Download EYE 2

RECEPTORS IN
ANIMALS
RICHARD LLOPIS GARCIA
Adapted by MH A2 BIOLOGY
FOCUSING
The eye is able to focus all rays of light
from one object to a single point on the
retina.
The ability of the eye to change focus
from near to distant objects is called
ACCOMODATION
FOCUSING
The amount of refraction taking place at
the cornea is more or less constant if
the curvature remains the same.
The curvature of the lens is altered by
the action of the ciliary muscles and so
the amount of refraction changes.
The Retina
Three main layers
Photoreceptors (cones and rods)
Associated neurons (bipolar)
Sensory neurons (ganglion)
Cones and Rods
Cones and Rods differ by:
• Visual acuity (degree of detail, “pixels”)
• Sensitivity ((the intensity of light
required to produce a generator
potential large enough to trigger an
action potential)
This is due to their connections with
bipolar neurons
Differences between cones
and rods
PROPERTY
CONES
RODS
Sensitivity
Low: light energy
transduced by a single
cone must produce a
generator potential
large enough to exceed
the threshold needed
for and action potential
(unlikely in low light
intensities)
High: in low light
intensity, generator
potential from several
rods can combine and
so the threshold is
more likely to be
exceeded and action
potential initiated
(SUMMATION).
Several rods are liked
to a single nerve cell
(via bipolar cells).
This is called retinal
convergence.
Differences between cones
and rods
PROPERTY
CONES
RODS
Acuity
High: each cone
is conected to a
single bipolar
cell, so in high
light intensities
each cone
stimulated
represent a
separate part of
the image which
can be seen in
detail
Low: several
rods are
conected to the
same bipolar
cell, so the
individual parts
of the image
represented by
each rod are
merged into one
(low detail
distintion)
Distribution of rods and
cones
Rods and cones are distributed
unevenly across the retina (please see
graph and diagram)
What conclusions can
you deduce from the
diagrams about the
distribution of rods
and cones in the eye?
Distribution of rods and
cones
The greatest concentration of cones is
found at the fovea in the centre of the
retina.
Looking straight at an object focuses
light intensity from it into the fovea,
enabling to be seen in great detail if the
light intensity is high
Distribution of rods and
cones
The greatest concentration of rods is
about 20 degrees away from the fovea.
In low light intensities, looking slightly to
the side of an object causes the light
rays to fall on this area of the retina.
Summation by the rods allows better
perception than if the light fell on the
fovea. (in dim light)
TRICHROMATIC
THEORY
We see everything by mixing only 3
colours in different proportions.
Each cone is sensitive to one of these
wavelengths.
Any particular colour is experienced
because the wavelength stimulates one,
two, or all three types TO A
DIFFERENT DEGREE.
Different types of cones
There are 3 different
types of cone,
sensitive to different
wavelenght of light
which are broadly
equivalent to the 3
primary colours (red,
blue and green.
Now look again to the
graph and suggest
where in the graph will
we be able to see the
yellow colour?
Different types of cones
Answer: a
wavelenght of 575
nm stimulates both
red and green cones
and it is interpreted
by the brain as the
yellow colour
How rods and cones are
stimulated (at the
molecular level)
When cones and rods are stimulated by
light,
A change occurs in a photosensitive
pigment.
This alters the membrane potential of
the cell, creating a generator potential.
The pigment in RODS is RHODOPSIN
Light energy is absorbed by a part of
rhodopsin called RETINAL
Retinal changes from cis retinal to a
trans retinal isomer (bleaching)
Trans retinal is not photosensitive
Rhodopsin
How things go back to
normal?
Retinal and opsin then join back together in
an enzyme catalysed reaction that
regenerates the photo senstive cis retinal
ready to be used again.
The same happens with IODOPSIN in
CONES but breaks less easily and joins back
together more slowly (cones better for High
Light Intensities)
Creating a nerve impulse
(Action Potential)
Bleaching causes excess Na+ channels to
close (resting potential is more –ve) 120mV
Less neurotransmitter is released at the
bipolar synapse
This stops the inhibition of the bipolar cell
A generator potential is formed in the bipolar
cell
If threshold is reached in the bipolar cell
(through a summation of the generator
potentials) then an action potential is
produced in the bipolar and ganglion cells
So RODS are more
sensitive
20 times more rods than cones
Found outside the fovea (periphery of the
retina)
More sensitive because they converge onto
the SAME BIPOLAR NEURONE.
i.e. even the small responses from rods will
be detected by the brain.
They are not good at providing clarity or
detail. (try to see an object from the corner of
your eye)
But Cones let you see in
more detail.
Mostly packed together in the FOVEA
They give good VISUAL ACUITY
(clarity)
And more accurately and in more detail
Because each cone synapses with its
own individual bipolar synapse.
Remember that also let you see in
colour. (draw diagram of connections)
Click on the hyperlink to
see an eye dissection
Eye Dissection Complete.wmv