Neurons are called excitable cells because their membrane is in a polarized state due to differential concentration gradient of sodium and potassium ions. Axolemma is selectively permeable in nature. In the resting state, the potential difference across the cell membrane of neurons is negative inside the cell with respect to the outside. The membrane is said to be polarised. Resting membrane potential is about -70 mV. This potential is maintained by the active transport of ions against their gradient by the sodium-potassium pump.