hyperpolarize

Hyperpolarize is a term used across disciplines to describe increasing the degree of polarization beyond a baseline state. In biology, it most often refers to a change in a cell’s membrane potential, while in physics and chemistry it describes creating a larger population difference between quantum spin states than exists at thermal equilibrium.

In neuroscience, hyperpolarization describes a membrane potential that becomes more negative than the resting potential. This makes neurons less likely to fire an action potential. Hyperpolarization can result from the opening of potassium or chloride channels, often as part of inhibitory synaptic signaling (for example, activation of GABA receptors permits Cl− influx or K+ efflux). The hyperpolarized state contributes to the afterhyperpolarization phase following an action potential and helps regulate neuronal excitability and timing of signaling.

In magnetic resonance and related fields, hyperpolarization refers to artificially increasing spin polarization to amplify signal strength. Techniques include dynamic nuclear polarization (DNP), spin-exchange optical pumping (SEOP) for noble gases, and parahydrogen-induced polarization (PHIP). Hyperpolarized samples provide signals far stronger than those at thermal equilibrium, enabling enhanced magnetic resonance imaging (MRI) and spectroscopy, especially for metabolic imaging with hyperpolarized 13C tracers. The advantages include faster, more sensitive measurements, but the enhanced polarization is temporary, decaying with spin-lattice relaxation (T1) and requiring specialized equipment and rapid data acquisition.

Overall, hyperpolarization describes a state of heightened polarization achieved either by physiological processes that increase membrane negativity or by technical methods that boost spin alignment for improved analytical signals.