hyperpolarisationactivated

Hyperpolarization, or hyperpolarisation, is a term used in multiple scientific fields to describe a state in which a system is more polarized than typical baseline conditions. In neuroscience, membrane hyperpolarization refers to a transient change of a cell's membrane potential to a more negative value than the resting potential. This occurs when potassium channels open, allowing K+ to leave the cell, or when inhibitory neurotransmitter receptors increase chloride conductance, driving the potential toward the equilibrium potential of Cl-. Hyperpolarization reduces neuronal excitability by increasing the threshold for action potential initiation and can underlie inhibitory postsynaptic potentials and the afterhyperpolarization that follows an action potential.

In magnetic resonance and related spectroscopy, hyperpolarization describes artificially increasing the population difference between nuclear spin states beyond the Boltzmann distribution. This creates greatly enhanced signal intensities in short-lived experiments. Techniques include dynamic nuclear polarization (DNP), para-hydrogen-induced polarization (PHIP), and spin-exchange optical pumping (SEOP). Hyperpolarized spins relax back to normal polarization over characteristic timescales determined by T1, often limiting practical use to minutes in in-vivo imaging.

Hyperpolarization methods have applications in medical imaging, materials science, and chemical kinetics, improving sensitivity for detecting metabolites, tracking processes in real time, and enabling observations that are impractical with conventional signals. The term therefore encompasses both physiological processes that transiently reduce excitability and physical techniques for signal enhancement.