Module 1 Lesson 2 - 6

Rods and Cones Initiate Nerve Impulses

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Rods and cones contain photoreceptor proteins that absorb light and initiate chemical changes. Cones contain three pigments called photopsins. Rods have only one type of pigment called rhodopsin.

The opsin pigments (photopsins and rhodopsin) contain a derivative of vitamin A called retinal. When an opsin pigment  absorbs light, the retinal molecule changes its shape and initiates a nerve impulse to be processed by the occipital lobe of the cerebrum. Until the retinal molecule reverts to its original shape, the opsin pigment cannot respond to light again. In other words, the opsin pigment is bleached and cannot send signals in response to wavelengths of light.

Retinal molecules cannot be synthesized by humans and it must be converted from dietary vitamin A.

Knowing this, could you explain why your eyes become strained in poor lighting, why night blindness is caused by vitamin A deficiency, and why you should eat your carrots? Carrots contain beta-carotene. Once ingested, beta-carotene is converted into vitamin A in the intestines. Because the only source of vitamin A comes from diet, regular consumption of  various foods containing vitamin A, such as fish, liver, leafy greens, and carrots, is important.

Light and Dark Adaptation

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Rhodopsin is extremely sensitive to light and dark changes. Even starlight causes some molecules of rhodopsin to become bleached! It is sensitive to shape and movement but not good for colour vision. In a dark room, rods are active and that is why everything looks black and white.

In high intensity light, rhodopsin is broken down almost as fast as it is remade. In such a situation, the rods become non-functional and the cones take over completely. Cones are not as sensitive to light as rods are, but cones are sensitive to colours (red, blue, and green).

For example, think about what happens when the lights are turned on after a movie. You are blinded momentarily and all you see is white light because the rods are bleached. Your pupil constricts in a reflex to protect your retina from the bright light. Within a minute or so, the cones become sufficiently stimulated to take over again. During the next few minutes, visual acuity and colour vision improve.

On the other hand, adaptation to low light occurs when you go from a well-lit area into a dark one. At first, you see nothing but velvety darkness because your cones have stopped functioning due to lack of light, and your rods are not functioning because they have been bleached by the bright light in the well-lit area. Now in the dark, rhodopsin is remade and accumulated so that low intensity light can again stimulate the rods.

During adaptation to both light and dark, reflexive changes occur in the pupil of the eye. The pupils are constricted in bright light to protect the retina. In a dark room, the pupils are dilated to allow more light to stimulate the rods.