Extracellular Potassium in the Mammalian Central Nervous System

Potassium, calcium, and magnesium are three inorganic ions that occur in significant quantity in extracellular fluid of the mammalian brain and have powerful effects on the functioning of nervous tissue. In different ways all three influence the excitability of neurons and the release of transmitters from presynaptic terminals. Two different points of view have evolved concerning the regulation of these ions in the central nervous system (CNS). Some authors have emphasized the narrow range of the activity of these ions in the healthy brain and have concluded that stability of brain function requires stability of the extracellular activity of these ions. Others have argued that in the course of evolution the mammalian brain must have found an advantageous use for these powerful agents in the normal regula­ tion of neuronal excitability. According to this view, programmed varia­ tions in K+o, Ca2+o, and perhaps also Mg2+o may be an integral component in the normal function of the central nervous system. Due to the recent development of potassium-selective microelectrodes (184) there has been a new surge of interest in the functional significance of potassium distribution in the nervous system. This selective review may be supplemented by consulting more general reviews on brain electrolytes (69, 72, 74, 83, 90, 158, 175).