What Is Resting Membrane Potential?
All living cells have an electrical potential, or voltage, across their outer membranes. This voltage, called resting membrane potential, is created by ions inside and outside of the cell. Ions are electrically charged atoms, or molecules, so if the concentration of an ion differs on either side of a cellular membrane, electrical voltage can result. Voltage is the separation of positive and negative charges across a resistive barrier, and in the case of cells, the cell membrane provides the resistive barrier.
In most cells, there is a higher concentration of potassium (K+) inside than outside the cell and a higher concentration of chloride (Cl-) and sodium (Na+) outside the cell than inside. It takes energy for cells to maintain different internal ion concentrations than the extracellular milieu, because ions can enter and exit the cell by diffusing through the membrane, and by moving though protein channels. Cells use a sodium/potassium pump to constantly expel three sodium ions from the cell, each time two potassium ions are moved inside the cell. The cell membrane is fairly permeable to potassium ions, however, so potassium ions diffuse out of the cell, leaving behind them a net negative charge at the interior side of the cell membrane. This net negative charge means that the resting membrane potential of the average cell is about -70 milivolts.
The resting membrane potential sets the stage for communication and movement in neurons and muscle cells. An action potential is the mechanism neurons use to send signals to other neurons, or to muscle cells. When a nerve is stimulated, protein channels in the membrane open, and let sodium into the cell, making the membrane potential more positive. This depolarization of the membrane spreads down the length of a neuron until it reaches the point at which the neuron connects with a muscle cell, or another neuron. At that point, the depolarization cause the release of signaling molecules, which, in turn, cause an action potential in the muscle cell or second nerve cell.
Shortly after the sodium channels open, potassium channels open as well, letting potassium out of the cell and making the membrane potential more negative again (repolarization). The cycle of depolarization and repolarization caused by the opening of sodium and potassium channels is called an action potential. Potassium channels let enough potassium out of the cell that the membrane potential is actually a little more negative than -70 milivolts after the completion of an action potential. A resting membrane potential of -70 milivolts returns quickly, however, because of the constant action of sodium/potassium pumps, which maintain physiologically appropriate levels of sodium and potassium in the cell. Once resting membrane potential has been restored, the cell is competent to undergo another action potential.
Written by Emily Updegraff
