Hyperpolarization occurs when the difference in electrical potential between two sides of a cellular membrane changes significantly, resulting in a large electrical potential across the membrane. Specifically, the value of the electrical potential across the membrane becomes more negative, meaning that the charge on the inside of the cell's membrane is more negative than the charge on the outside of the membrane. This process is commonly observed in neuroscience as neurons are activated through processes involving changes in electrical potential. The opposite of hyperpolarization is depolarization, in which a cell's potential becomes more positive, meaning that there is significantly less negative charge inside of the cell membrane.
Electrochemical processes are generally responsible for the occurrence of hyperpolarization across cellular membranes. The concentrations of various chemicals on different sides of a membrane can cause an electrical potential to develop across the membrane. Generally, when the electrical potential reaches a certain point, some biological process will be initiated, such as the firing of a neuron. After this point, the membrane tends to return to its resting potential, or the electrical potential before any stimuli caused the electrochemical event to occur. In neurons, this process happens continuously; stimuli cause polarization to occur over a membrane, and when the degree of that polarization crosses a certain threshold, the neuron fires and returns to its resting potential.
A neuron will not fire until its electrical potential overcomes a certain threshold. Upon reaching the threshold, the electrical potential increases drastically, allowing the neuron to send off an electrical signal to other parts of the body. Hyperpolarization occurs after this spike in potential; the electrochemical potential briefly becomes negative, dropping below the the resting potential, before returning to the resting potential. Usually, this stage of hyperpolarization lasts for only a brief fraction of a second.
Hyperpolarization and electrical potentials across membranes in general involve the transfer of electrons in ions. An ion is an atom that has either a positive or a negative charge. Potassium and chlorine ions are commonly involved in electrochemical potentials; their relative concentrations determine the magnitude of the electrochemical cellular potential. In the resting stage, potassium lies within the cellular membrane; upon exposure to a stimulus, the potassium rushes out and negative chlorine ions flow into the cell through the membrane. Occasionally, sodium and calcium ions cause electrochemical cellular potentials across cellular membranes as well.