6.1.1 Photoreceptor Cells

Topic 6: Photoreceptor Cells
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT Point(s)
 identify photoreceptor cells as those containing light sensitive
pigments and explain that these cells convert light images into
electrochemical signals that the brain can interpret
 explain that colour blindness in humans results from the lack of
one or more of the colour-sensitive pigments in the cones
Once the eye has focused light on the retina, the light signal is
transformed into an electrical impulse which is carried by the
optic nerve to the brain. We are going to look in detail at this
We know the retina is the innermost coat of the eyeball and is very
thin (1/10mm thick). It consists of several layers of nerve cells,
one of which is the layer of visual receptors-the rods and cones.
 Of all the nerve cells that make up the retina, only the rods and
cones respond directly to light.
This is the reason we call them photoreceptors. In humans, each
retina contains approximately 125 million rods and 6 to 7 million
Position of Photoreceptors
The many layers of nerve cells in
the retina are arranged ‘back to
front’ compared with what one
would expect. The rods and
cones, which are sensitive to
light, are the last layer of cells
that the light reaches. Light
coming into the eye passes
through the entire retina before
striking the rods and cones,
which are closest to the choroid
Position of Photoreceptors
The photoreceptors generate
impulses, which travel back
along the various neuron layers
of the retina to the optic nerve,
which carries the signals to the
Light Images to Electrochemical Signals
The rods and cones, when
stimulated by light, perform 3
main functions:
1. They absorb light energy
(this involves the visual
2. They convert light energy
into electrochemical energy,
generating a nerve impulse
3. They transmit this nerve
impulse towards the bipolar
cells of the retina
Position of Photoreceptors
There are 5 main layers of nerve
cells or neurones that are directly
involved in the transmission of
impulses in the retina:
1. The photoreceptor layer
2. Bipolar cell layer
3. Ganglion cells layer
4. Horizontal cells
5. Amacrine cells
Light Images to Electrochemical Signals
Bipolar Cell Layer
These sensory neurones receive
electrochemical signals from the
rods and cones and transmit the
signal from these photoreceptors
to the next layer of cells, the
ganglion cells.
Light Images to Electrochemical Signals
Ganglion Cell Layer
The neurons in this layer receive
electrochemical signals from the
bipolar cells. The distal end of
ganglion cells is extended into
long processes that go on to form
the fibres of the optic nerve.
These neurons are responsible
for carrying electrochemical
signals from the retina to the
Light Images to Electrochemical Signals
Horizontal Cells
These cells occur at the junction
between photoreceptors and
bipolar cells. They connect one
group of rod and cone cells with
another and then link them to
bipolar cells.
Light Images to Electrochemical Signals
Amacrine Cells
These cells occur at the junction between bipolar cells and ganglion
cells. Amacrine cells are involved in processing or ‘summarising’
incoming visual information.
www.webexhibits.org -
Interesting Information
Studies of the eye show that
several rods make synaptic
contact with one bipolar
neurone, which in turn connects
a single cell body of an optic
nerve fibre. This is termed
retinal convergence and leads
to a decrease in visual acuity in
rods. Visual acuity is the ability to
see a clear and precise image.
Interesting Information
Retinal convergence is, however, an
advantage in giving the eye
increased sensitivity to small
amounts of light. If light of a low
intensity hits a single rod cell it, as a
single cell, may not receive enough
energy to initiate a nerve impulse
in a bipolar cell. However, if several
rods each receives a small amount
of light, they may have enough
combined energy to initiate this
response in the bipolar cell.
Interesting Information
Cones, which show little or no convergence, therefore have greater
visual acuity, but are less sensitive to light.
Interpretation of the Visual Signal
Although some information is processed in the retina, most of the
interpretation of visual stimuli occurs in the brain, based on
variables such as:
 How strong the light is
 How many rods and/or cones are stimulated
 Contrast enhancement
 Recognition of horizontal, vertical and diagonal lines
 The combination of cones stimulated (leading to colour
 Differences in the image that falls on the retina of the left and
right eye (depth perception)
Rods and Cones
For this next section, draw a chart like the one below and fill in
each section as we go through the next few slides.
Rods and Cones
Both rods and cones are elongated
cells that contain an outer
segment (closer to the choroid
layer of the eye) joined to an
inner segment that leads to the
conducting part of the cell. The
conducting part of the cell
comprises a cell body containing a
nucleus and an extension or
process called the foot. This
process conducts impulses to the
next layer of neurones in the
Rods and Cones
Rods and cones are named after the shape of their outer segments.
 In rods, this segment is long and narrow.
 Cones tend to have a shorter outer segment that is conical
(cone-shaped). Most cones are broader than rods.
Rods and Cones
Rods and cones contain visual
pigments, chemical substances
that absorb light energy. These
pigments, sometimes collectively
termed visual purple, are stacked
in layers of flattened membranes
in the outer segment of each
Rods and Cones
Rhodopsin is the only pigment
present in rods. Cones contain
iodopsins. There are three
different types of iodopsin, one
found in each type of cone cell.
Each type is sensitive to a
different wavelength of light.
Rods and Cones
The cone cells are therefore
responsible for colour vision,
while the rod cells can only see
in black and white. The role of
the visual pigments is to absorb
light energy, which the rod or
cone cell then converts into an
electrochemical signal that the
brain can interpret.
Distribution and Function
Rods are evenly distributed across
most of the retina, but are absent
from the fovea. As a result, rods
are responsible for most
peripheral vision, including the
detection of movement. The rods
are not very tightly packed in the
retina and many rods may connect
with one bipolar neuron. This
retinal convergence results in the
rods having poorer visual acuity.
Distribution and Function
Rods are extremely sensitive to
light, responding best to low
light intensities. They can be
stimulated (bleached) by very
small quantities of light energy.
The pigment can also be rapidly
regenerated. The resulting
sensitivity makes them able to
operate well in semi-darkness.
They are used for night vision
and to detect light and shadow
Distribution and Function
Cones are distributed in groups
throughout the retina, but there
are fewer around the periphery
of the retina, most being
concentrated in the macula
(yellow spot), an area of the
retina that gives the central 10
degrees of vision. The fovea is a
small pit in the middle of the
macula and contains cones only.
Distribution and Function
The cones in the fovea are very densely packed and show no renal
convergence. As a result they have a high degree of visual acuity,
perceiving images central to the visual field clearly and precisely.
Distribution and Function
Cones are responsible for colour
vision. Each cone contains one
of three types of iodopsin
pigment, and each type of
iodopsin is sensitive to one of
the primary colours of light.
Distribution and Function
The model of trichromatic
colour vision proposes that all
colours perceived by the eyes are
a combination of the three basic
colour, red, green and blue, to
which the cones are sensitive.
The brain interprets the
combination of different types of
cones that have been stimulated
as the different colours we see.
Distribution and Function
Because cones are less sensitive to light than rods are, they require
larger quantities of light to stimulate or bleach them. As a result,
cones function best in high intensity light, giving them daytime
Distribution and Function
Cones take longer to regenerate
once they have been bleached by
light. This bleaching effect is
experienced when you are
exposed to a ‘blinding’ flash of
light. For example, when a bright
flash of a light such as a camera
flash goes off and we cannot seem
to see anything for a brief period,
usually only a fraction of a second.
Colour Blindness
 A person who is termed ‘colour
blind’ is not truly colour blind, but
is usually able to see only two of the
three primary colours of light. They
lack one or more of the coloursensitive pigments in the cones
Because of this, they perceive colours
differently. Such individuals have
dichromatic vision and interpret all
colours based on combinations of the
two primary colours that they are able
to see.
Colour Blindness
People who are red-green colour blind find it difficult to distinguish
between red and green objects placed adjacent to each other. They
can often still distinguish red objects from green objects if there is a
difference in the brightness of the objects.
Colour Blindness
It is extremely rare that a person is monochromatic, unable to
distinguish colours and seeing most things in shades of black, white
and grey.
Colour Blindness
Not all people who have defective genes for colour vision are colour
blind. Some people may be colour deficient. A mutation in a gene
for any one of the cone pigments may simply cause a change in the
peak of spectral sensitivity of that cone. People who are colour
deficient may not see the number one on a colour vision plate like
the one below.
-Students to complete DOT Point

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