How the Retina Works
Vision Begins with a Highly Ordered Series of Nerve Cells in the Eye
Sep 14, 2009
The ability of a human to see, to perceive light, shape, form and color, begins in the eye. Light entering the eye activates a series of nerve cells in a highly ordered parallel array at the back of the eye. Visual abilities begin here, in the structure known as the retina.
When light enters the eye, the job of the cornea and the ocular lens at the front (anterior end) of the eye is to focus the visual image on the retina at the back (posterior end) of the eye. This highly specialized structure actually occupies a large percentage of the inside surface of the eyeball behind the lens.
Photoreceptors Respond to Light
When the light entering the eye strikes the retina, the light activates specialized light-responsive nerve cells known as photoreceptors. There are two types of photoreceptor neurons, rods and cones. The process of vision begins with these two cell types. Cones are responsible for the highest acuity and for the ability to see color, in fact there are three populations of cone cells commonly referred to as red, green, and blue cones. As for the rod photoreceptor cells, they are much more sensitive to light than cones in general, and therefore are responsible for vision in very dim light (what is known as scotopic vision). The rods though work best in the blue range of the light spectrum and thus, though we can see in dim lighting, we notice that color perception is greatly reduced.
Massive Parallel Processing
After photoreceptor neurons are activated by light, they create an action potential which allows them to signal to the next set of nerve cells in the chain, the bipolar cells. Bipolar cells then transmit the signal to the nerve cells that send the visual signal out of the retina and towards the brain, the retinal ganglion cells. Interestingly, all of these neurons are located in very definable layers in the structure of the retina. In addition to these different types of neurons, there are two other neurons, amacrine cells and horizontal cells, which, while not terribly well understood, are believed to act to modify the sensitivity and signaling of the bipolar and retinal ganglion cells.
In one of the more interesting features of the retina, the arrays of the main signaling cells are in a very specific order, front to back, that might at first seem backwards. The retinal ganglion neurons, which are the final cell in the chain before signals are sent to the brain, are actually the cells closest to the lens, and thus the front of the eye. The bipolar cells largely form the middle layer. The photoreceptors are actually the furthest away from the lens, and the source of light, being closest to the structures that make up the globe of the eye. Across from the photoreceptors is a specialized layer known as the pigment epithelium, which helps to finally absorb the light energy and keep it from scattering around the inside of the eye. The axons of the retinal ganglion cells exit the back of the eye and form the optic nerve which allows for visual signals to make their way to the brain.
For a very informative and fun examination of the eye and how it works visit the US National Eye Institute site, See All You Can See.
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