Figure 926

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Nearsightedness, farsightedness, and their correction. %

images of far objects focus at a point in front of the retina. This is a nearsighted, or myopic, eye, which is unable to see distant objects clearly. In contrast, if the eye is too short for the lens, images of distant objects are focused on the retina but those of near objects are focused behind it. This eye is farsighted, or hyperopic, and near vision is poor. The use of corrective lenses for near- and farsighted vision is shown in Figure 9-26.

Defects in vision also occur where the lens or cornea does not have a smoothly spherical surface, a condition known as astigmatism. These surface imperfections can usually be compensated for by eyeglasses.

The lens separates two fluid-filled chambers in the eye, the anterior chamber, which contains aqueous humor, and the posterior chamber, which contains the more viscous vitreous humor (see Figure 9-22). These two fluids are colorless and permit the transmission of light from the front of the eye to the retina. The aqueous humor is formed by special vascular tissue that overlies the ciliary muscle. In some instances, the aqueous humor is formed faster than it is removed, which results in increased pressure within the eye. Glaucoma, the leading cause of irreversible blindness, is a disease in which the axons of the optic nerve die, but it is often associated with increased pressure within the eye.

The amount of light entering the eye is controlled by muscles in the ringlike, pigmented tissue known as the iris (see Figure 9-22), the color being of no importance as long as the tissue is sufficiently opaque to prevent the passage of light. The hole in the center of the iris through which light enters the eye is the pupil. The iris is composed of smooth muscle, which is innervated by autonomic nerves. Stimulation of sympathetic nerves to the iris enlarges the pupil by causing the radially arranged muscle fibers to contract. Stimulation of parasympathetic fibers to the iris makes the pupil smaller by causing the sphincter muscle fibers, which circle around the pupil, to contract.

These neurally induced changes occur in response to light-sensitive reflexes. Bright light causes a decrease in the diameter of the pupil, which reduces the amount of light entering the eye and restricts the light to the central part of the lens for more accurate vision. Conversely, the iris enlarges in dim light, when maximal illumination is needed. Changes also occur as a result of emotion or pain.

Photoreceptor Cells

The photoreceptor cells in the retina are called rods and cones because of the shapes of their light-sensitive tips. Note in Figure 9-27 that the light-sensitive portion of the photoreceptor cells—the tips of the rods and cones—faces away from the incoming light, and the light must pass through all the cell layers of the retina before reaching the photoreceptors and

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

Back of Retina

Front of retina

Back of Retina

Front of retina

Humor Bipolar

Bipolar cell

Light Path

Ganglion cell (axons become optic nerve)

FIGURE 9-27

Organization of the retina. Light enters through the cornea, passes through the aqueous humor, pupil, vitreous humor, and the front surface of the retina before reaching the photoreceptors. The membranes that contain the photoreceptors form discrete discs in the rods but are continuous with the plasma membrane in the cones, which accounts for the comblike appearance of these latter cells. Two other neuron types, depicted here in purple and orange, provide lateral inhibition between neurons of the retina. %

Redrawn from Dowling and Boycott.

Cone

Bipolar cell

Light Path

Ganglion cell (axons become optic nerve)

FIGURE 9-27

Organization of the retina. Light enters through the cornea, passes through the aqueous humor, pupil, vitreous humor, and the front surface of the retina before reaching the photoreceptors. The membranes that contain the photoreceptors form discrete discs in the rods but are continuous with the plasma membrane in the cones, which accounts for the comblike appearance of these latter cells. Two other neuron types, depicted here in purple and orange, provide lateral inhibition between neurons of the retina. %

Redrawn from Dowling and Boycott.

stimulating them. A pigmented layer (the choroid), which lies behind the retina (see Figure 9-22), absorbs light and prevents its reflection back to the rods and cones, which would cause the visual image to be blurred. The rods are extremely sensitive and respond to very low levels of illumination, whereas the cones are considerably less sensitive and respond only when the light is brighter than, for example, twilight.

The photoreceptors contain molecules called photopigments, which absorb light. There are four different photopigments in the retina, one (rhodopsin) in the rods and one in each of the three cone types. Each photopigment contains an opsin and a chromophore.

Opsin is a collective term for a group of integral membrane proteins, one of which surrounds and binds a chromophore molecule (Figure 9-28). The chro-mophore, which is the actual light-sensitive part of the photopigment, is the same in each of the four photopigments and is retinal, a derivative of vitamin A. The opsin differs in each of the four photopigments. Since each type of opsin binds to the chromophore in a different way and filters light differently, each of the four photopigments absorbs light most effectively at a different part of the visible spectrum. For example, one photopigment absorbs wavelengths in the range of red light best, whereas another absorbs green light best.

Vander et al.: Human I II. Biological Control I 9. The Sensory Systems I I © The McGraw-Hill

Physiology: The Systems Companies, 2001 Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

FIGURE 9-28

The arrangement of the opsin and retinal (the chromophore) in the membrane of the photoreceptor discs of a cone. The opsin actually crosses the membrane seven times, not three as shown here.

FIGURE 9-28

The arrangement of the opsin and retinal (the chromophore) in the membrane of the photoreceptor discs of a cone. The opsin actually crosses the membrane seven times, not three as shown here.

Within the photoreceptor cells, the photopigments lie in specialized membranes that are arranged in highly ordered stacks, or discs, parallel to the surface of the retina (Figures 9-27 and 9-28). The repeated layers of membranes in each photoreceptor may contain over a billion molecules of photopigment, providing an effective trap for light.

Light activates retinal, causing it to change shape. This change triggers a cascade of biochemical events that lead to hyperpolarization of the photoreceptor cell's plasma membrane and, thereby, decreased release of neurotransmitter (glutamate) from the cell. Note that in the case of photoreceptors the response of the cell to a stimulus (light) is a hyperpolarizing receptor potential and a decrease in neurotransmitter release. The decrease in neurotransmitter then causes the bipolar cells, which synapse with the photoreceptor cell, to undergo a hyperpolarization in membrane potential.

After its activation by light, retinal changes back to its resting shape by several mechanisms that do not depend on light but are enzyme mediated. Thus, in the dark, retinal has its resting shape, the photoreceptor cell is partially depolarized, and more neurotransmit-ter is being released.

When one steps back from a place of bright sunlight into a darkened room, dark adaptation, a temporary "blindness," takes place. In the low levels of illumination of the darkened room, vision can only be supplied by the rods, which have greater sensitivity than the cones. During the exposure to bright light, however, the rods' rhodopsin has been completely activated. It cannot respond fully again until it is restored to its resting state, a process requiring some tens of minutes. Dark adaptation occurs, in part, as enzymes regenerate the initial form of rhodopsin, which can respond to light.

Neural Pathways of Vision

The neural pathways of vision begin with the rods and cones. These photoreceptors communicate by way of electrical synapses with each other and with second-order neurons, the only one of which we shall mention being the bipolar cell (Figures 9-27 and 9-29). The bipolar cells synapse (still within the retina) both upon neurons that pass information horizontally from one part of the retina to another and upon the ganglion cells. Via these latter synapses, the ganglion cells are caused to respond differentially to the various characteristics of visual images, such as color, intensity, form, and movement. Thus, a great deal of information processing takes place at this early stage of the sensory pathway.

The distinct characteristics of the visual image are transmitted through the visual system along multiple, parallel pathways by two types of ganglion cells, each type concerned with different aspects of the visual stimulus. Parallel processing of information continues all the way to and within the cerebral cortex, to the highest stages of visual neural networks. (Note that in this discussion of the visual pathway, the terms "respond" and "response" denote not a direct response to a light stimulus—only the rods and cones show such responses—but rather to the synaptic input reaching the relevant pathway neuron as a consequence of the original stimulus to the rods and cones.)

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

The Sensory Systems CHAPTER NINE

The Sensory Systems CHAPTER NINE

Stimulus (Light)

Retina

Rods and cones n

Bipolar cells ar

Ganglion cells ar

Ganglion cells

Optic nerve

/

Thalamus

Lateral geniculate nucleus

J

Pathway

Visual cortex

Visual association cortex

Cortex

Visual cortex

Visual association cortex

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