by the
ciliary body
to adjust the refraction of the
light passing through the lens. This process of ad-
justing the shape of the lens (anywhere from very
flat to round) for focusing is called
accommoda-
tion
. Accommodation can keep an image focused
on the retina, regardless of whether you are look-
ing at a textbook on your desk or an instructor
at the front of a lecture hall. Abnormally shaped
eyeballs can lead to vision problems, which can be
with eyeglasses, contact lenses, or surgery.
photopigment
A
substance that can
absorb light and
undergo structural
changes that can lead
to the development of
a receptor potential.
corrected
How do your eyes provide visual information
to the brain? In the retinal photoreceptors, light
interacts with a
photopigment
called
rhodopsin
.
Light causes rhodopsin to break up into two piec-
es,
retinal
and
opsin
. The chemical changes in
rhodopsin lead to electrochemical changes in the
photoreceptor membrane, which in turn lead to
action potentials. The rhodopsin molecule must
be reassembled by an enzyme before it can respond to light
again (see Figure 8.9c).
c.
Stimulation of
a
nerve impulse by light
Rod disc
^
Rhodopsin
r
molecule
3.
Enzyme
converts
retinal back
to original
configuration
Original
retinal
shape
t
Colored
photopigment
(rhodopsin)
Light
1. Change in shape
causing nerve
impulse of retinal
Altered
retinal
2.
Altered retinal
separates
from opsin
(bleaching)
Colorless products
■ M n iin orcircm
1. The image you see
a. is inverted on the retina.
b. is reflected on the retina.
c. is sent to the brain as an image.
2. Vision problems such as nearsight-
edness or farsightedness are caused
by
a. abnormally shaped eyeballs.
b. an inability to shape or accommo-
date the lens.
c. insufficient quantities of rhodopsin.
\
3.The photoreceptors of the eye
a. focus an inverted image on the
retina.
b. transduce the light into nerve
impulses.
c. shape the lens to allow the eye to
focus.
Vision involves Photoreceptors in the Eyes 239
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