Resting potential is the difference in voltage across a cell membrane, and it is sometimes referred to as resting voltage. Certain types of cells, such as neurons and muscle cells, use resting potential to enact changes within the cell and the body. Action potentials, muscle contraction, and establishing or changing equilibrium processes in the cell all involve the membrane’s resting potential.
There are varying concentrations of ions in the cytosol, or cell interior, as well as inside different cellular compartments and organelles. Since ions are either positively or negatively charged, they create a charge difference between these different compartments, forming a difference in electric potential. Often, cells will want to maintain this difference across a membrane by using protein ion pumps and channels. When an electric potential difference is maintained, it is called the resting potential.
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The ions that are most involved in creating and maintaining a resting voltage for a membrane are sodium (Na) and potassium (K) ions. Generally, the concentration of K+ is greater inside the cell than outside, while the concentration of Na+ is greater outside of the cell than inside. This difference is maintained by a membrane protein pump called the Na+/K+-ATPase, which uses adenosine triphosphate (ATP) for energy to maintain the relative concentrations. The pump incorporates three Na+ ions into the cell for every two K+ ions it exports, giving the cell’s interior a more negative charge. This resting potential is especially important for neurons, which use the voltage difference to fire action potentials.
In neurons and other cells in the nervous system, an action potential is generated when the resting potential is disturbed. The action potential begins with an influx of Na+ ions into the cell through certain ion channels, which creates a depolarization of the membrane potential once a certain threshold has been met. Here, the action potential is generated and the electrical signal is transmitted through the neuron. Following the spike in Na+, more voltage-gated ion channels open, releasing K+ from the cell, a step in the action potential is known as hyperpolarization, where the membrane potential drops below the normal resting voltage. The cell then reestablishes its resting potential using the Na+/K+-ATPase in the process of repolarization.
Calcium (Ca) ions are also important in maintaining the resting membrane potential in muscle cells. The Ca2+ ions are stored in an organelle called the sarcoplasmic reticulum, which contains protein pumps to maintain high concentrations of Ca2+ inside the compartment. When a muscle cell is told to contract, an electrical signal triggers the sarcoplasmic reticulum by using the resting potential. The compartment is then able to open, releasing Ca2+ ions into the cell, which bind to the fibers that allow the muscle to contract.