Excitable cells (muscle cells and neurons) respond to and carry electrical signals. All cells have a resting membrane potential, a slight negative charge on the inner surface of their cell membranes. The concentration of particles inside cells is very high, so Na ions must be pumped out of cells by the Na-K ATPase to maintain osmotic balance. At the same time, ion channels (which are required for ions to pass across membranes) for K ions are open at rest. K ions can leave the cell (down their concentration gradient), but are kept inside the cell by the negative membrane potential.
The end result is a high concentration of Na ions outside the cell and a negative charge on the inside of the cell membrane. If cells were permeable to Na ions, they would enter the cell and collapse the negative charge on the inside of the cell. However, cells are not very permeable to Na ions. This means that Na ions can be used for signaling. If Na ion channels are opened, Na enters the cells and makes the membrane potential less negative. This is called depolarization.
When depolarization of the cell membrane of an excitable cell is sufficient, an action potential will fire. An action potential is a long-distance, all-or-none electrical signal that passes along axons of neurons and the sarcolemma of muscle fibers. Muscle fibers, cardiac pacemakers, and axons all have action potentials with different characteristics. Those of axons are sharp spikes. Muscle fibers have more prolonged action potentials. Cardiac pacemakers have action potentials that spontaneously fire again and again.
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