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level: 15.3 Passage of an action potential

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level questions: 15.3 Passage of an action potential

QuestionAnswer
Action potential move along neuron as a wave of depolarisation- When an action potential happens, some of the S ions that enter neuron diffuse sideways - This causes S ion channels in the next region of neuron to open - S ions diffuse into that part - Causing wave of depolarisation to travel along neuron - The wave moves away from the parts of the membrane in the refractory period coz these parts cant fire an action potential
Passage of an action potential along an unmyelinated axon (1)- At resting potential the conc. of S ion outside axon membrane is higher than inside - P ion conc. higher inside than outside membrane - Overall conc. of + ions is greater outside than inside - Axon membrane is polarised
Passage of an action potential along an unmyelinated axon (2)- As stimulus causes a sudden influx of S ions hence a reversal of charge on axon membrane - This is the action potential - Membrane is depolarised
Passage of an action potential along an unmyelinated axon (3)- Localised electrical currents established by influx of S ions cause opening of S voltage-gated channels further along axon - The resulting influx of S ions in this region causes depolarisation
Passage of an action potential along an unmyelinated axon (4)- Behind new region of depolarisation, S voltage-gated channels close - P ones open - P ions leave axon along their electrochemical gradient - The depolarisation moves along membrane
Passage of an action potential along an unmyelinated axon (5)- The action potential (depolarisation) is spread in the same way further along axon - Outward movement of P ions has continued to extent that axon mem. behind AP has returned to original charged state (+ outside, - inside) - It has been repolarised
Passage of an action potential along an unmyelinated axon (6)- Repolarisation of axon allows S ions to be actively transported out - Returning axon to its resting potential in readiness for new stimulus if it comes
Passage of an action potential along an myelinated axon (1)- In myelinated axons the fatty sheath of myelin around axon acts as an electrial insultor - Preventing APs from forming - At intervals of 1-3mm there are breaks in myelin insulation, nodes of Ranvier - AP can occur at these points
Passage of an action potential along an myelinated axon (2)- Localised circuits arise between adjacent nodes of Ranvier - APs in effect jump from node to node in a process called saltatory conduction - So, AP passes along a myelinated neuron faster than unmye. axon of same diameter - Coz in unmyel. neuron, depolarisation have to take place all the way along axon, so takes time