How does an action potential occur? When a neuron is not transmitting a signal there is a separation of charge across its membrane. This is called the resting potential. Sodium is in high concentration outside and potassium is in high concentration inside the cell. The cytoplasm is negative relative to the exterior because of the presence of negatively charged proteins, amino acids and other substances. When a sufficiently large stimulus reaches a section of membrane, sodium is able to diffuse in across the membrane following its concentration gradient. This depolarizes the membrane and is the stimulus for similar changes in adjacent membrane. Thus the action potential travels down the axon in a chain reaction. After sodium diffuses in, potassium diffuses out following its own concentration gradient. This restores the membrane’s polarity, but sodium and potassium have switched positions. The sodium potassium pump restores these ions to their original positions and the neuron is now ready for another action potential.