Visual Pigments

Both rods and cones contain light-sensitive pigments that decompose when they absorb light energy. The lightsensitive pigment in rods is called rhodopsin (ro-dop'sin), or visual purple, and it is embedded in membranous disks that are stacked within these receptor cells (fig. 12.40). A single rod cell may have 2,000 interconnected discs, derived from the cell membrane. In the presence of light, rhodopsin molecules break down into molecules of a colorless protein called opsin and a yellowish organic molecule called retinal (retinene) that is synthesized from vitamin A.

In darkness, sodium channels in portions of the receptor cell membranes are kept open by a nucleotide called cyclic guanosine monophosphate (cGMP). When rhodopsin molecules absorb light, they change shape and release opsin, in mere trillionths of a second. The released opsin then becomes an active enzyme. This enzyme activates a second enzyme (transducin), which, in turn, activates still another enzyme (phosphodiesterase). The third enzyme of this series breaks down cGMP, and as the concentration of cGMP decreases, sodium channels close, and the receptor cell membrane hyperpo-larizes (see chapter 10, p. 376). The degree of hyperpolari-zation is directly proportional to the intensity of the light stimulating the receptor cells.

The hyperpolarization reaches the synaptic end of the cell, inhibiting release of neurotransmitter. Through a complex mechanism, decreased release of neurotransmit-ter by photoreceptor cells either stimulates or inhibits nerve impulses (action potentials) in nearby retinal

- Disks of membrane within cell h?M

'Mitochondria quires cellular energy, which ATP provides (see chapter 4, p. 114). Under these conditions, the rods continue to function and the cones remain unstimulated. Hence, we see only shades of gray in dim light.

The light sensitivity of an eye whose rods have converted the available opsin and retinal to rhodopsin increases about 100,000 times, and the eye is said to be dark adapted. A person needs a dark-adapted eye to see in dim light. For example, when going from daylight into a darkened theater, it may be difficult to see well enough to locate a seat, but soon the eyes adapt to the dim light and vision improves. Later, leaving the theater and entering the sunlight may cause discomfort or even pain. This occurs at the moment that most of the rhodopsin decomposes in response to the bright light. At the same time, the light sensitivity of the eyes decreases greatly, and they become light adapted.

Too little vitamin A in the diet reduces the quantity of retinal, impairing rhodopsin production and sensitivity of the rods. The result is poor vision in dim light, called night blindness.

Rod cell

-Nucleus

Synaptic ending

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