Hyperpolarisaation

Hyperpolarisation describes two related phenomena in different fields. In neuroscience and electrophysiology, it denotes a change in a cell’s membrane potential toward a more negative value than the resting potential, typically following inhibitory synaptic input or the opening of potassium channels. This increase in the magnitude of the negative membrane potential places the cell farther from the threshold for action potential generation, reducing excitability. Hyperpolarisation can occur during inhibitory postsynaptic potentials, after-action potentials during repolarization, or via ion channel activity that increases chloride conductance.

In magnetic resonance and related fields, hyperpolarisation refers to engineered increases in the population difference between spin states beyond what is present at thermal equilibrium. This leads to signal enhancements in NMR and MRI. Techniques include dynamic nuclear polarization (DNP), para-hydrogen-induced polarisation (PHIP) and its variants such as SABRE, spin-exchange optical pumping (SEOP) for noble gases like 129Xe and 3He, and other methods that transfer polarisation from electrons or parahydrogen to nuclei. Hyperpolarised substances provide greatly enhanced signal strength for spectroscopy and imaging, enabling studies of metabolic processes, molecular structure, and materials.

Limitations: Hyperpolarisation is typically non-permanent; the enhanced polarisation decays with a characteristic relaxation time (T1) and may require rapid data acquisition. In neuroscience, hyperpolarisation is transient and central to neuronal signalling. In magnetic resonance, generating hyperpolarised agents requires specialised equipment and handling.