212
Chapter 7
as the tissue is suffi ciently opaque to prevent the passage of
light. The iris is composed of two rings of smooth muscle that
are innervated by autonomic nerves. Stimulation of sympa-
thetic nerves to the iris enlarges the pupil by causing radially
arranged muscle fi bers to contract. Stimulation of parasym-
pathetic fi bers to the iris makes the pupil smaller by causing
the sphincter muscle fi bers, which circle around the pupil, to
contract.
These neurally induced changes occur in response to
light-sensitive refl exes. 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. The constriction of the pupil
also protects the retina from damage induced by very bright
light, such as direct rays from the sun. Conversely, the pupil
enlarges in dim light, when maximal illumination is needed.
Changes also occur as a result of emotion or pain. Abnormal
or absent response of the pupil to changes in light can indicate
damage to the midbrain from trauma or tumors, or can also
be a telltale sign when a person is under the infl uence of nar-
cotics like heroin.
Photoreceptor Cells and Phototransduction
Figure 7–27
shows a detailed view of the retina. The photore-
ceptor cells have a tip, or
outer segment,
composed of stacked
layers of membrane, called
discs.
The discs hold the chemical
substances that respond to light. The photoreceptors also have
an
inner segment
that contains the nucleus, mitochondria,
Back of
retina
Front of
retina
Rod
Cone
Horizontal cell
Bipolar cell
Amacrine cell
Ganglion cell (axons
become optic nerve)
Light Path
Pig
ment ep
i
t
h
e
li
u
m
horoi
d
Figure 7–27
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 photoreceptor cells. The membranes that contain the light-sensitive proteins 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. Horizontal and
amacrine cells, depicted here in purple and orange, provide lateral inhibition between neurons of the retina. At the lower left is a scanning
electron micrograph of rods and cones.
Redrawn from Dowling and Boycott.
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