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Ocular anatomy and physiology — what Stage A asks

Most basic science questions in Stage A rest on a few structural facts: the cornea is clear thanks to a stroma of regularly arranged type I collagen and an endothelium that pumps out water, the lens lives on anaerobic glycolysis, aqueous is produced in the ciliary body and leaves mainly through the trabecular meshwork, and the outer retina is fed by the choroid and the inner retina by the retinal artery.

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Why it matters in the exam

The cornea: epithelium, Bowman layer, stroma (about 90% of the thickness, type I collagen in parallel lamellae at regular spacing), Descemet membrane (the basement membrane of the endothelium) and endothelium. The endothelium does not regenerate in humans; it keeps the cornea "dry" with Na/K-ATPase pumps, and when the cell count falls greatly — oedema. Normal central thickness is about 540 microns. The cornea provides about two thirds of the eye's refractive power (about 43 dioptres) and the lens about 20 dioptres. The tear film: lipid from the meibomian glands, water from the lacrimal gland, mucin from goblet cells.

Aqueous is actively secreted by the non-pigmented epithelium of the ciliary body (with carbonic anhydrase), at about 2.5 µL per minute, and leaves mainly through the trabecular meshwork into Schlemm canal and the episcleral veins, and partly through the uveoscleral route. The lens — without vessels or nerves, with epithelium only on the anterior side, keeps growing throughout life; most of its ATP comes from anaerobic glycolysis. The receptors: rods (rhodopsin, peak sensitivity around 500 nm, night vision) and cones (day and colour vision, three types). The fovea has cones only.

Phototransduction: light converts 11-cis retinal to all-trans, transducin activates PDE6, cGMP falls, sodium channels close and the receptor hyperpolarises — receptors "switch off" in light. The RPE engulfs receptor tips, recycles vitamin A and forms the outer blood–retina barrier. Dual blood supply: the central retinal artery for the inner layers, the choroid for the receptors — which is why the fovea looks red in CRAO. The light reflex: retina → optic nerve → pretectal nucleus → Edinger-Westphal nuclei on both sides → third nerve → ciliary ganglion → sphincter muscle. Embryology: lens and corneal epithelium from surface ectoderm, retina and RPE from neuroectoderm, corneal stroma and endothelium from neural crest cells.

Key points

• Stroma: regular type I collagen; endothelium: a pump that does not regenerate.
• Cornea about 43 dioptres, lens about 20.
• Aqueous: ciliary body, carbonic anhydrase; trabecular outflow.
• Lens: anaerobic glycolysis.
• Rhodopsin around 500 nm; hyperpolarisation in light.

Summary table

In the full version for subscribers

• A simple algorithm
• Memory aid
• Exam traps
• 5 review cards
• Enough for IMLE

The full version is open to subscribers of: Ophthalmology, Medical licensing exam (IMLE).

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Sources

• AAO Basic and Clinical Science Course, Section 2: Fundamentals and Principles of Ophthalmology, 2024-2025 — Ocular anatomy, biochemistry and physiology
• Adler's Physiology of the Eye, 11th edition (2011) — Cornea, aqueous humour, lens and phototransduction
• Kanski's Clinical Ophthalmology, 9th edition (2020) — Ocular anatomy overview

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.