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.