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Nerve Impulses - Action Potentials An impulse or action potential is a temporary and local reversal of the resting potential, arising when an axon is stimulated. During an action potential, the membrane potential falls until the inside of the membrane becomes positively charged with respect to the exterior (-70mv to +40mv). The membrane is then said to be depolarised. The change in potential across a membrane comes about because of ion channels in the membrane that, when open, allow particular ions to pass. One type of protein is permeable to sodium ions and another type is permeable to potassium ions. During the resting potential these channels are closed. When an impulse is transmitted they start to open. Sodium channels open first, allowing a large number of sodium ions to flow in. The axoplasm becomes progressively more positive with respect to the outside of the membrane. Then, almost instantly, the permeability of the membrane to sodium...

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