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AQA A level Biology receptors summary sheet
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A summary sheet for the receptors topic in line with the AQA a level biology spec. Simple, concise notes including diagram.
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Receptors:Sensory receptors are specialised cells in the nervous system that detect physical stimuli and
convert them into electrical signals (action potentials). They are usually specific to one type of
stimulus as they have specialised structures specific to one type of physical property.
Pacinian corpuscles
Detect changes in pressure in skin.
Pressure increase causes deformation of the concentric rings.
This opens stretch mediated sodium ion channels in the membrane so sodium ions enter
the sensory neurone which can generate an action potential.
Photoreceptors
The retina contains photoreceptors which detect light. There are 2 types – rods and cones.
1) Many rod cells are connected to one bipolar neurone which gives them a lower visual acuity
but a higher sensitivity to light.
2) Only one cone cell is connected to each bipolar neurone, giving them higher visual acuity
but a lower sensitivity to light.
Bipolar neurones are a type of relay neurone – they connect sensory neurones to motor neurones.
Rod cells Cone cells
Monochromatic – only detect one wavelength Trichromatic – there are 3 types, each of which
of light responds to either red, green or blue light.
Highly sensitive to light (i.e. can detect light Low sensitivity to light
even at low intensities)
Low visual acuity (ability to distinguish between High visual acuity
2 points)
Uses the pigment rhodopsin to detect light Uses the pigment iodopsin to detect light
Numerous A lot less abundant than rod cells
Evenly distributed on the retina but absent on Concentrated on the fovea
the fovea
How photoreceptors convert light into an electrical impulse:
1. Light is absorbed by pigments in cone and rod cells (rhodopsin and iodopsin).
2. The absorption creates a change in the membrane permeability of these pigments, causing
Na+ ions to flood into the cell.
3. Na+ ions entering the cell causes the potential difference to be relatively positive. If the
potential difference reaches +55mV (generator potential), then an action potential is
generated along the bipolar neurone.
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