Retina

At a glance

English

Retina

Latin (TA2)

Retina

System

Eye

Region

Head

Side

Paired, left and right

About the retina

The retina is the innermost of the three coats of the eye. It lines the vitreous chamber from the optic disc at the back to the ora serrata, a wavy margin just behind the ciliary body, where the light-sensitive part ends; a thin non-sensory layer continues forward over the ciliary body and iris. Developmentally it is an outgrowth of the diencephalon, so it truly belongs to the central nervous system, and the optic nerve is really a brain tract. It has two components: an outer retinal pigment epithelium, a single sheet of melanin-filled cells resting on the choroid, and an inner neural retina.

The neural retina is built in layers. Photoreceptors lie at its outer side, against the pigment epithelium, so light must pass through the full thickness of the tissue before reaching them. Their signals pass inwards to bipolar cells and then to ganglion cells, whose axons sweep across the inner surface towards the disc and form the optic nerve. Horizontal and amacrine cells link neighbouring pathways sideways, and tall Müller glial cells span the whole depth and form its inner and outer limiting membranes. In histology ten layers are named, alternating rows of cell bodies with zones of synapses.

There are two kinds of photoreceptor. Rods are extremely sensitive and serve vision in dim light, without colour. Cones work in brighter light, give colour vision and fine detail, and are crowded into the fovea, the central pit of the macula, a small yellowish area temporal to the disc. At the optic disc there are no photoreceptors, which creates the blind spot. A few ganglion cells contain their own pigment and report overall brightness for the pupil reflex and the body clock.

Supply is double. The central retinal artery, a branch of the ophthalmic artery, enters with the optic nerve and nourishes the inner layers, while the outer layers, including the photoreceptors, depend on diffusion from the choriocapillaris of the choroid. Blockage of the central artery causes sudden painless loss of vision. In retinal detachment, fluid separates the neural retina from the pigment epithelium. Diabetes and high blood pressure damage the retinal vessels, macular degeneration destroys central vision, retinitis pigmentosa causes night blindness and narrowing fields, and retinoblastoma is a childhood cancer of the retina.

Function and supply

Function

Detects light with rods and cones, processes the signals through its layered neurons, and sends the result to the brain along the optic nerve; also signals light levels for the pupil reflex and circadian timing.

Blood supply

The central retinal artery from the ophthalmic artery supplies the inner layers; the outer layers rely on the choriocapillaris supplied by the posterior ciliary arteries. Drainage is by the central retinal vein.