HUMAN BIOLOGY

SenSory SyStemS 279

What are the eye’s photoreceptors?

• Rods and cones in the retina are the eye’s photoreceptors.


• Rods detect dim light. Cones detect bright light and provide our
  sense of color.


• The eye analyzes information on the distance, shape, brightness,
  position, and movement of a visual stimulus.


• Visual signals move through layers of neurons in the retina
  before moving on to the brain.

taKe-Home message

The retina begins processing visual signals

In an early embryo, its retinas arise from its developing
brain. As a result, anatomically speaking, the retina is an
extension of the brain. Perhaps it is not surprising, then,
that cells in the retina process visual signals before they are
sent on to the brain’s vision centers.
Neurons in the eye are organized in layers above the rods
and cones. As you can see in Figure 14.19, signals flow from
rods and cones to bipolar interneurons, then to interneurons
called ganglion cells. Signals also travel to horizontal cells and
amacrine cells. These neurons jointly strengthen or weaken
the signals before they reach ganglion cells. The axons of
ganglion cells form the two optic nerves to the brain.

Signals move on to the visual cortex

The part of the outside world you actually see is called
the “visual field.” The right side of each retina intercepts
light from the left half of the visual field and the left side
intercepts light from the right half. Signals from each eye
“criss-cross” (Figure 14.20). That is, the optic nerve leading
out of each eye delivers signals from the left visual field to
the right cerebral hemisphere, and signals from the right go
to the left hemisphere.
Axons of the optic nerves end in an island of gray matter
in the cerebrum (the lateral geniculate nucleus). Its layers
each have a map corresponding to receptive fields of the
retina. Each map’s interneurons deal with one aspect of a
visual stimulus—its form, movement, depth, color, texture,
and so on. After initial processing all the visual signals
travel rapidly, at the same time, to different parts of the
visual cortex. There, final processing produces the sensa-
tion of sight.

rods

cones

horizontal cells

bipolar cells

amacrine cells

incoming

rays of

light

ganglion cells (axons

get bundled into one

of two optic nerves)

Figure 14.19 Animated! Photoreceptors connect with

sensory neurons in the retina. (© Cengage Learning)

to

optic nerve

optic

nerve

lateral

geniculate nucleus

retina

visual

cortex

Figure 14.20 Sensory signals criss-cross as they travel

from the retina to the brain. (© Cengage Learning)

F i g u r e 14.18 The fovea contains densely packed rods

and cones. This image shows the location of the fovea

and the start of the optic nerve.

start of an

optic nerve

in back of

the eyeball

fovea

© Ophthalmoscopic image from Webvision

http://webvision.med.utah.edu/

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