Human Physiology, 14th edition (2016)

300 Chapter 10

11- cis form ( fig. 10.39 ). The all- trans form is more stable, but
only the 11- cis form is found attached to opsin. In response to
absorbed light energy, the 11- cis -retinal is converted to the all-
trans isomer, causing it to dissociate from the opsin. This dis-
sociation reaction in response to light initiates changes in the
ionic permeability of the rod plasma membrane and ultimately
results in the production of nerve impulses in the ganglion
cells. As a result of these effects, rods provide black-and-white
vision under conditions of low light intensity.
The retinal pigment epithelium is needed for the visual
cycle of retinal. Photoreceptors lack the enzyme cis-trans
isomerase, which is needed to re-isomerize (reconvert) retinal
from the all- trans form back into the 11- cis form. After the
absorption of light has caused the formation of the all- trans
form of retinal, the all- trans retinal dissociates from the opsin
and is transported from the photoreceptors into the closely
associated pigment epithelial cells. There, it is re-isomerized
into the 11- cis form and then transported back to the photo-
receptors. Now, the 11- cis retinal can again bind to the opsin
and form the active photopigment, able to respond to light. It is
this recycling between the photoreceptors and the retinal pig-
ment epithelium that is known as the visual cycle of retinal.

Dark Adaptation

The bleaching reaction that occurs in the light results in a low-
ered amount of rhodopsin in the rods and lowered amounts
of visual pigments in the cones. When a light-adapted person
first enters a darkened room, therefore, sensitivity to light is
low and vision is poor. A gradual increase in photoreceptor
sensitivity, known as dark adaptation, then occurs, reaching
maximal sensitivity in about 20 minutes. The increased sensi-
tivity to low light intensity is partly due to increased amounts
of visual pigments produced in the dark. Increased pigments
in the cones produce a slight dark adaptation in the first five
minutes. Increased rhodopsin in the rods produces a much
greater increase in sensitivity to low light levels and is partly

Figure 10.39 The photodissociation
of rhodopsin. ( a ) The photopigment rhodopsin
consists of the protein opsin combined with
11– cis -retinal (retinene). ( b ) Upon exposure to
light, the retinal is converted to a different form,
called all- trans, and dissociates from the opsin.
This photochemical reaction induces changes
in ionic permeability that ultimately result in
stimulation of ganglion cells in the retina.

(a)

(b)

Opsin

Opsin

11-cis-retinal

all-trans-retinal

CH 3

C

C

H 2

H 2 C

H 2 C

H 3 CCH

HC

O

CH 3

CH 3

CH 3

H

C

CCC

HH

H

CH

C

C

C

C

11

CH 3

C

C

H 2

H 2 C

H 2 C

O

CH 3 CH 3

CH 3

CH 3

HHH

C

CCCCC

HH HH

C

CCCC

11

CLINICAL APPLICATIONS

Retinitis pigmentosa is a group of inherited diseases that

cause degeneration of photoreceptors. Many genes appear

to be involved, and forms of this disease are inherited as

autosomal dominant, autosomal recessive, and X-linked

traits. In one autosomal dominant form, scientists discov-

ered a single base change in a gene for the opsin protein that

leads to degeneration of the photoreceptors. Rods degen-

erate before cones, leading first to a loss of night vision. A

loss of day vision follows, and progresses over time from

the periphery (where rods predominate) to the center (where

cones predominate) of the visual field. As a result, people

with retinitis pigmentosa increasingly have “tunnel vision.”

This contrasts with people who have macular degeneration

(discussed later), who lose vision in the central area of the

retina (the fovea centralis) and who must try to see from the

“corners of the eyes.”

responsible for the adaptation that occurs after about five min-

utes in the dark. In addition to the increased concentration

of rhodopsin, other more subtle (and less well understood)

changes occur in the rods that ultimately result in a 100,000-

fold increase in light sensitivity in dark-adapted as compared

to light-adapted eyes.

Electrical Activity of Retinal Cells

The only neurons in the retina that produce all-or-none action

potentials are ganglion cells and amacrine cells. The photo-

receptors, bipolar cells, and horizontal cells instead produce

only graded depolarizations or hyperpolarizations, analogous

to EPSPs and IPSPs.

The transduction of light energy into nerve impulses fol-

lows a cause-and-effect sequence that is the inverse of the usual