SN3 SHORT NOTE 5M ★ High-yield

Describe the layers of the retina. Compare rods and cones and add a note on the fovea.

Anatomy · 1st Professional — Pre-Clinical

General Structure

The retina is the innermost, neural coat of the eyeball, developed from the optic cup, an outgrowth of the diencephalon — making it part of the brain rather than a peripheral organ.

The outer layer of the optic cup becomes the pigment epithelium; the inner layer becomes the neural retina
The potential space between them is the site of retinal detachment, which is a separation of the two layers of the cup rather than of the retina from the choroid

The Ten Layers

No.	Layer	Contents
1	Pigment epithelium	Cuboidal cells with melanin; phagocytose shed photoreceptor discs
2	Layer of rods and cones	Outer segments of the photoreceptors
3	External limiting membrane	Junctions with Muller cells
4	Outer nuclear layer	Cell bodies of rods and cones
5	Outer plexiform layer	Synapses between photoreceptors and bipolar cells
6	Inner nuclear layer	Bipolar, horizontal, amacrine and MULLER cells
7	Inner plexiform layer	Synapses between bipolar and ganglion cells
8	Ganglion cell layer	Cell bodies of ganglion cells
9	Nerve fibre layer	Axons of ganglion cells → the optic nerve
10	Internal limiting membrane	Foot processes of Muller cells

Rods and Cones

Feature	Rods	Cones
Number	About 120 million	About 6 million
Pigment	Rhodopsin (one type)	Iodopsins — three types
Function	Dim light (scotopic); no colour	Bright light (photopic); colour; high acuity
Sensitivity	High	Low
Distribution	Absent at the fovea; maximal about 20 degrees from it	Concentrated at the fovea; few peripherally
Convergence	Many rods to one ganglion cell — high sensitivity, low acuity	One cone to one ganglion cell at the fovea — high acuity

Specialised Regions

Macula LUTEA — a yellow area lateral to the optic disc, containing xanthophyll
Fovea centralis — a depression at its centre; only cones, no rods; the inner layers displaced; avascular, nourished from the choroid; the point of greatest acuity
Optic disc — where the axons leave; NO photoreceptors, so it is the blind spot; the only part where the central retinal vessels are visible
Ora serrata — the anterior limit of the neural retina

Applied Aspects

Retinal detachment — separation of the neural retina from the pigment epithelium; the photoreceptors lose their choroidal nutrition and die, so it is an emergency. Patients describe floaters, flashes and a curtain across the field
Central retinal artery occlusion — sudden painless blindness with a pale retina and a cherry-red spot at the fovea, where the intact choroidal circulation shows through the thin avascular tissue
Diabetic retinopathy — microaneurysms, dot and blot haemorrhages, hard exudates, then neovascularisation; the commonest cause of blindness in working-age adults
Age-related macular degeneration destroys central vision while peripheral vision is retained, because the fovea is affected first
Retinitis pigmentosa — rod degeneration first, so night blindness and tunnel vision precede loss of central vision
The optic disc is examined for papilloedema and cupping; it is the only place in the body where blood vessels and a part of the central nervous system can be seen directly

All questions in Systemic Histology

Every Anatomy chapter →