Constraints on the PLM. The steady-state intracellular ion concentrations ([Conc.], top), voltage (middle), and volume (bottom) are plotted as a function of the NKA pump rate (p). The concentrations are stacked as in Fig. 2. The steady-state solutions shown are the analytical Keener–Sneyd solution given in the Appendix. The hyperpolarization induced by the action of the NKA drives Cl⁻ out of the cell. Water moves out of the cell, preserving osmotic and charge balance, leading to a decrease in volume and an increase in the concentration of X^z. As p is increased further, K⁺ progressively substitutes for Na⁺. Osmotic balance is shown again by the fact that the sum of all intracellular ions at any p equals the extracellular osmotic strength. Electroneutrality is also preserved at all p; the sum of anions = sum of cations; z = −1. The volume is normalized by that at p = 2 (% Norm. vol.). The figure has been modified from Kay (2017), with a temperature of 37°C rather than 25°C.