Receptor cells
Receptor
Stimulus
Photoreceptor
Light
Chemo receptor
Chemicals
Thermoreceptor
Temperature
Mechanoreceptor
Physical
Proprioreceptor
Position/movement
Role of receptors- Pacinian Corpuscle
Learning Objective:
In order to be successful in this lesson you must be able to:
explain how a Pacinian corpsule produces a
generator potential in a response to a specific
stimulus.
Role of receptors- Pacinian Corpuscle
describe the
structure of a
Pacinian
corpuscle
explain the stimulus
which Pacinian
corpuscles respond
to
PROGRESS
explain how a
Pacinian corpsule
produces a
generator potential
in aresponse to a
specific stimulus.
Pacinian corpuscle
A touch/pressure receptor.
Layers of
membrane
Terminal of
sensory
neurone
To CNS
The Pacinian corpuscle is a primary
receptor because it is stimulated
directly.
https://www.youtube.com/watch?v=IW8OnV
8J2Qw
When pressed, the change in pressure on
the membranes passes to the core and
causes increased permeabiltiy to sodium
ions Na+ causing depolarisation leading to
a generator potential. If this exceeds
the threshold then a nerve impulse is
generated.
Produce a flow chart to explain
how a Pacinian corpuscle
produces a generator potential
Slow pressure changes or prolonged
pressure does not cause a response.
•
•
•
A biologist investigated the stimulation of a Pacinian corpuscle in the skin of a
fingertip.
She used microelectrodes to measure the maximum membrane potential of a
Pacinian corpuscle and its sensory neurone when different pressures were
applied to the fingertip.
Figure 4 shows the Pacinian corpuscle, its sensory neurone and the
position of the microelectrodes
Explain how applying pressure to the Pacinian corpuscle produces
changes in membrane potential recorded by microelectrode P.
[3 marks]
Rods and cones
The lining of the eyeball is the retina. It
contains light receptors. The receptors send
nerve impulses along sensory neurones in the
optic nerve to the brain.
There are two types of photoreceptors – rods
and cones.
rhodopsin
iodopsin
They are at the back of the retina.
Light must pass through other
structures (bipolar cells, blood vessels
etc) to reach the rods and cones.
They contain photosensitive pigments
called rhodopsin and iodopsin. The
pigments are bleached by light but the
bleaching is reversible.
After bleaching rhodopsin regenerates
slowly but iodopsin regenerates quickly.
They both work in the same way in
rods and cones. When stimulated the
rhodopsin/iodopsin molecule changes
shape and a transmitter substance is
released to bipolar cells.
Rods
Retina has ca
120 million
rods. They
are in all parts
of the retina
Used for night vision.
except the
fovea.
Respond to low light levels and are
used for seeing in the dark/dim light
Rhodopsin =
the pigment.
Made of a
protein called
opsin and a non
protein called
retinal.
Retinal is a
derivative of
vitamin A.
Light causes rhodopsin to change shape
so it splits into opsin and retinal. Rod
cells produce transmitter when not
stimulated and stop producing when
stimulated.
The transmitter is sent to bipolar
cells. Bipolar cells are neurones
which transmit the impulse to the
next layer of cells. Several rods
connect to each bipolar cell making it
more likely to be activated by dim
light.
In the dark bleached pigment turns
slowly back to rhodopsin. This is
called dark adaptation and is why it
takes a while to be able to see when
you enter a dark place.
Rhodopsin is very stable n the dark.
In strong light it breaks down
quickly.
Cones
Retina has ca 5
to 6 million
cones. They
are in all parts
of the retina
but especially in Used for colour vision
in high intensity light.
the fovea.
In the fovea the surface layer is much
thinner. Cone cells are very small so
this area has the best definition.
Iodopsin = the
pigment.
Iodopsin differs
from rhodopsin in
the protein part
of the molecule.
Each cone is
connected to only
1 bipolar cell.
There are 3 types of cone responding
to different wavelengths of light - the
3 primary colours of light.
Red 570nm
Green 535nm
Blue 445nm
Each type of cone has a broad
response to different wavelengths so
the responses overlap to distinguish
other colours such as yellow.
550nm light stimulates both red and
green cones and yellow is seen.
The pigment is only broken down by
high intensity light and iodopsin is
quickly regenerated.
Colour blind people may lack cones
responding to some wavelengths.
Red green colour blindness may result
from a lack of:
red sensitive cones = protanopia
or
green sensitive cones = deuteranopia
Rods
1 connects to
1 bipolar cell
120 million in eye
Cones
In all retina
but not fovea
6 million in eye
In all retina
lots in fovea
Good sensitivity, 1
photon -> response
iodopsin
Good acuity
Poor acuity
15 to 45 connect to
1 bipolar cell
Poor sensitivity, 100s
photon -> response
rhodopsin
Rods
Cones
120 million in eye
6 million in eye
15 to 45 connect to
1 bipolar cell
1 connects to
1 bipolar cell
In all retina
but not fovea
In all retina
lots in fovea
Good sensitivity, 1
photon -> response
Poor acuity
rhodopsin
Poor sensitivity, 100s
photon -> response
Good acuity
iodopsin