Light passes through the retina to reach the photoreceptors at the back
Mnemonic
Ten layers, but the useful version is “light travels backwards through the retina”: light must pass through the entire neural retina to reach the photoreceptors at the back, which is why the fovea is a pit, with the inner layers swept aside for maximum acuity.
From the vitreous inwards to the choroid:
- Internal limiting membrane
- Nerve fibre layer
- Ganglion cell layer
- Inner plexiform layer
- Inner nuclear layer
- Outer plexiform layer
- Outer nuclear layer
- External limiting membrane
- Photoreceptor layer, rods and cones
- Retinal pigment epithelium
“Rods for night and periphery, cones for colour and the fovea”: about 120 million rods and 6 million cones, with the fovea containing cones only.
Retinal detachment separates the photoreceptors from the pigment epithelium, at the embryological cleft between the two layers of the optic cup, which is why the plane is consistent and why detachment causes rapid photoreceptor death without prompt reattachment.
Expansion
Layers, from the vitreous inwards to the choroid:
- Nerve fibre layer (ganglion cell axons, becoming the optic nerve)
- Ganglion cell layer
- Inner plexiform layer
- Bipolar cell layer
- Outer plexiform layer
- Photoreceptor nuclei
- Rods and cones outer segments
- Retinal pigment epithelium
Photoreceptors
| Rods | Cones | |
|---|---|---|
| Number | ~120 million | ~6 million |
| Sensitivity | High, for dim light | Low |
| Acuity | Low | High |
| Colour | No | Yes, three types |
| Distribution | Peripheral, none at the fovea | Concentrated at the fovea |
The fovea achieves maximum acuity by containing only cones, each with a one to one connection to a bipolar and ganglion cell, and by displacing the overlying layers aside so light reaches the photoreceptors directly.
The optic disc has no photoreceptors, giving the blind spot.
The retinal pigment epithelium absorbs stray light, recycles retinal and phagocytoses shed outer segments; its failure underlies macular degeneration, and its adhesion is lost in retinal detachment.