Measurement of Metabolic Activity Associated with Ion Shifts

Brain “function” comprises the activities that transfer and integrate information wit~undefinedhin and among brain cells. Such information consists of changes in electrical potentials that exist across cell membranes. Electrical potentials are characteristic of both neurons and glia because of the asymmetrical distribution of ions, particularly Na⁺ and K⁺ , across the membranes of these cells, between the intracellular and extracellular milieu. Under normal circumstances, the intracellular K⁺ activity is more than 30 times greater than that in the extracellular space, whereas the extracellular Na⁺ activity is 10 or more times greater than that within these cells (e.g., Katz, 1966; Katzman and Pappius, 1973). These transmembrane ion gradients produce a situation equivalent to a battery between cells and their external environment, the inside of brain cells being negative with respect to their outside. Whenever ion gradients change, such as occurs in response to ionic or neurotransmitter-mediated changes in membrane conductances (permeability), there are changes in transmembrane electrical potentials. These voltage shifts are the “information” of the nervous system.