Retina

Retina

Also known as: neural tunic, neurosensory retina, bipolar cells, ganglion cells, horizontal cells, amacrine cells, Müller cells, ora serrata

The light-sensitive inner tunic of the eye — an outgrowth of the brain — in which photoreceptors transduce light and a three-neuron chain with lateral interneurons processes the image before ganglion-cell axons leave as the optic nerve.

Show on body map Eye — Horizontal Section

Overview

Photoreceptors≈92 million rods + ≈4.6 million cones

Ganglion cells≈1.2 million (≈0.7–1.5 million)

Thickness≈0.1–0.3 mm

Retinal layers10

The optic (visual) part of the retina lines the posterior eye from the optic disc to the scalloped ora serrata; anteriorly it continues as the non-photosensitive ciliary and iridial parts. It is ≈0.1 mm thick at the ora, ≈0.23 mm at the equator, and thickest (≈0.3 mm) around the fovea, where it thins to ≈0.1 mm at the foveola itself. Developmentally and functionally it is part of the CNS: its outer layer (from the outer wall of the optic cup) is the retinal pigment epithelium and its inner layer the neurosensory retina; they are held together only loosely, so they can separate (retinal detachment).

The retina is "inverted": light passes through the transparent inner layers (ganglion, bipolar cells) before reaching the photoreceptors at the back, which are apposed to the RPE that nourishes them. The vertical "through" pathway is photoreceptor → bipolar cell → ganglion cell; horizontal cells (outer plexiform layer) and amacrine cells (inner plexiform layer) mediate lateral interactions such as centre–surround antagonism, and Müller cells are the principal glia spanning the retina.

The retina performs substantial computation: ≈96 million photoreceptors converge on ≈1.2 million ganglion cells (≈80:1 overall, but nearly 1:1 for foveal cones, giving high acuity, and up to hundreds:1 for peripheral rods, giving high sensitivity). Ganglion cells have centre–surround receptive fields (ON-centre or OFF-centre) and fall into classes — midget (parvocellular), parasol (magnocellular), bistratified (koniocellular) and intrinsically photosensitive (melanopsin) cells.

Location: Innermost layer of the posterior eye wall, between the choroid and the vitreous body; extends from the optic disc to the ora serrata (≈6–7 mm behind the limbus).

Parts & subdivisions

1. Retinal pigment epithelium (RPE)
Single layer of pigmented cuboidal cells on Bruch membrane; outer blood–retina barrier; phagocytoses outer segments; recycles retinal.
2. Photoreceptor layer
Outer and inner segments of rods and cones.
3. External (outer) limiting membrane
Not a true membrane — a row of adherens junctions between Müller cells and photoreceptor inner segments.
4. Outer nuclear layer
Cell bodies (nuclei) of rods and cones.
5. Outer plexiform layer
Synapses between photoreceptor terminals (rod spherules, cone pedicles), bipolar and horizontal cell dendrites; ribbon synapses. Forms the oblique Henle fibre layer around the fovea.
6. Inner nuclear layer
Cell bodies of bipolar, horizontal, amacrine and Müller cells. Supplied by the central retinal artery (its outer part also by choroidal diffusion).
7. Inner plexiform layer
Synapses between bipolar axons, amacrine processes and ganglion cell dendrites; divided into OFF (outer) and ON (inner) sublaminae.
8. Ganglion cell layer
Cell bodies of retinal ganglion cells (and displaced amacrine cells); several cells thick around the fovea, absent at the foveola.
9. Nerve fibre layer
Unmyelinated ganglion cell axons converging on the optic disc; arcuate pattern around the fovea (basis of arcuate field defects in glaucoma).
10. Internal (inner) limiting membrane
Basal lamina formed by Müller cell end-feet, facing the vitreous.
Bipolar cells
Second-order neurons. Rod bipolars are all ON; cone bipolars are ON (mGluR6 — sign-inverting, depolarise to light) or OFF (AMPA/kainate — hyperpolarise to light). Generate graded potentials only.
Horizontal cells
Interneurons in the outer plexiform layer providing inhibitory feedback to photoreceptors — creates the antagonistic surround of receptive fields (lateral inhibition, edge enhancement).
Amacrine cells
Diverse (≈30 types) inner-plexiform interneurons (GABA, glycine, dopamine, ACh); shape temporal responses, direction selectivity, and route rod signals (AII amacrine) into cone pathways.
Retinal ganglion cells
Output neurons — the only retinal cells firing action potentials; ≈1.2 million axons form the optic nerve. ≈1–2% are melanopsin-containing intrinsically photosensitive RGCs projecting to the suprachiasmatic nucleus and pretectum.
Müller cells
Radial glia spanning ILM to OLM: K⁺ buffering (spatial siphoning), glutamate recycling, glycogen store, structural support and light-guiding.

Anatomy

Arterial supply: Dual supply: the central retinal artery (branch of the ophthalmic artery, an end artery) supplies the inner two-thirds (ILM to inner nuclear layer) via four branch arterioles on the retinal surface; the choriocapillaris supplies the outer third (photoreceptors and RPE) by diffusion. The fovea is avascular and depends entirely on the choroid. A cilioretinal artery (from the posterior ciliary circulation) supplying part of the macula is present in ≈15–30% of eyes.
Venous drainage: Central retinal vein → superior ophthalmic vein or directly to the cavernous sinus.
Histology: Ten layers (see parts). Inner blood–retina barrier = tight junctions of retinal capillary endothelium; outer barrier = tight junctions of the RPE.
Development: Inner layer of the optic cup → neurosensory retina (ganglion cells differentiate first, photoreceptors last); outer layer → RPE. The fovea continues to mature until ≈4 years of age.

Function

Transduce light into electrical signals (photoreceptors)
Process contrast, colour, motion and adaptation before signals leave the eye
Transmit visual information via the optic nerve to the brain
Provide non-image-forming light signals for circadian entrainment and the pupil reflex (ipRGCs)

Clinical significance

Central retinal artery occlusion

Sudden, painless, profound monocular visual loss, usually embolic (carotid plaque, cardiac) or from giant cell arteritis (check ESR/CRP in older patients). Fundus: pale oedematous retina with a cherry-red spot at the fovea; RAPD. A transient version is amaurosis fugax — a TIA of the eye warranting urgent stroke work-up.

Central retinal vein occlusion

Painless visual loss with a "blood and thunder" fundus (widespread haemorrhages, dilated tortuous veins, disc swelling). Risk factors: hypertension, diabetes, glaucoma, hyperviscosity. Complications: macular oedema, neovascular glaucoma.

Retinal detachment and retinitis pigmentosa

Detachment separates photoreceptors from their RPE/choroidal support (see system clinical notes). Retinitis pigmentosa is an inherited rod–cone dystrophy: night blindness, progressive tunnel vision, "bone-spicule" pigmentation and attenuated vessels.

Hypertensive retinopathy

Arteriolar narrowing, AV nicking, flame haemorrhages, cotton-wool spots and hard exudates; papilloedema indicates malignant (accelerated) hypertension.

Mnemonics

Layers (outer → inner)

"Really Patient Ophthalmologists Often Observe Inner Images Giving New Insight" — RPE, Photoreceptors, OLM, ONL, OPL, INL, IPL, GCL, NFL, ILM.

Test yourself

Which retinal cells are the only ones that normally generate action potentials and form the optic nerve?

Ganglion cells
Bipolar cells
Horizontal cells
Müller cells

Answer: Ganglion cells

Photoreceptors, bipolar and horizontal cells use graded potentials; retinal ganglion cells fire action potentials, and their ≈1.2 million axons converge at the optic disc to form the optic nerve.

Why does the fovea provide the highest visual acuity?

It contains the highest density of rods
It is supplied directly by the central retinal artery
It lies at the optic disc where ganglion axons converge
Inner retinal layers are displaced and densely packed cones connect almost 1:1 to ganglion cells

Answer: Inner retinal layers are displaced and densely packed cones connect almost 1:1 to ganglion cells

The foveola is rod-free, packed with slender cones (≈200 000/mm²), the overlying layers and vessels are pushed aside, and each cone has a "private line" via a midget bipolar to a midget ganglion cell. It is avascular and nourished by the choroid.