Polarization, depolarization, and repolarization are terms commonly used to describe electrical changes across the membranes of excitable cells, such as neurons and muscle cells.
Polarization refers to the state of a cell when it is at rest. In this state, the inside of the cell has a negative charge relative to the outside. This occurs because there is a higher concentration of positively charged ions (like sodium) outside the cell and negatively charged ions (like potassium and various organic anions) inside. The difference in charge creates a resting membrane potential, usually around -70mV in neurons.
Depolarization is the process that occurs when a stimulus causes the membrane potential to become less negative (or more positive). This can happen through the influx of sodium ions into the cell when voltage-gated sodium channels open in response to a stimulus. If the membrane potential reaches a certain threshold (approximately -55mV), an action potential is triggered, leading to a rapid increase in positive charge inside the cell.
Repolarization follows depolarization and is the process of returning the cell to its resting state. This involves the opening of potassium channels, which allows potassium ions to flow out of the cell, restoring the negative internal environment. During repolarization, the membrane potential goes back toward the resting value of around -70mV. However, due to the prolonged opening of potassium channels, it may temporarily become more negative than the resting potential, a phase known as hyperpolarization.
In summary, polarization establishes the resting state of the cell, depolarization represents the rapid change in membrane potential during action potential generation, and repolarization restores the cell to its resting state, ready to respond to future stimuli.