👁️ Eyeball & phototransduction core
Light journey: cornea → aqueous humor → pupil → lens → vitreous → retina. At the retina, light traverses multiple layers before reaching the photoreceptor outer segments (rods & cones).
⚡ Phototransduction (simplified): Photons → photopigment isomerization → closure of Na⁺ channels → hyperpolarization → reduced neurotransmitter release. In darkness, photoreceptors are depolarized and release more neurotransmitter.
Rods
- 1 type (rhodopsin)
- Achromatic, low-light sensitivity
- Night vision, motion perception
Cones
- 3 types (red, green, blue)
- Chromatic, bright-light
- Object recognition, visual acuity
🥕 Vitamin A: essential for photopigment synthesis. Deficiency → night blindness (nyctalopia). Humans cannot synthesize it; dietary intake is crucial.
Glaucoma – key concepts
- Open-angle (chronic): decreased aqueous drainage (often via canal of Schlemm) → elevated IOP → painless progressive visual loss.
- Narrow-angle (acute): mechanical blockade of drainage angle → rapid IOP rise (painful, emergency). Treated with cholinomimetics, carbonic anhydrase inhibitors, mannitol.
💡 Visual reflexes high yield
Pupillary light reflex
Pathway: Light → retina → CN II → pretectal nuclei (bilateral) → Edinger-Westphal nuclei (EW) → CN III → ciliary ganglion → pupillary sphincter (M3) → miosis.
- Direct: light in one eye → constriction of that pupil.
- Consensual: light in one eye → constriction of the opposite pupil.
- This reflex does not require visual cortex; cortically blind individuals still have it.
CN II (afferent)
→
Pretectal nuclei (bilateral)
→
Edinger-Westphal (bilateral)
→
CN III → ciliary ganglion
→
Sphincter pupillae → miosis
Accommodation-convergence reaction
When shifting gaze from far to near:
- Accommodation: ciliary muscle contracts (parasympathetic) → suspensory ligaments relax → lens becomes more convex (increased refractive power).
- Convergence: medial recti contract (CN III) → adduction of both eyes.
- Miosis: pupillary sphincter contracts → decreased aperture, increased depth of field.
🩺 Argyll Robertson pupil: bilateral small pupils that accommodate but do not react to light. Classic for neurosyphilis (also MS, pineal tumors). Site: pretectal region rostral to superior colliculi.
🧠 Visual pathways & classic lesions exam
Optic chiasm & tracts
- Nasal retinal fibers cross at chiasm → contralateral optic tract.
- Temporal retinal fibers remain ipsilateral.
- Optic tract contains: ipsilateral temporal + contralateral nasal fibers.
- Major projection: lateral geniculate nucleus (LGB). Also → pretectal (reflex), superior colliculus (gaze), suprachiasmatic (circadian).
📌 Lens inverts image: nasal retina → temporal visual field; temporal retina → nasal visual field.
Visual radiations & cortical areas
- Meyer loop (temporal lobe): carries inferior retinal fibers → superior visual field (contralateral).
- Parietal radiations: carry superior retinal fibers → inferior visual field (contralateral).
- Primary visual cortex (V1, area 17) lies around calcarine sulcus: cuneus (superior bank) receives upper retina; lingual gyrus (inferior bank) receives lower retina.
Lesion patterns – high yield
| Lesion site | Visual field defect | Key feature |
|---|---|---|
| Optic nerve | Monocular blindness (ipsilateral) | Loss of direct light reflex; consensual preserved |
| Optic chiasm (compression) | Bitemporal heteronymous hemianopia | Pituitary adenoma, craniopharyngioma |
| Optic tract | Contralateral homonymous hemianopia | e.g., right tract → left visual field loss |
| Meyer loop (temporal) | Contralateral superior quadrantanopia | "Pie in the sky" |
| Parietal radiations | Contralateral inferior quadrantanopia | "Pie on the floor" |
| Visual cortex (area 17) | Contralateral homonymous hemianopia with macular sparing | Macula has collateral supply (MCA) |
🧩 Cortical blindness: bilateral occipital lesions → blindness but pupillary light reflexes remain intact (because reflex arc leaves before LGB).
- Central scotoma: macular involvement (optic neuritis, MS).
- Optic chiasm compression: often from pituitary macroadenoma → bitemporal hemianopia.
- Homonymous hemianopia: lesions of optic tract, LGB, or radiations (if complete).
- Macular sparing: posterior cerebral artery (PCA) occlusion → occipital infarct, but macula spared due to MCA collateral.
💊 Ocular pharmacology drugs
| Structure | Receptor | Stimulation effect | Blockade effect |
|---|---|---|---|
| Pupillary sphincter | M3 (PANS) | Miosis (contraction) | Mydriasis (relaxation) |
| Radial dilator (iris) | α (SANS) | Mydriasis | Miosis |
| Ciliary muscle | M3 (PANS) | Accommodation (near vision) | Relaxation (far vision) |
| Ciliary epithelium | β (SANS) | Aqueous secretion ↑ | Aqueous secretion ↓ |
💡 Parasympathetic dominance: the eye is predominantly parasympathetic; muscarinic antagonists (e.g., atropine) cause mydriasis and cycloplegia.
🩺 Clinical correlates & pupillary abnormalities
Marcus Gunn pupil (relative afferent)
Afferent defect (CN II) – swinging flashlight test: affected pupil dilates apparently when light is swung from the normal eye to the affected side. Seen in MS, optic neuritis.
Horner syndrome
Oculosympathetic pathway lesion → miosis, ptosis, anhidrosis, apparent enophthalmos.
Adie pupil
Dilated, sluggish to light but better to accommodation. Ciliary ganglion lesion. Often women, loss of knee jerks.
Transtentorial herniation
CN III compression → fixed, dilated pupil, "down and out" eye, ptosis.
🧠 USMLE high-yield: Argyll Robertson = light-near dissociation. Marcus Gunn = afferent defect. Cortical blindness = intact reflexes. Macular sparing = PCA vs MCA.
📌 Summary: lesions and light reflex
- Optic nerve lesion → no direct reflex, consensual intact.
- Optic tract or above (radiations, cortex) → light reflex preserved (because pretectal fibers branch prior).
- Bitemporal hemianopia from chiasm – pituitary tumor.
- Homonymous quadrantanopia: localize to temporal (superior) vs parietal (inferior).