Neuroscience - Vision and Eye movements

The 6 extraocular muscles in each eye precisely position the globe so that light falls on the fovea, the area of highest visual acuity. 👀 Coordinated conjugate eye movements keep both eyes aligned on the same target, allowing binocular single vision. 💡 Disruption of this system causes diplopia, strabismus, or impaired gaze control.

• 🧠 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

• ⚡ Saccades: rapid, ballistic eye movements that shift fixation from one target to another, such as during reading or scanning a room.
  Generated mainly by the frontal eye fields, parietal eye fields, and superior colliculus, with burst neurons in the PPRF for horizontal movements and riMLF for vertical movements.
• 🎥 Smooth pursuit: slow, continuous tracking of a moving target.
  Requires cortical motion areas such as MT/MST, cerebellar modulation, and brainstem gaze pathways.
• 🌀 Vestibulo-ocular reflex (VOR): stabilises gaze during head movement by moving the eyes in the opposite direction to head rotation.
  Driven by the semicircular canals → vestibular nuclei → ocular motor nuclei.
  💡 This is why you can keep your eyes on a target while your head moves.
• 🔍 Vergence / convergence: disconjugate movements that align both eyes for near vision.
  Part of the near triad: convergence + accommodation + miosis.

⚠️ Strabismus & Diplopia

• Strabismus = misalignment of the visual axes.
• Diplopia occurs when the eyes are misaligned and both foveae are no longer directed at the same target.
• Common causes include:
  • 🧠 CN III, IV, or VI palsies
  • 🔗 Internuclear ophthalmoplegia (INO) due to MLF lesion
  • 💪 Myasthenia gravis
  • 🦋 Thyroid eye disease
  • 🚨 Brainstem stroke

↔️ Horizontal Gaze Control

• 🧩 Key Structures
  • PPRF (paramedian pontine reticular formation): horizontal saccade generator
  • Abducens (VI) nucleus: contains:
    • motor neurons to the ipsilateral lateral rectus
    • internuclear neurons projecting via the MLF to the contralateral III nucleus for medial rectus activation
  • MLF (medial longitudinal fasciculus): coordinates conjugate horizontal gaze
  • Vestibular nuclei: integrate head-movement signals into eye movements
• ➡️ Mechanism of rightward gaze
  • The right PPRF activates the right VI nucleus.
  • This causes:
    • right lateral rectus contraction
    • signal via the left MLF to the left III nucleus → left medial rectus contraction
  • The result is conjugate gaze to the right.
• 🚨 Classic lesion patterns
  • PPRF lesion → ipsilateral gaze palsy
  • MLF lesion → INO: impaired adduction of the affected eye with abducting nystagmus of the opposite eye
  • Abducens nucleus lesion → ipsilateral horizontal gaze palsy

↕️ Vertical Gaze Control

• Vertical gaze centres are located mainly in the rostral midbrain.
• riMLF (rostral interstitial nucleus of the MLF) generates vertical saccades.
• Interstitial nucleus of Cajal helps maintain vertical gaze position.
• These pathways coordinate:
  • CN III → superior and inferior recti, inferior oblique
  • CN IV → superior oblique
• 🚨 Dorsal midbrain (Parinaud) syndrome classically causes:
  • upgaze palsy
  • convergence-retraction nystagmus
  • light-near dissociation

👓 Binocular Vision

• Binocular single vision requires accurate alignment of both eyes on the same target.
• This depends on:
  • PPRF + abducens nucleus for horizontal gaze
  • Midbrain convergence centres for near gaze
  • MLF for internuclear coordination
  • Vestibular inputs to stabilise gaze during head movement
• Example: if the head turns suddenly to the right, the vestibular system generates compensatory leftward eye movement to keep vision stable.

👁️‍🗨️ Pupils

• 🔹 Miosis (constriction): parasympathetic pathway
  • Edinger–Westphal nucleus → CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae
• 🔸 Mydriasis (dilation): sympathetic pathway
  • Hypothalamus → ciliospinal centre (C8–T2) → superior cervical ganglion → long ciliary nerves → dilator pupillae
• 💡 Light reflex
  • Afferent limb: CN II
  • Signal passes to the pretectal nuclei
  • Then bilaterally to the Edinger–Westphal nuclei
  • This produces both:
    • direct light reflex
    • consensual light reflex
  • RAPD (Marcus Gunn pupil) indicates an afferent pathway defect.
• 📚 Accommodation reflex (near triad)
  • Convergence of both eyes
  • Lens accommodation via ciliary muscle
  • Miosis

🧠 Retina

• Light passes through the inner retinal layers before reaching the photoreceptors (rods and cones).
• Unlike most neurons, photoreceptors hyperpolarise in response to light, reducing glutamate release.
• This signal is then processed through:
  • bipolar cells
  • ganglion cells — the first cells in the visual pathway to generate action potentials
• Horizontal cells and amacrine cells provide lateral modulation, improving contrast and visual processing.
• 🎯 Fovea:
  • cone-rich
  • avascular
  • thin inner layers
  • site of highest visual acuity and colour vision
• ⚫ Optic disc:
  • site where ganglion cell axons leave the eye
  • contains no photoreceptors
  • creates the physiological blind spot

💡 Key summary:
👁️ Eye movements depend on the precise coordination of CN III, IV, and VI, brainstem gaze centres, the MLF, and vestibular input.
↔️ Horizontal gaze is organised through the PPRF–VI nucleus–MLF–III nucleus pathway.
↕️ Vertical gaze depends mainly on the rostral midbrain.
👁️‍🗨️ Pupil reflexes test both afferent (CN II) and efferent parasympathetic (CN III) pathways.
🧠 The retina is not just a camera sensor — it is active neural tissue that performs early visual processing before signals even leave the eye.