hyperpolarization

Hyperpolarization is a term used in neuroscience and physics/chemistry to describe a state in which a system's polarization exceeds its thermal equilibrium level. In magnetic resonance contexts, hyperpolarization refers to a non-equilibrium excess of nuclear spin polarization that yields signal intensification in NMR and MRI. In neuroscience, it denotes a transient increase in the negativity of a cell's membrane potential, below the resting potential, which reduces neuronal excitability.

In neuroscience, hyperpolarization typically follows inhibitory inputs. Activation of GABA_A receptors allows Cl- ions to flow into the neuron, while certain potassium channels permit K+ efflux. The resulting outward current or inward chloride current drives the membrane potential away from threshold, producing an inhibitory postsynaptic potential. Hyperpolarization can also occur as part of intrinsic afterhyperpolarization after action potentials. The hyperpolarized state increases the distance to the threshold for triggering action potentials and can regulate spike timing, integration of synaptic inputs, and rebound excitability.

In the physics and chemistry sense, hyperpolarization means creating a population imbalance where more spins occupy the lower-energy state than predicted by Boltzmann distribution at a given temperature and magnetic field. Techniques such as dynamic nuclear polarization transfer polarization from electrons to nuclei (DNP), spin-exchange optical pumping (SEOP) to polarize noble gases, and parahydrogen-induced polarization (PHIP) or SABRE can produce substantial enhancements in NMR/MRI signal. These methods enable highly sensitive detection of metabolic processes and molecular structure, albeit with technical requirements and relaxation limits.

Hyperpolarized states are typically non-equilibrium and relax back to thermal equilibrium with characteristic relaxation times. Applications include research and clinical imaging, with benefits in sensitivity but limitations in duration, stability, and accessibility.