88 Light Transduction

How do we experience light? When light reaches the back of the eye, it enters the retina. The retina is composed of many layers and contains specialized cells known as photoreceptors. There are two types of photoreceptors: rods and cones. Rods are sensitive to vision in low light and do not sense the color of light, whereas cones are sensitive to brighter light and allow us to perceive color in normal lighting.

Light transduction happens in the outer segments of the rods and cones. This means that light travels through several layers (ganglion cells, bipolar and amacrine cells) before it does anything! Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the rods and cones. When light (of the right wavelength/color) hits 11-cis retinal, it changes conformation (to all-trans), acting as a switch to start an enzyme cascade in the cell, which eventually changes the rate at which photoreceptors release neurotransmitters.

After absorbing light, all-trans retinal needs to be turned back into 11-cis retinal. So it breaks off its opsin, finds its way to the pigment epithelium, gets bent back into shape, and finds its way back to an opsin. This is why photoreceptor outer segments need to be close to the pigment epithelium, which is the back layer behind the retina where visual pigments are replenished.

Another way humans experience light is through visual acuity, which is the sharpness of vision. The exact center of the retina in the eye, called the fovea, has the greatest visual acuity and lacks supporting cells and blood vessels. The fovea only contains photoreceptor cells, which results in the least amount of incoming light to be absorbed by other retinal structures. As the light enters further away from the fovea, the visual acuity decreases; showing a linear relationship between the distance from the fovea and the sharpness of visual acuity. A commonly used example to explain this relationship is that when a person stares at a word on the center of the page, they often can clearly see the word. Whereas, if the person were to use their peripheral retina to try to read a word on the side of the page, the word is likely blurry and undetectable. This example demonstrates that light transduction through the fovea in the retina results in the greatest visual acuity in comparison to other areas of the retina.