hyperpolarisere

Hyperpolarisation (or hyperpolarization) refers to a polarization state that exceeds the normal equilibrium polarization of a system. In biology, membrane hyperpolarization is a change in a cell's membrane potential that makes the inside more negative relative to the outside, typically by the efflux of potassium ions or influx of chloride ions. In neurons, hyperpolarization follows inhibitory synaptic activity, such as activation of GABAergic receptors, and increases the threshold for firing action potentials. It plays a key role in controlling neuronal excitability, shaping signal integration, and maintaining rhythmic activity. In cardiac muscle, hyperpolarization influences pacemaker activity and refractoriness. Hyperpolarization is transient and is followed by repolarization to resting potential.

In physics, chemistry, and medicine, hyperpolarisation describes a deliberate enhancement of spin polarization beyond thermal equilibrium, used to boost signal in magnetic resonance techniques. Methods include dynamic nuclear polarization (DNP), which transfers electron spin polarization to nuclei at low temperature and high magnetic field, often increasing NMR and MRI signals by orders of magnitude. Other approaches include parahydrogen-induced polarization (PHIP) and its SABRE variant, which exploit chemically induced spin order, and optical pumping of noble gases to produce hyperpolarized 129Xe or 3He for lung imaging and spectroscopy. Hyperpolarization requires fast signal acquisition and rapid transfer, because the enhanced polarization decays with the longitudinal relaxation time T1.

Hyperpolarisation techniques are used to improve sensitivity in biomedical imaging, materials science, and chemical analysis. They complement traditional steady-state NMR by enabling observation of low-concentration species and fast dynamic processes.