Change of potassium activity associated with membrane current flow

Potassium-sensitive liquid ion exchanger microelectrodes are capable of recording fast concentration changes. Under suitable conditions, changes in extracellular potassium can be quantitatively measured during spike activity or voltage clamp pulses of exposed neurons. Comparison between transferred charge during outward current and the amount of extra potassium outside the cell reveals a deficit in net outward current, which is brought about by a long-lasting inward current. This inward current is facilitated by preceding depolarization and its appearance adds to a true inactivation of the potassium system. This is an example of how ion exchanger microelectrodes can be used to differentiate between time ionic fluxes underlying membrane currents. They are also useful in examining relations between neuronal activity and extracellular changes in potassium ion concentration in the central nervous system. Due to restricted diffusion in its extracellular space transient changes in potassium activity remain quite localized for considerable time. This makes it possible to identify discrete fields of activated neurons and to compare changes of neuronal excitability with variations of extracellular potassium activity.