus (porus acousticus)
• Length: ~24 mm from the pontomedullary sulcus to the porus of the internal auditory canal (IAC)
• At this point the nerve lacks a fibrous sheath (no epineurium) and is covered only by arachnoid

4. INTRACANALICULAR SEGMENT (within the IAC)

• Traverses the internal auditory canal
• At the fundus of the IAC, the facial nerve occupies the anterosuperior quadrant (Bill's bar separates it from the superior vestibular nerve above)
• The nervus intermedius joins here
• Enters the fallopian canal (bony canal) at the fundus

5. INTRATEMPORAL SEGMENTS (Fallopian Canal)

A. Labyrinthine Segment (~4 mm)

• Shortest and narrowest segment of the entire facial nerve
• Runs from the fundus of the IAC to the geniculate ganglion, superior to the cochlea
• Opens into the geniculate fossa (just deep to the suprameatal ridge)
• Contains the geniculate ganglion - cell bodies of taste and somatic sensory neurons
• Bone here is thin and often dehiscent (~25% of ears)
• First branch given off: Greater Superficial Petrosal Nerve (GSPN)
  • Exits at the facial hiatus, runs along the middle fossa floor
  • Carries preganglionic parasympathetics to the pterygopalatine ganglion → lacrimal gland and nasal glands

B. Tympanic (Horizontal) Segment (~13 mm)

• Begins at the 1st genu (at the geniculate ganglion) - nerve turns sharply posteriorly ~90°
• Runs in the medial wall of the tympanic cavity (middle ear)
• Courses superior to the cochleariform process and superior to the oval window niche
• Most common site of congenital dehiscence of the bony canal (especially above the oval window)
• Second branch: Nerve to stapedius (emerges near the pyramidal eminence)

C. Mastoid (Vertical) Segment (~20 mm)

• Begins at the 2nd genu at the pyramidal eminence (just anteroinferior to the lateral semicircular canal)
• Descends vertically through the mastoid bone
• Third branch: Chorda tympani - arises midway in the descending portion
  • Re-enters the middle ear, crosses the tympanic membrane (between malleus and incus)
  • Exits via the petrotympanic fissure to join the lingual nerve
  • Carries taste from anterior 2/3 tongue + preganglionic parasympathetics to submandibular ganglion → submandibular and sublingual glands
• Terminates at the stylomastoid foramen

6. EXTRATEMPORAL SEGMENT (outside the temporal bone)

1. Posterior auricular nerve - first extratemporal branch, supplies occipitalis, posterior auricular muscles
2. Branch to posterior belly of digastric
3. Branch to stylohyoid muscle

• Upper division (temporofacial trunk)
• Lower division (cervicofacial trunk)

Five Terminal Branches (mnemonic: To Zanzibar By Motor Car):

Summary: Complete Pathway at a Glance

Primary Motor Cortex (precentral gyrus, lower 1/3)
        ↓  Corticobulbar tract
Genu of Internal Capsule → Cerebral peduncle → Pons
        ↓
Facial Motor Nucleus (caudal pons) + Superior Salivatory Nucleus + NTS
        ↓
Loops around CN VI nucleus (facial colliculus) → exits at CPA (pontomedullary junction)
        ↓  Intracranial segment (24 mm)
Internal Auditory Canal (IAC) - anterosuperior quadrant at fundus
        ↓
Fallopian Canal begins:
  [1] Labyrinthine segment (4 mm) → Geniculate ganglion → GSPN branch given off
  [2] 1st genu → Tympanic/Horizontal segment (13 mm) → Stapedius nerve branch
  [3] 2nd genu → Mastoid/Vertical segment (20 mm) → Chorda tympani branch
        ↓
Stylomastoid foramen
        ↓  Posterior auricular nerve, digastric branch, stylohyoid branch
Parotid gland → Pes anserinus division
        ↓
5 terminal branches: Temporal | Zygomatic | Buccal | Marginal mandibular | Cervical
        ↓
Muscles of facial expression + stapedius + stylohyoid + posterior digastric

• UMN lesion (e.g., cortical stroke): Contralateral lower face weakness; forehead spared (bilateral cortical supply)
• LMN lesion (e.g., Bell's palsy, acoustic neuroma): Ipsilateral entire hemiface weakness including forehead
• Bell's palsy typically involves the geniculate ganglion / labyrinthine segment (reactivation of HSV-1), causing facial palsy + possible hyperacusis + loss of taste + dry eye

Lesion of Cortex / Internal Capsule - Upper Motor Neuron (UMN) Facial Palsy

The Core Concept: WHY the Forehead is Spared

• Upper face neurons (frontalis, orbicularis oculi) in the facial motor nucleus receive bilateral corticobulbar input - both from the ipsilateral AND contralateral cortex
• Lower face neurons (muscles around the mouth) receive predominantly contralateral (crossed) input only

• The lower face loses its only cortical input → weakness
• The upper face still has intact input from the other hemisphere → spared

What Happens: Feature by Feature

1. Pattern of Facial Weakness

"With supranuclear lesions, there is paresis of voluntary movements of the muscles of facial expression contralateral to the affected hemisphere, with relative sparing of the frontalis and orbicularis oculi. The muscles around the mouth are especially affected." - Localization in Clinical Neurology, 8e

2. Side of Weakness

• Left cortex/internal capsule lesion → Right lower face weakness

3. Voluntary vs. Emotional Facial Movement - Important Dissociation

"Volitional facial paresis without emotional is far more common... and results from interruption of corticobulbar pathways due to lesions of the lower precentral gyrus, internal capsule, cerebral peduncle, or upper pons. The reverse pattern - emotional facial paresis without volitional - is observed with lesions anterior to the precentral gyrus, especially the right hemisphere." - Localization in Clinical Neurology, 8e

"Fibers mediating emotional facial movements do NOT descend in the internal capsule in their course to the facial motor nuclei." - Localization in Clinical Neurology, 8e

Cortex vs. Internal Capsule: Differences

"Pure motor hemiplegia - without sensory abnormalities or cortical signs such as aphasia or neglect - can be localized to the contralateral corticobulbar and corticospinal tracts, most commonly in the internal capsule." - Neuroanatomy through Clinical Cases, 3rd Ed.

Associated Signs (UMN Pattern)

• Hemiparesis/hemiplegia - contralateral face, arm, and leg
• Hyperreflexia - contralateral limbs (may be absent acutely - "spinal shock" equivalent)
• Upgoing plantar (Babinski sign) - contralateral
• Spasticity - develops over weeks (UMN release phenomenon)
• Dysarthria - from weakness of lip and tongue muscles
• No Bell's phenomenon - eye can still close (orbicularis spared)
• No loss of taste, no dry eye, no hyperacusis - these require actual facial nerve damage (LMN)

Bilateral UMN Lesions (Pseudobulbar Palsy)

• Bilateral facial diplegia - weakness of both sides
• Spastic dysarthria ("Donald Duck" speech)
• Dysphagia
• Pseudobulbar affect - pathological laughing and crying disproportionate to emotion (uninhibited, uncontrollable)
• Brisk jaw jerk

Summary Comparison: UMN vs. LMN Facial Palsy

Common Causes of Cortex/Internal Capsule Lesions Causing This Pattern

• Ischemic stroke - most common; internal capsule supplied by lenticulostriate arteries (branches of proximal MCA) → lacunar infarcts
• Hemorrhage - hypertensive bleed into internal capsule or basal ganglia
• Tumor - infiltrating the motor strip or IC
• Demyelination (MS) - plaque in corona radiata or IC
• Trauma - cortical contusion

Signs & Symptoms: Cortical vs. Internal Capsule Lesions

The Fundamental Difference

• Cortex: Motor fibers are spread out over a large area (the homunculus). A small lesion hits only part of the body map.
• Internal capsule: ALL motor and sensory fibers from the entire cortex converge into a tiny compact bundle. Even a tiny lesion hits everything - face + arm + leg equally.

1. CEREBRAL CORTEX LESION

Motor Signs

• Weakness is focal and disproportionate - follows the homunculus
• A small lesion causes weakness of only one body part (e.g., just the hand, just the face, just the leg)
• Full hemiplegia (arm + leg) requires a large, extensive cortical lesion (e.g., entire MCA territory)
• MCA territory (lateral cortex): face + arm + hand most affected; leg relatively spared
• ACA territory (parasagittal): leg most affected; arm and face spared
• Arm extensors and leg flexors preferentially weak (UMN pattern)

The "Cortical Signs" - Hallmark of Cortical Lesions

"Lesions of the cerebral cortex often result in disorders of language and visual-spatial integration (neglect), cortical sensory disturbances (agraphesthesia), apraxia, and seizure. The presence of any of these findings in addition to hemiparesis is suggestive of a lesion within the cerebral cortex." - Frameworks for Internal Medicine

Sensory Signs (Cortical)

• Contralateral hemisensory loss - primary sensory cortex (parietal)
• Mainly cortical/discriminative sensations lost: 2-point discrimination, stereognosis, graphesthesia, proprioception
• Basic pain/temperature may be intact (thalamus handles those)

2. INTERNAL CAPSULE LESION

Structure of the Internal Capsule - Recap

[Caudate nucleus]  [Lentiform nucleus (Putamen + Globus pallidus)]
         \               /
     Anterior limb    Genu    Posterior limb
          |             |           |
    Frontopontine  Corticobulbar  Corticospinal
      fibers        (face)       (arm + leg)
                               + Sensory radiations
                               + Optic radiations

Signs by Location Within the Internal Capsule

Classic Internal Capsule Lesion (Posterior Limb): Pure Motor Hemiplegia

• Contralateral hemiplegia - face + arm + leg equally affected (proportionate)
• No sensory loss (if lesion spares the posterior part of posterior limb)
• No aphasia, no neglect, no agnosia (no cortical signs)
• Dysarthria - common (corticobulbar fibers at the genu)
• Hyperreflexia + Babinski sign contralaterally
• Spasticity develops over days-weeks

"More caudally, the descending motor fibers converge in the posterior limb of the internal capsule, so that even a small lesion there will cause pure motor hemiplegia in which the face, arm, hand, leg, and foot are affected to approximately the same degree." - Adams & Victor's Neurology, 12th Ed.

Lacunar Syndromes of the Internal Capsule (Classic 4)

Side-by-Side Comparison Table

Quick Memory Aid

CORTEX lesion  =  Focal weakness  +  CORTICAL SIGNS (AAAN)
                  • Aphasia
                  • Agnosia (astereognosis, agraphesthesia)
                  • Apraxia
                  • Neglect
                  + Seizures possible

INTERNAL CAPSULE =  PURE MOTOR HEMIPLEGIA (face + arm + leg)
                  • NO cortical signs
                  • Dysarthria common
                  • Tiny lesion = BIG deficit (all fibers packed together)

Hyperacusis

Definition

Mechanism - Why Does It Happen?

Normal physiology of the stapedius muscle:

• When a loud sound arrives, the stapedius reflexively contracts (the acoustic/stapedial reflex)
• This contraction stiffens the ossicular chain (malleus-incus-stapes)
• A stiffer chain transmits less energy to the cochlea - it dampens the sound
• This protects the inner ear from overstimulation by loud sounds

What happens when the facial nerve is damaged:

• Stapedius is paralyzed - it cannot contract
• The ossicular chain remains loose/floppy even when loud sounds arrive
• Sound vibrations are transmitted to the cochlea without any damping
• Normal sounds are perceived as excessively loud and distorted
• Patient describes sounds as "booming," "tinny," or uncomfortably loud in that ear

"Because the stapedius muscle dampens vibration of the stapes bone in the middle ear, paralysis of the facial nerve usually also includes hyperacusis on the ipsilateral side." - Rheumatology (Elsevier)

"In patients with injury to the facial nerve (e.g., Bell's palsy), the stapedial reflex is impaired, and the patient complains that sounds in that ear have a 'booming' quality (hyperacusis)." - Kandel's Principles of Neural Science, 6th Ed.

Localizing Value of Hyperacusis

                PROXIMAL                                    DISTAL
Geniculate  ──────────────────────────────────────────────────────►
ganglion       Labyrinthine  │  Tympanic   │  Nerve to    │  Chorda   │ Stylomastoid
                             │             │  Stapedius   │  Tympani  │ Foramen
                             │             │  (branch)    │ (branch)  │

The key rule:

Hyperacusis = lesion is PROXIMAL to the nerve to stapedius branch No hyperacusis = lesion is DISTAL to the nerve to stapedius branch

"Lesions within this segment of the facial nerve (proximal to nerve to stapedius) result in ipsilateral peripheral facial paralysis, impaired taste, and hyperacusis." - Localization in Clinical Neurology, 8e

"Lesions within the facial canal between the exit of the nerve to stapedius and the exit of the chorda tympani cause facial paralysis and impaired taste - but as the lesion is DISTAL to the nerve to stapedius, hearing is SPARED." - Localization in Clinical Neurology, 8e

In Bell's Palsy

1. Ipsilateral LMN facial palsy (entire half face including forehead)
2. Hyperacusis (nerve to stapedius affected - proximal to it)
3. Loss of taste on anterior 2/3 tongue (chorda tympani affected)
4. Decreased lacrimation (GSPN affected - proximal to it)
5. Retroauricular pain (somatic sensory component of CN VII)

Acoustic Reflex Testing

• A loud tone is delivered to one ear
• Impedance change from stapedius contraction is measured in the other ear (or same ear)
• Absent acoustic reflex on the ipsilateral side = stapedius paralysis = confirms hyperacusis/CN VII lesion proximal to nerve to stapedius
• This is an objective test - does not rely on patient reporting

Quick Summary

Stapedius (CN VII branch) → contracts → dampens ossicular chain → protects cochlea

Facial nerve lesion → stapedius paralysis → no dampening → all sounds amplified
                                                          → HYPERACUSIS

Localizing value:
  Hyperacusis PRESENT  →  Lesion ABOVE nerve to stapedius (geniculate, labyrinthine, IAC, CPA*)
  Hyperacusis ABSENT   →  Lesion BELOW nerve to stapedius (mastoid segment distal, parotid)
  *CPA lesions: usually NO hyperacusis because stapedius branch is spared at that level

Stapedial (Acoustic) Reflex

What It Is

Anatomy Recap: The Stapedius Muscle

• Smallest skeletal muscle in the human body
• Attached to the neck of the stapes (third ossicle)
• Housed inside the pyramidal eminence on the posterior wall of the middle ear
• Innervated by the nerve to stapedius - a branch of CN VII (facial nerve), arising from the mastoid/vertical segment just below the second genu

Reflex Arc - The Complete Pathway

Stimulus: Loud sound (~70-100 dB HL in a normal ear)

TWO arcs exist depending on which ear is stimulated:

A. Ipsilateral Arc (stimulus and recording in the SAME ear)

Loud sound → Cochlea (outer/inner hair cells)
    ↓
CN VIII (Cochlear nerve / Acoustic nerve)
    ↓
Ipsilateral Ventral Cochlear Nucleus (VCN)
    ↓
Trapezoid body
    ↓
Ipsilateral Facial Motor Nucleus (OR Medial Superior Olive → Facial nucleus)
    ↓
Ipsilateral CN VII (Facial nerve) → Nerve to stapedius branch
    ↓
Ipsilateral Stapedius muscle contracts
    ↓
Stiffens ipsilateral ossicular chain → Sound damped

B. Contralateral Arc (stimulus in one ear, recording/contraction in the OTHER ear)

Loud sound in RIGHT ear → Cochlea
    ↓
Right CN VIII → Right Ventral Cochlear Nucleus
    ↓
Crosses midline via Trapezoid body
    ↓
LEFT Medial Superior Olive
    ↓
LEFT Facial Motor Nucleus
    ↓
LEFT CN VII → Left Nerve to stapedius
    ↓
LEFT Stapedius muscle contracts

"The contralateral acoustic reflex arc includes the acoustic nerve and ventral cochlear nucleus, the medial superior olive, the contralateral motor nucleus of CN VII, and the contralateral stapedius muscle." - Cummings Otolaryngology

"The sensory pathway is through the cochlear nerve and nucleus to the reticular formation adjacent to the facial motor nucleus and from there to the stapedial motor neurons, which run in the facial nerve." - Kandel's Principles of Neural Science, 6th Ed.

Normal Threshold

• Reflex occurs at 70-100 dB HL in normal hearing ears
• In cochlear hearing loss with recruitment (loudness intolerance): reflex may occur at levels as low as 60 dB SL (occurs at lower sensation levels because of abnormal loudness growth)

Clinical Testing: Tympanometry / Impedance Audiometry

Localizing Value: Acoustic Reflex Patterns

Pattern Analysis Table

Localizing CN VII Lesions with Stapedial Reflex

     GSPN    Nerve to    Chorda      Stylomastoid
     branch  Stapedius   Tympani      Foramen
       |         |          |             |
──────────────────────────────────────────────────►  Facial nerve (distal)
                ↑
         If lesion HERE (proximal to nerve to stapedius):
         → Stapedial reflex ABSENT
         → Hyperacusis present

                             ↑
                  If lesion HERE (distal to nerve to stapedius):
                  → Stapedial reflex PRESENT (stapedius branch intact)
                  → No hyperacusis

"The stapedius reflex provides valuable clinical information regarding the level of a facial nerve lesion. If the lesion is proximal, an absent stapedius reflex may be expected. If the lesion is distal to the stapedius branch, an intact reflex may be expected. Stapedius reflex measurement should be included ROUTINELY in the evaluation of patients with facial paralysis." - Cummings Otolaryngology

In Bell's Palsy: Prognostic Value

• Reflex PRESENT = the lesion is incomplete / distal → better prognosis for recovery
• Reflex ABSENT = lesion at or proximal to the nerve to stapedius → more severe, may indicate degeneration → poorer prognosis, higher chance of incomplete recovery

Quick Summary

STAPEDIAL REFLEX ARC:
  Loud sound → CN VIII → VCN → (cross via trapezoid body) →
  Facial motor nucleus → CN VII → Nerve to stapedius → Stapedius contracts
  → Ossicular chain stiffens → Sound damped → Cochlea protected

BILATERAL: One loud sound triggers BOTH stapedii simultaneously

CLINICAL USE:
  1. Differentiate cochlear vs. retrocochlear hearing loss
  2. Detect brainstem lesions (contralateral absent, ipsilateral present)
  3. Localize CN VII lesions (absent = proximal; present = distal to stapedius branch)
  4. Prognosis in Bell's palsy