Unified account of the variable effects of carbon dioxide on nerve cells.

When the abdominal ganglion of Aplysia californica is exposed to 5 percent carbon dioxide, certain neurons are depolarized, others hyperpolarized, and some are unaffected. The effect of increased carbon dioxide is due solely to the concomitant fall in extracellular pH, which causes an increase in membrane chloride conductance of responsive cells. The directional change of the membrane potential in different neurons is determined by the relative values of the chloride equilibrium and the resting potentials. The chloride equilibrium potentials are calculated after direct measurement of the intracellular chloride activity with a chloride microelectrode.