A nerve impulse is a wave of energy transduction, an electrochemically actuated change in the polarity of the neuron that travels as a wave. The basis of a nerve impulse is the movement of ions across the cell membrane. Prior to transmitting an impulse, the neuron inhabits a state of polarization called its resting potential in which the inside of the cell is electrically negative compared to the outside of the cell. The resting potential is the result of the activity of sodium-potassium pumps in the neuron cell membrane. These enzyme pumps move Na+ out of the cell and K+ into the cell. The typical resting potential is about -70 mV. A nerve impulse is a wave of depolarization travelling down the membrane. Depolarization is accomplished by the opening of voltage-gated channels or chemical gated channels. The first channels to open are Na+ channels, and because free energy change driving the spontaneous inrush derives from both the voltage gradient and the concentration gradient, depoloarization overshoots zero achieving the action potential of +40 mV. The positive potential triggers voltage gated K+ channels to open in the same region and voltage-gated Na+ channels to open in an adjacent region, propagating the impulse. K+ ions rush out of the cell; and the potential falls rapidly to a degree even below the original resting potential. What follows is the refractory period, in which the cell returns to the original configuration of the resting period and during which it cannot transmit another impulse.