Today we are covering from the
specification:
The sensory nervous system
Receptors are specialized cells that can detect changes in
the body’s internal and external environment. Most receptor
cells are only sensitive to one type of stimulus.
Receptors convert the energy
of the stimulus into the start
of a nerve impulse known as
a generator potential.
For example, mechanoreceptors
detect changes in mechanical
energy, such as pressure.
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mechanoreceptor
in the skin
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• Read the article ‘How are you feeling?’.
• Answer these questions:
1. What is the main difference between a
Meissner’s corpuscle and a Merkel’s disc?
2. What is a ‘two point discrimination test’?
3. Explain how a mechanoreceptor converts
mechanical energy into electrical energy.
4. Explain how a Pacinian corpuscle can
produce a generator potential.
5. List some examples of the uses of haptic
technology.
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Mechanoreceptors in the skin
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Receptors - Pressure
A Pacinian corpuscle
Yteach resource
Pacinian corpuscle
Layers of
connective
tissue
separated by a
gel
Blood capillary
Stretch-mediated sodium channels (Permeability change
when shape changes)
Resting Potential:
Positive outside – negative
inside
Pressure:
Distorts & opens Na+
channels
Generator potential:
Inflow of Na+ depolarises
membrane
Photoreceptors in the retina
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How do rod cells produce impulses?
Rod cells allow vision in dim light due to the
presence of a pigment called rhodopsin,
which is found in membrane-bound vesicles.
When rhodopsin absorbs light it splits into
its constituent parts, opsin and retinal. This
is called bleaching. Low intensity light is
sufficient to cause this breakdown.
vesicles
containing
rhodopsin
The presence of opsin causes a change in the
permeability of the rod cell to sodium, which
initiates a generator potential. Rhodopsin can
reform in the absence of further light stimulation.
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How do cone cells produce impulses?
Cone cells are sensitive to high light intensities due
to the presence of the pigment iodopsin.
In bright light, iodopsin is broken down
into its constituent parts, generating an
action potential in the ganglion cell.
vesicles
containing
iodopsin
There are three different types of cone cell, each
containing a different form of iodopsin. Each form
of iodopsin absorbs a different wavelength of light
– green, blue or red.
The colour seen depends on the relative degree of
stimulation of the three different types of cone cell.
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Photoreceptors
These are found in the retina. There are two
types Rods and Cones and they are arranged
as shown:
Outer !
Inner
Light
Pigmented Layer
To Optic
Nerve
Ganglion Cells
Bipolar
Neurones
Rod
Cone
Light receptors
Both rod and cone cells act as transducers by
converting light energy into the electrical energy
of a nerve impulse.
Rod cells
Cannot distinguish different wavelengths of light
and therefore produce images only in black and
white.
Rod cells are more numerous than cones.
Rod cells
Many rod cells share a single sensory neurone. Rod
cells can therefore respond to light of very low
intensity.
This is because a certain threshold value has to be
exceeded before a generator potential is created
in the bipolar cells to which they are attached.
Rod cells
A number of rod cells are attached to a single
bipolar cell (= retinal convergence), there is a much
greater chance that the threshold value will be
exceeded than if only a single rod cell were
attached to each bipolar cell.
As a result, rod cells allow us to see in low light
intensity (i.e. at night), although only in black and
white.
Changes in the electrical potential of a receptor
when stimulated by three separate stimuli. Only the
third stimulus produces a generator potential high
enough to trigger a nerve impulse.
A rod cell
LIGHT
Rhodopsin
(pigment in
rod cells
broken down)
opsin
Signal from
Bipolar cell
A rod cell
As many rod cells are
joined to the same
bipolar cells, only a single
impulse will be stimulated.
This means that they
cannot distinguish
between the separate
sources of light that
stimulated them.
2 dots close together will
appear as a single blob.
Rod cells therefore have
low visual acuity.
Cone cells
Cone cells are of three different types, each
responding to a different wavelength of light.
Depending on the proportion of each type that is
stimulated, we can perceive images in full colour.
Each cone cell usually has its own bipolar cell
connected to a sensory neurone. This means that
often the generator potential is not exceeded. As
a result, cone cells only respond to high light
intensity and not to low light intensity.
Cone cells
Cone cells contain a different pigment to rod cells
(iodopsin). This requires a higher light intensity to
be broken down and create a generator potential.
As cone cells are attached to their own bipolar
cell, if 2 adjacent cells are stimulated, the brain
receives 2 separate impulses.
Cone cells give very accurate vision, they have
good visual acuity.
Iodopsin
pigment
Rod cells
Cone Cells
Rod-shaped
Cone-shaped
One type
Red, green &
blue types
Greater
Fewer
numbers than numbers than
cones
rods
Distributed
more in the
periphery
Fewer at
periphery,
concentrated
in fovea
Poor acuity
Good acuity
High
sensitivity
Low
sensitivity
Rhodopsin
pigment
Iodopsin
pigment
Cone cells
Light is focussed by the lens on a point known as
the fovea. The fovea therefore receives the
highest intensity of light.
Therefore cone cells, but not rod cells, are found
at the fovea. The concentration of cone cells
diminishes further away from the fovea. At the
peripheries of the retina, where light intensity is
at its lowest, only the rod cells are found.
Wavelengths of light absorbed by different
cones
Distribution of Rods and
Cones
• Fovea – area of retina where light is
focused – highest concentration of
cones – few elsewhere.
• Rods most concentrated either side of
fovea but still many elsewhere.
• No rods or cones at blind spot – this is
where the optic nerve exits the eye.
Colour Blindness
• If you have normal vision
you will see a figure seven
in reddish brown dots.
• People with red-green
colour blindness will not
see the 7, why?
• These people lack red sensitive cones, but the
green stimulated cones are stimulated by the red
light, so all dots appear green
Further Questions
1. Explain why brightly coloured objects
often appear grey in dim light.
2. At night, it is often easier to see a star
in the sky by looking slightly to the
side of it rather than directly at it.
Suggest why this is so.