<P> The resting potential must be established within a cell before the cell can be depolarized . There are many mechanisms by which a cell can establish a resting potential, however there is a typical pattern of generating this resting potential that many cells follow . The cell uses ion channels, ion pumps, and voltage gated ion channels to generate a negative resting potential within the cell . However, the process of generating the resting potential within the cell also creates an environment outside the cell that favors depolarization . The sodium potassium pump is largely responsible for the optimization of conditions on both the interior and the exterior of the cell for depolarization . By pumping three positively charged sodium ions (Na) out of the cell for every two positively charged potassium ions (K) pumped into the cell, not only is the resting potential of the cell established, but an unfavorable concentration gradient is created by increasing the concentration of sodium outside the cell and increasing the concentration of potassium within the cell . Although there is an excessive amount of potassium in the cell and sodium outside the cell, the generated resting potential keeps the voltage gated ion channels in the plasma membrane closed, preventing the ions that have been pumped across the plasma membrane from diffusing to an area of lower concentration . Additionally, despite the high concentration of positively - charged potassium ions, most cells contain internal components (of negative charge), which accumulate to establish a negative inner - charge . </P> <P> After a cell has established a resting potential, that cell has the capacity to undergo depolarization . During depolarization, the membrane potential rapidly shifts from negative to positive . For this rapid change to take place within the interior of the cell, several events must occur along the plasma membrane of the cell . While the sodium--potassium pump continues to work, the voltage - gated sodium and calcium channels that had been closed while the cell was at resting potential are opened in response to an initial change in voltage . As the sodium ions rush back into the cell, they add positive charge to the cell interior, and change the membrane potential from negative to positive . Once the interior of the cell becomes more positively charged, depolarization of the cell is complete, and the channels close again . </P> <P> After a cell has been depolarized, it undergoes one final change in internal charge . Following depolarization, the voltage - gated sodium ion channels that had been open while the cell was undergoing depolarization close again . The increased positive charge within the cell now causes the potassium channels to open . Potassium ions (K) begin to move down the electrochemical gradient (in favor of the concentration gradient and the newly established electrical gradient). As potassium moves out of the cell the potential within the cell decreases and approaches its resting potential once more . The sodium potassium pump works continuously throughout this process . </P> <P> The process of repolarization causes an overshoot in the potential of the cell . Potassium ions continue to move out of the axon so much so that the resting potential is exceeded and the new cell potential becomes more negative than the resting potential . The resting potential is ultimately re-established by the closing of all voltage - gated ion channels and the activity of the sodium potassium ion pump . </P>

Channel of transmission of blood to the heart in the human body