Ionic Changes Responsible for Post-tetanic Hyperpolarization

Abstract

AFTER repetitive stimulation, the membrane potential of nerve fibres is transiently increased. It has been suggested by Ritchie and Straub¹ that this hyperpolarization could be explained on the basis of two assumptions: that the ionic pump responsible for the coupled efflux of sodium ions and influx of potassium ions² becomes hyperactive following stimulation because of the raised intra-cellular sodium ion concentration³; that the extracellular space adjacent to the membrane is confined by a diffusional barrier. Following activity, the hyperactive pump would deplete the potassium in this space and thus raise the membrane potential of the nerve fibre above its resting level. A consequence of this hypothesis is that, in the absence of extracellular potassium, the sodium pump would be unable to lower the potassium concentration further and there would be no post-tetanic hyperpolarization, as Ritchie and Straub¹ indeed found in mammalian C-fibres. On the other hand, an increased post-tetanic hyperpolarization in potassium-free solutions has since been reported both in the same fibres⁴ and in frog nodes of Ranvier⁵.