Anatomy and Histology of the Visual System
This domain tests structure-function relationships from the tear film and cornea through the retina, optic nerve, chiasm, and postchiasmal visual pathways. Expect layered anatomy, vascular supply, extraocular muscle actions, and lesion localization.
Core rules
- Cornea has five layers: epithelium, Bowman layer, stroma, Descemet membrane, and endothelium. The endothelium is neural-crest derived and uses Na+/K+-ATPase to maintain deturgescence.
- Retina layers, inner to outer: internal limiting membrane, nerve fiber layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, external limiting membrane, photoreceptor layer, and retinal pigment epithelium. The fovea is cone-only and avascular.
- Visual pathway: retinal ganglion cells -> optic nerve -> optic chiasm -> optic tract -> lateral geniculate nucleus -> optic radiations -> primary visual cortex. Nasal fibers cross at the chiasm.
- Cranial nerves: III supplies medial rectus, superior rectus, inferior rectus, inferior oblique; IV supplies superior oblique; VI supplies lateral rectus.
Worked example
A pituitary mass compresses the optic chiasm. Reasoning: crossing nasal retinal fibers subserve the temporal visual fields. Compression produces bitemporal heteronymous hemianopia. A right optic tract lesion instead produces a left homonymous hemianopia because postchiasmal fibers carry information from the left visual field.
Traps that cost marks
- Confusing the fovea with the optic disc: the fovea has no rods and no major vessels; the disc is the axon exit and blind spot.
- Reversing cylinder axis conventions when describing corneal astigmatism.
- Assigning superior oblique action to elevation; its primary action is intorsion, with depression in adduction.