The sodium–potassium pump and membrane voltage
The Na⁺/K⁺-ATPase uses ATP to move three Na⁺ ions out of a cell and two K⁺ ions in. It maintains ion gradients and contributes directly to voltage, but it is not the sole or usually the main immediate cause of neuronal resting potential.
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One transport cycle
The sodium–potassium pump (Na⁺/K⁺-ATPase) uses energy from ATP hydrolysis to move ions against their concentration gradients. Each cycle exports three Na⁺ ions and imports two K⁺ ions, leaving one net positive charge moved outward. Holmgren and colleagues used rapid voltage jumps to observe charge movements consistent with three sequential sodium-release steps. The Nature paper studies the transport cycle; it does not claim that the pump alone sets the entire resting voltage.
How gradients contribute to voltage
Membrane voltage depends on ion concentrations on both sides of the membrane and on how permeable the membrane is to each ion. In many neurons at rest, the membrane is more permeable to K⁺, so its gradient has a strong influence; lower Na⁺ permeability contributes as well. The pump counteracts the gradual leak of ions and preserves those gradients. Its 3:2 exchange also creates a direct electrogenic current that makes the inside slightly more negative. A University of St Andrews neuroscience tutorial describes this direct contribution as usually only a few millivolts and generally not the dominant immediate cause. See its Goldman-equation discussion.
Worked thought experiment: what if the pump stops?
Imagine a hypothetical neuron whose resting membrane is more permeable to K⁺ than to Na⁺. Its voltage is weighted toward the K⁺ diffusion gradient, with Na⁺ permeability shifting it somewhat; the active pump adds a smaller direct negative contribution. If the pump is inhibited briefly, existing gradients do not vanish instantly. Over time, however, leaked Na⁺ and K⁺ are no longer restored, so the gradients and cell function change. This is a conceptual example, not a measured voltage for a particular cell.
Conditions that change the interpretation
Voltage depends on cell type, channel expression and opening, extracellular ion concentrations, activity history, and measurement time. As cellular activity increases, the pump has to work harder to maintain the ion gradients. Thus “the pump creates the voltage” conflates two roles. More precisely, the pump maintains long-term ion gradients; those gradients and selective membrane permeability largely shape voltage, while the pump’s direct electrical effect adds to it and varies by condition.
Sources
- Holmgren et al., sequential Na⁺ release by Na⁺/K⁺-ATPase (Nature, 2000) — 3 Na⁺ out, 2 K⁺ in, and measured charge movements
- W. J. Heitler, Origin of the Resting Membrane Potential — permeability, ion gradients, and direct pump effect