Organization of the retina

The retina is the innermost layer of the eye. It consists of five distinct neuron types, inter-connected in circuits which are repeated millions of times.

The light-sensitive photoreceptors that lie at the back of the retina adjacent to melanin-having pigmented epithelial cells, form synapses with a range of retinal inter-neurons; horizontal cells, bipolar cells, and amacrine cells. The Bipolar cells synapse with the output neurons of the retina, ganglion cells, the axons of which form the optic nerve, and only become myelinated as they leave the eye through the optic disc. However all the cells in the retina (except the pigment cells) are neurons, the ganglion cells are only able to fire the action potentials. The Photoreceptors and retinal interneurons signal by the way of passively conducted synaptic potentials. Each human retina has roughly 108 photoreceptors, but an output through only about 106 optic nerve axons. This is a huge convergence and displays that considerable processing of visual input is completed by the retina to achieve this level of data compression.

The gaze of the eye is generally adjusted therefore the images are brought to focus at the fovea. This area of the retina (diameter 1.5 mm) has the maximum visual acuity. This is due to the following:

- Its very high density of photoreceptors

- The displacement of overlying layers of the retina to the side therefore the light hits the photoreceptors directly

- The lack of blood vessels

- The fovea being at the optical axis of the eye therefore image distortion by the optics (example spherical or chromatic aberration) is minimal.

About 4 mm of the fovea towards the nose lays the optic disc, where the optic nerve fibers and retinal blood vessels pierce the retina. This area lacks the photoreceptors and accounts for the blind spot which occurs in the visual fields.

For areas outside the fovea light passes throughout the full thickness of the retina before striking the light-sensitive photoreceptors. The Muller glial cells offset a few of the demerits this poses. These funnel-shaped cells have their wide end at the retinal surface and extend long slender procedures to the photoreceptor layer. The Muller cells act as light guides, rather like fiber-optic cables, in that they channel light throughout the neuronal layers of the retina to the photoreceptors at the rear. They do this more proficiently for light being brought to focus than for light which has suffered multiple reflections within the eye. Therefore they enhance the signal-to-noise ratio. The Muller cells are tuned to visible light therefore radiation in the near IR and UV leak out to be absorbed by surrounding neurons instead of excite photoreceptors. Additionally, the Muller cells refract light therefore blue light is brought to similar focus as red light, correcting a lens defect known as chromatic aberration.