Answer
The resting membrane potential (RMP) of a cell is the voltage difference between the outside of the cell ( outer surface of the plasmalemma) and the inside surface of the cell membrane . In a resting. unstimulated cell this will vary, but it is usually negative. In a resting muscle cell it will be about -90 mV and in an unstimulated neuron it will be about -70 mV.
Work Step by Step
The plasmalemma is selectively permeable, the large of negative ions in the cytosol ( sarcoplasm) are trapped. These include charged proteins, nucleic acids, and phosphate ions. This gives the cell a negative charge; in other words the cell is polarized. The concentration of Na+ ions is higher in the ECF than in the intracellular fluid, but the concentration of K+ ions is higher in the sarcoplasm than in the extracellular fluid. The Na+ ions leak down their electrochemical gradient into the cell, and the K+ ions tend to leak down their chemical gradient out of the cell. However, the NA+/K+ ATPase pump continuously kicks out three Na+ ions for every two K+ ions that it brings into the cell and thus it maintains the voltage difference established by the selective permeability of the cell to negative and positive ions.
An action potential disrupts the RMP, depolarizes and may even hyperpolarize the cell.
