ποΈ Eyeball & phototransduction core
Light journey: cornea β aqueous humor β pupil β lens β vitreous β retina. At the retina, light traverses multiple layers before reaching the photoreceptor outer segments (rods & cones).
β‘ Phototransduction (simplified): Photons β photopigment isomerization β closure of NaβΊ channels β hyperpolarization β 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
π₯ Vitamin A: essential for photopigment synthesis. Deficiency β night blindness (nyctalopia). Humans cannot synthesize it; dietary intake is crucial.
Glaucoma β key concepts
- Open-angle (chronic): decreased aqueous drainage (often via canal of Schlemm) β elevated IOP β painless progressive visual loss.
- Narrow-angle (acute): mechanical blockade of drainage angle β rapid IOP rise (painful, emergency). Treated with cholinomimetics, carbonic anhydrase inhibitors, mannitol.
π‘ Visual reflexes high yield
Pupillary light reflex
Pathway: Light β retina β CN II β pretectal nuclei (bilateral) β Edinger-Westphal nuclei (EW) β CN III β ciliary ganglion β pupillary sphincter (M3) β miosis.
- Direct: light in one eye β constriction of that pupil.
- Consensual: light in one eye β constriction of the opposite pupil.
- This reflex does not require visual cortex; cortically blind individuals still have it.
CN II (afferent)
β
Pretectal nuclei (bilateral)
β
Edinger-Westphal (bilateral)
β
CN III β ciliary ganglion
β
Sphincter pupillae β miosis
Accommodation-convergence reaction
When shifting gaze from far to near:
- Accommodation: ciliary muscle contracts (parasympathetic) β suspensory ligaments relax β lens becomes more convex (increased refractive power).
- Convergence: medial recti contract (CN III) β adduction of both eyes.
- Miosis: pupillary sphincter contracts β decreased aperture, increased depth of field.
π©Ί 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.
π§ Visual pathways & classic lesions exam
Optic chiasm & tracts
- Nasal retinal fibers cross at chiasm β contralateral optic tract.
- Temporal retinal fibers remain ipsilateral.
- Optic tract contains: ipsilateral temporal + contralateral nasal fibers.
- Major projection: lateral geniculate nucleus (LGB). Also β pretectal (reflex), superior colliculus (gaze), suprachiasmatic (circadian).
π Lens inverts image: nasal retina β temporal visual field; temporal retina β nasal visual field.
Visual radiations & cortical areas
- Meyer loop (temporal lobe): carries inferior retinal fibers β superior visual field (contralateral).
- Parietal radiations: carry superior retinal fibers β 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 β 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 β 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) |
𧩠Cortical blindness: bilateral occipital lesions β 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 β bitemporal hemianopia.
- Homonymous hemianopia: lesions of optic tract, LGB, or radiations (if complete).
- Macular sparing: posterior cerebral artery (PCA) occlusion β occipital infarct, but macula spared due to MCA collateral.
π Ocular pharmacology drugs
| Structure | Receptor | Stimulation effect | Blockade effect |
|---|---|---|---|
| Pupillary sphincter | M3 (PANS) | Miosis (contraction) | Mydriasis (relaxation) |
| Radial dilator (iris) | Ξ± (SANS) | Mydriasis | Miosis |
| Ciliary muscle | M3 (PANS) | Accommodation (near vision) | Relaxation (far vision) |
| Ciliary epithelium | Ξ² (SANS) | Aqueous secretion β | Aqueous secretion β |
π‘ Parasympathetic dominance: the eye is predominantly parasympathetic; muscarinic antagonists (e.g., atropine) cause mydriasis and cycloplegia.
π©Ί Clinical correlates & pupillary abnormalities
Marcus Gunn pupil (relative afferent)
Afferent defect (CN II) β 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 β 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 β fixed, dilated pupil, "down and out" eye, ptosis.
π§ USMLE high-yield: Argyll Robertson = light-near dissociation. Marcus Gunn = afferent defect. Cortical blindness = intact reflexes. Macular sparing = PCA vs MCA.
π Summary: lesions and light reflex
- Optic nerve lesion β no direct reflex, consensual intact.
- Optic tract or above (radiations, cortex) β light reflex preserved (because pretectal fibers branch prior).
- Bitemporal hemianopia from chiasm β pituitary tumor.
- Homonymous quadrantanopia: localize to temporal (superior) vs parietal (inferior).