Ocular Anatomy and Physiology Flashcards

The blood-retinal barrier is maintained by tight junctions between retinal pigment epithelial cells (outer barrier) and which cell type that forms the inner barrier?

Answer: Retinal capillary endothelial cells

The inner blood-retinal barrier is formed by tight junctions (zonulae occludentes) between retinal capillary endothelial cells, which restrict paracellular transport of macromolecules from the bloodstream into the retina. The outer barrier is formed by RPE tight junctions. While pericytes and astrocytes support barrier function, they do not themselves constitute the structural tight-junction barrier.

A patient's accommodative amplitude is measured at 3.00 D. Using Donder's table norms, this amplitude is most consistent with an expected age of approximately:

Answer: 50 years

According to Donder's table of expected accommodative amplitude by age, approximately 50 years corresponds to a mean amplitude of about 2.5–3.5 D. At age 40, the expected amplitude is roughly 5–6 D; at age 60, it drops to less than 1 D. A 3.00 D amplitude is characteristic of early to mid-presbyopia, placing the patient around age 48–52.

The choroidal watershed zone, an area particularly vulnerable to ischemia in conditions such as anterior ischemic optic neuropathy, is located between territories supplied by which two arterial systems?

Answer: Medial and lateral posterior ciliary arteries

The choroid is perfused by multiple posterior ciliary arteries (PCAs), typically medial and lateral divisions. The watershed zone lies between the terminal perfusion territories of the medial and lateral PCAs, creating a region of relatively poor collateral circulation. This zone, located in the peripapillary choroid and optic nerve head, is highly susceptible to ischemia when perfusion pressure drops, which is the mechanism underlying non-arteritic anterior ischemic optic neuropathy (NAION).

Which phenomenon best explains why a patient with a unilateral dense nuclear sclerotic cataract may report improved near vision (so-called 'second sight') before eventually losing it?

Answer: Progressive myopic shift increases the effective power of the crystalline lens

Nuclear sclerotic cataracts increase the refractive index of the lens nucleus, producing a progressive myopic shift (index myopia). This shift moves the far point closer, temporarily improving unaided near vision for a previously emmetropic or hyperopic patient — a phenomenon called 'second sight of the aged.' As the cataract matures further, light scatter and opacity ultimately degrade vision at all distances.

The axons of which retinal ganglion cell subtype project preferentially to the koniocellular layers of the lateral geniculate nucleus and are thought to carry short-wavelength (blue-yellow) color opponent signals?

Answer: Bistratified ganglion cells

Small bistratified retinal ganglion cells receive ON input from short-wavelength (S-cone) bipolar cells in their inner dendritic tier and OFF input from medium/long-wavelength (M/L-cone) bipolar cells in their outer tier, creating a blue-ON/yellow-OFF chromatic opponent signal. These cells project to the koniocellular (K) layers interleaved between the magnocellular and parvocellular layers of the LGN, distinct from the M-pathway (parasol cells → magnocellular LGN) and P-pathway (midget cells → parvocellular LGN).

Aqueous humor production by the ciliary epithelium involves which transport mechanism as the rate-limiting step responsible for approximately 80–90% of total aqueous secretion?

Answer: Active transport via Na⁺/K⁺-ATPase in the non-pigmented ciliary epithelium

The non-pigmented ciliary epithelium (NPCE) drives the majority of aqueous secretion via active transport, primarily through Na⁺/K⁺-ATPase pumps on the basolateral surface, which establish sodium and osmotic gradients that draw water into the posterior chamber. Carbonic anhydrase II and IV also contribute by generating bicarbonate ions, but the Na⁺/K⁺-ATPase is the dominant, rate-limiting mechanism — which is why carbonic anhydrase inhibitors (e.g., acetazolamide) and beta-blockers (reducing cAMP-driven pump activity) both lower IOP. Passive ultrafiltration accounts for the minority of production.