Module 1 The Nervous System

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Rods and Cones Initiate Nerve Impulses

Rods and cones contain pigments called photopsins that absorb light and start chemical changes. Cones contain three different pigments called iodopsins. Rods have only one type of pigment called rhodopsin. When rhodopsin absorbs light it is bleached and it breaks down into opsin, a colourless protein, and a derivative of Vitamin A called retinal. This chemical change in the rods initiates a nerve impulse in the bipolar cells, which transfer the impulse to the ganglions. The ganglions, which make up the optic nerve, then transmit the impulse to the occipital lobe of the cerebrum to be processed.

Your eye then restores opsin and retinal to rhodopsin, but this takes energy in the form of ATP. This reverse reaction also requires vitamin A. Knowing this, could you explain why your eyes get strained in poor lighting, why nightblindness is caused by Vitamin A deficiency, and why you should eat your carrots?

Light and Dark Adaptation

Rhodopsin is extremely sensitive to light. Even starlight causes some molecules of rhodopsin to become bleached. In high intensity light, rhodopsin is broken down almost as fast as it is remade. In such a situation, the rods become nonfunctional and the cones take over completely. For example, think about what happens when the lights are turned on after a movie. You are momentarily “blinded” and all you see is white light. Your pupil constricts in a reflex in order to protect your retina from the bright light. Within a minute or so, the cones become sufficiently stimulated to take over once again. Over 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 out by the bright light. Once 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 Pathway to the Brain

Try This

Did you know that each eye sees thing differently?  Each eye has it’s own area of “blindness”, the blind spot. To find your blind spot, try the activity in Figure 12.17 on page 416.

The information you receive from each eye is communicated through the optic nerve to the brain. Your brain processes this information into one image for you without gaps or blind spots. The optic nerves enter the optic chiasm on the ventral, front of the brain (see Lesson 2). In the X-shaped optic chiasma some axons from each eye cross over to the opposite side of the brain while others continue to their respective side of the brain. The crossing over of about half of the sensory fibres ensures that each half of the occipital lobe receives the same image or part of the visual field as viewed by each eye. However, keep in mind that each eye will see the image from a slightly different angle. Study Figure 12.18 on page 416, or the illustration provided here very carefully in order to better understand vision interpretation by the brain.

Depth perception is possible due to the “fusing” or the superimposing of the slightly different images from the two eyes by the visual cortex. This creates a three-dimensional image. Thus, by working together, the eyes provide slightly different angles of view that allow the brain to estimate distance.

The image that was reversed and inverted in the lens is at his point interpreted correctly.