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7.2 The resting potential and the nerve impulse

Neurons transmit electrical signals along the axon, away from the cell body. At rest the inside of the membrane is negative relative to the outside; the resting membrane potential is approximately -70 mV. This gradient is maintained by the Na+/K+ ATPase, which moves three sodium ions out of the cell and two potassium ions in for each ATP hydrolysed, and by the greater resting permeability of the membrane to potassium.

When a stimulus depolarises the membrane to threshold (about -55 mV), voltage-gated sodium channels open and sodium enters the cell, producing the rising phase of the action potential. Repolarisation follows as sodium channels inactivate and voltage-gated potassium channels open. During the absolute refractory period no second action potential can be generated, which limits firing frequency and keeps conduction one-directional along the axon…