Fred’s Head from APH, a Blindness Blog

Fred’s Head, offered by the American Printing House for the Blind, contains tips, techniques, tutorials, in-depth articles, and resources for and by blind or visually impaired people. Our blog is named after the legendary Fred Gissoni, renowned for answering a seemingly infinite variety of questions on every aspect of blindness.

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Wednesday, April 25, 2007

What is the Retina?

We hear a lot about the retina, but what is it? I found the following explanation on The Foundation for Retinal Research website and wanted to share it with you.

The retina, as the brain, is part of the Central Nervous System. It is a thin, transparent tissue that is attached inside the back part of the eye. Its main function is to capture light images, begin their processing and pass them down the optic nerve to the brain. Structurally, the retina is stratified, i.e., most cells are in distinct bands or layers. In fact, one can think of the retina as a layer cake. The following is a short description of the important cell types of the retina.

Photoreceptor Cells

The most important cell is the photoreceptor neuron. Its main function is to capture the light energy in a visual image and convert it to an electrical response. This is done is a specialized part of the photoreceptor cells called the outer segment. Visual proteins are concentrated in the outer segment. These are the proteins that actually capture the light energy. Once the photoreceptor neuron converts the photic energy to an electrophysiological signal, it passes this signal on to secondary neurons in the next layer of the retina (e.g., bipolar cells) and ultimately to the brain.

There are two main types of photoreceptor cells in most animal retinas. These are called rods and cones. Rod cells are, as the name implies, rod-shaped. They are designed to mainly function in dim light and in peripheral vision. Cone cells are more cone shaped. They serve in central vision, bright-light vision and in color vision. There is a concentration of cone cells in a highly specialized, region of the retina called the macula. Most of our central and sharp vision uses macular cone cells. Interestingly, the photoreceptor cells point towards the back of the eye, necessitating light to pass through all the other retinal layers before striking the photoreceptors.

Retinal Pigment Epithelium (RPE) Cells

Juxtaposed to the layer of photoreceptor cells is a single cell layer of RPE cells. Perhaps, think of them as frosting on the retinal "cake". They are tightly intertwined with the outer segments of the photoreceptor cells. The RPE cell layer functions in maintaining proper operation of the photoreceptor cells which are thought to have the highest metabolic activity of any cell type in the human body. Thus, RPE cells bring nutrients and oxygen to photoreceptor cells and remove waste products. RPE cells also are heavily pigmented (melanin granules), allowing for capture of stray light. Last but not least, RPE cells are intrinsic to vision in that they participate in the visual cycle with photoreceptor cells. They store the vitamin A (retinoids) needed in vision and also contain enzymes that chemically alter vitamin A to forms used in photoreceptor outer segments in the visual process. When RPE cells are not functioning properly, photoreceptor cells are usually quickly affected resulting in retinal degeneration.

On the other side of the RPE cell layer from the photoreceptors is a dense network of blood vessels called the choroid. It is from this blood vessel system that RPE cells get the nutrients to pass on to photoreceptor cells.

Other Retinal Cell Types

Beneath the photoreceptor cells are several stratified cell layers. Within these layers are secondary neurons such as bipolar cells, amacrine cells and ganglion cells. These cells are all connected through structures called synapses. The function of these cells is to begin the processing and integration of the visual signals. These signals are finally passed to the brain through the optic nerve. The optic nerve consists of many long, thin processes (axons) of ganglion cells.

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