Neurological - Vision and Eye movements

Eye Movements

The six extraocular muscles per eye precisely position the globe to direct light onto the fovea for optimal vision. Coordinated conjugate movements maintain binocular alignment.

• Extraocular Muscles & Innervation
  • Lateral rectus: abducts eye → CN VI (abducens).
  • Medial rectus: adducts eye → CN III (oculomotor).
  • Superior rectus: elevates, adducts, intorts → CN III.
  • Inferior rectus: depresses, adducts, extorts → CN III.
  • Superior oblique: depresses, abducts, intorts → CN IV (trochlear).
  • Inferior oblique: elevates, abducts, extorts → CN III.

• Principal Types of Eye Movements - All serve to stabilize or shift images on the fovea.
  • Saccades: Rapid, ballistic shifts to foveate new targets (e.g., reading, scanning).
    Generated by frontal eye fields (Brodmann area 8), parietal eye fields, superior colliculus; burst neurons in PPRF (horizontal) and riMLF (vertical); high metabolic demand.
  • Smooth Pursuit: Slow, continuous tracking of moving objects (velocity matching up to ~30–50°/s).
    Involves middle temporal (MT)/medial superior temporal (MST) areas, cerebellum (flocculus/paraflocculus), and brainstem pathways.
  • Vestibulo-Ocular Reflex (VOR): Compensates for head rotation to maintain gaze stability (gain ~1).
    Driven by semicircular canals → vestibular nuclei → oculomotor nuclei; testable with head impulse test or doll's head maneuver.
  • Vergence/Convergence: Disconjugate adduction for near targets; linked to accommodation and miosis (near triad).

• Strabismus & Diplopia: Misalignment causing double vision.
  Causes: CN III/IV/VI palsies, internuclear ophthalmoplegia (INO: MLF lesion), myasthenia gravis, thyroid eye disease, brainstem stroke.

Horizontal Gaze Control

• Key Structures:
  • Paramedian pontine reticular formation (PPRF): horizontal saccade generator (burst neurons).
  • Abducens (VI) nucleus: contains motoneurons (lateral rectus) + internuclear neurons (project via MLF to contralateral III for medial rectus).
  • Medial longitudinal fasciculus (MLF): coordinates conjugate horizontal gaze.
  • Vestibular nuclei: integrate head movement signals.
• Mechanism (e.g., rightward gaze): Right PPRF excites right VI nucleus → right lateral rectus + MLF to left III nucleus → left medial rectus → conjugate right gaze.
  Lesion examples: PPRF lesion → ipsilateral gaze palsy; MLF lesion → INO (adduction deficit + abducting nystagmus).

Vertical Gaze Control

• Centers in rostral midbrain: rostral interstitial nucleus of MLF (riMLF) for vertical saccades; interstitial nucleus of Cajal for vertical gaze holding.
• Coordinates CN III (superior/inferior recti) and CN IV (superior oblique).
• Lesions: dorsal midbrain (Parinaud's syndrome) → upgaze palsy, convergence-retraction nystagmus, light-near dissociation.

Binocular Vision

• Requires synchronous conjugate movements via:
  • PPRF + abducens nucleus (horizontal).
  • Midbrain convergence center (near midbrain reticular formation).
  • MLF for internuclear coordination.
  • Vestibular inputs for head-eye integration.
• Example: Sudden right head turn → right vestibular nucleus → excites right VI/left III via MLF → compensatory leftward eye deviation to stabilize gaze.

Pupils

• Miosis (Constriction): Parasympathetic → Edinger-Westphal nucleus → CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae.
• Mydriasis (Dilation): Sympathetic → hypothalamus → ciliospinal center (C8–T2) → superior cervical ganglion → long ciliary nerves → dilator pupillae.
• Light Reflex: Afferent CN II → pretectal olivary nucleus → bilateral Edinger-Westphal → bilateral miosis (direct + consensual).
  Afferent defect: relative afferent pupillary defect (RAPD/Marcus Gunn pupil).
• Accommodation Reflex (Near Triad): Visual cortex → midbrain → convergence (medial recti), lens accommodation (ciliary muscle), miosis.

Retina

• Light passes through inner layers (ganglion → bipolar → photoreceptors) before hitting rods/cones.
  Photoreceptors hyperpolarize in light → reduce glutamate release → bipolar cells → ganglion cells (first action potentials in visual pathway).
• Horizontal/amacrine cells modulate laterally for contrast/enhancement.
• Fovea: cone-dominated, avascular, thin inner layers → highest acuity/color vision.
  Optic disc: ganglion axons exit (blind spot, no photoreceptors).