HUMAN BIOLOGY

278 Chapter 14

cone cells Photoreceptors
that detect bright light.

fovea Area near the center
of the retina where visual
acuity (sharpness) is the
greatest.

rod cells Photoreceptors
that detect dim light.

From Visual signals to “sight”

n    our vision sense is based on the sensory pathway from the

retina to the brain.

n Links to nerve impulses 13.2, Chemical synapses

13.3, nerves 13.4, Visual processing in the brain 13.8

“Seeing” something is a multistep process that begins

when your eyes receive raw visual information. The infor-

mation then is transmitted to the brain and processed. The

result is conscious awareness of light and shadows, of col-

ors, and of near and distant objects in the world around us.

Rods and cones are the photoreceptors

Vision begins when light reaches the retina, at the back of

the eyeball. Between the retina and the choroid is a layer

of epithelium where visual pigments form. Millions of

photoreceptors called rod cells and cone cells rest on this

layer (Figure 14.17 and Table 14.2)

and have visual pigments embed-

ded in them. Rod cells are sensitive

to dim light. They detect changes in

light intensity across the visual field.

Their signals are the start of coarse

perception of motion. Cone cells

detect bright light. Their signals are

the start of sharp daytime vision

and color perception.

Visual pigments intercept light energy

Like sound, light energy travels in waves, and different

light wavelengths correspond to different colors. As you

can see in the lower part of Figure 14.17, there are stacks

of membrane disks in the light-sensitive part of rods and

cones. These disks are where visual pigments are found.

Visual pigments are proteins that change shape when

they absorb certain wavelengths, or colors, of light. They

consist of different versions of a protein called opsin

together with retinal, a light-absorbing substance that is

derived from vitamin A. Rods contain a single type of

visual pigment, called rhodopsin. It absorbs mainly blue to

green light. By contrast, depending on the type of opsin in

its pigment, a cone may be sensitive to red, green, or blue

light. Thus we say there are three types of cones—red,

green, or blue.

Changes in visual pigments are key to our vision

sense. When light stimulates a visual pigment, its opsin

changes shape. The change begins a process that converts

light energy to nerve impulses. In this process, a series of

chemical reactions slow the release of a neurotransmitter

that inhibits neurons next to the photoreceptor. When they

are no longer inhibited, the neurons start sending signals

about the visual stimulus on toward the brain. So-called

night blindness results when a person’s diet is deficient in

vitamin A, so too little retinal is available to form visual

pigments. The effect is most severe in rods.

Near the center of the retina is a tiny depression called

the fovea (Figure 14.18). It is packed with cones. As a result,

visual acuity, the ability to discriminate between two

objects, is greatest there. For example, the fovea’s dense

cluster of cones enables you to distinguish between neigh-

boring points in space—like the e and the period at the end

of this sentence.

Cell Type Sensitive To Related Perception

Rod Dim light Coarse perception of

movement

Cone Bright light Daytime vision and

perception of color

Table 14.2 Rods and Cones Compared

14.9

F i g u r e 14.17 Rods and cones contain visual pigments.

(© Cengage Learning)

rod cell stacked pigmented membranes

cone cell

Mary Martin/Science Source

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