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Summary OCR A-Level Biology 5.3.3 Generating Action Potentials
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Generating Action PotentialsSensory Receptors
Known as transducers because they convert one form of energy to another
Photoreceptor – light to electricity
Temperature receptor – heat to electricity
Pressure receptor – pressure to electricity
Sound receptor – vibrations to electricity
Mechanoreceptor – movement to electricity
Chemoreceptor – chemicals to electricity
The Pacinian Corpuscle is a pressure receptor that detects pressure on the skin. They are
sensitive only to changes in pressure which deform the rings of connective tissue, so when
pressure is constant, they stop responding
Generating Action Potentials
Membrane starts in a resting state – polarised with a relative charge inside of the cell of
-60mV compared to the outside due to the higher concentration of sodium ions (Na +)
outside the cell
Deformation of membrane (due to pressure) opens Na + channels
Na+ ions diffuse into the cell, producing a generator potential
Membrane depolarises, becoming less negative compared to the outside of the cell
If a threshold of -50mV is reached, positive feedback will ensure an action potential is
generated
Voltage gated Na+ ions channels open and more Na+ ions flood in
The inside of the cell becomes positively charged compared to the outside
The potential difference across the membrane reaches a maximum of +40mV
Na+ ion channels close and potassium ion (K+) channels open
K+ ions diffuse into the cell, bringing the potential difference back to negative compared
to the outside of the cell – this is repolarisation
The K+ ion channels are slow to close and the potential difference overshoots slightly,
known as hyperpolarisation
There is a refractory period where sodium/potassium ion pumps return the potential
difference across the membrane to its resting state by pumping in 2 K + ions for every 3
Na+ ions pumped out
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