Factors determining the decay of K+ potentials and focal potentials in the central nervous system.

Post-tetanic undershoots in extracellular focal potential (deltaV) and K+ potential (EK) can be recorded in the cuneate nucleus and dorsal horn of Dial-anaesthetized or decerebrate cat. They are seen best at depths where the largest deltaEK and deltaV are recorded and they increase with the frequency and duration of stimulation. The very different time courses of undershoots of deltaV and deltaEK indicate two hyperpolarizing influences: first, electrogenic pumping, and later, a reduced external K+ concentration. The importance of active K+ removal in determining the amplitude and duration of deltaEK and deltaV is illustrated by their marked potentiation (as well as the disappearance of post-tetanic undershoots) induced by a lowering of blood pressure or local application of strophanthidin.