Describe the development and propagation of an action potential in response to a stimulus (6 marks)

1.     stimulus excites membrane so Na+ channels open, membrane becomes more permeable to Na+ so Na+ diffuse into neurone down electrochemical gradient. Inside of neuron becomes less -ve 2.     Depolarisation: if potential difference reaches threshold (c55mV) more Na+ channels open, more Na+ diffuses into neurone 3.      Repolarisation: at +30mV Na+ channels close and K+ channels open. Membrane is more permeable to K+ so K+ diffuses out of neuron down its concentration gradient. This pushes the membrane back to resting potential4.     Hyperpolarisation: K+ channels close too slowly so overshoot, too much K+ leaves the neuron and it becomes more negative than resting potential 5.     Resting potential: ion channels reset, sodium potassium pump returns membrane to resting potential (-70mV) by pumping Na+ out and K+ in.  

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