Hyperpolarizing

Hyperpolarizing refers to processes that make a cell's membrane potential more negative relative to its resting potential, thereby reducing excitability. In neurons, hyperpolarization is typically achieved when positively charged potassium ions exit the cell or when negatively charged chloride ions enter through specific ion channels, such as GABA_A receptors or glycine receptors. The result is an inhibitory postsynaptic potential (IPSP) that moves the membrane potential away from the threshold for action potential generation. Hyperpolarization can occur during the repolarization phase after an action potential and may exceed the resting potential, a state called afterhyperpolarization. The extent and duration depend on ion channel kinetics and the driving forces for K+ and Cl−. Hyperpolarizing mechanisms contribute to temporal coding, prevent excessive firing, and help restore membrane potential following excitation. Pharmacological agents that enhance inhibitory neurotransmission, such as GABAergic drugs, exert their effects by promoting hyperpolarization and neuronal silencing.

In a different context, hyperpolarization describes methods used in magnetic resonance imaging and spectroscopy to increase the population difference between nuclear spin states, thereby increasing signal intensity. Techniques include dynamic nuclear polarization (DNP) and spin-exchange optical pumping, used to create hyperpolarized substrates such as 13C-pyruvate or hyperpolarized noble gases (e.g., helium-3, xenon-129) for enhanced imaging of metabolism or lung structure. Hyperpolarized tracers provide substantially greater signal but decay as polarization relaxes, requiring rapid preparation and data acquisition. Together, hyperpolarization in physiology and imaging denotes processes that push systems toward greater negativity of membrane potential or enhanced nuclear polarization, respectively.