The Resting Membrane Potential: Why Your Cells Are Always Charged

Right now, every cell in your body is holding a charge. Not the neurons about to fire, not the muscle cells about to contract, all of them, all the time, including the skin cell on your fingertip that will never send an electrical signal anywhere in its life. The inside of that cell sits at roughly negative 70 millivolts relative to the outside. That’s the resting membrane potential, and the word “resting” is doing a lot of misleading work, because maintaining it is one of the most metabolically expensive things a cell does.

The physiologist Julius Bernstein proposed the basic idea in 1902 while working at the University of Halle in Germany: cell membranes are selectively permeable to ions, and that selective permeability creates a voltage difference across the membrane. Bernstein didn’t have the tools to prove the details. Those came half a century later, in 1952, when Alan Hodgkin and Andrew Huxley at Cambridge used the giant axon of a squid, thick enough to thread with electrodes, to measure exactly how sodium and potassium ions move across a membrane during a nerve impulse. Their work won a Nobel Prize in 1963 and gave biology its first mathematical model of how a cell generates and controls voltage. It’s still the model taught in every physiology course today.

A cell spends energy just to stay boring

Here’s the part that surprises people the first time they hear it: the resting potential isn’t a passive equilibrium. It’s an actively maintained, energy-consuming state. The main engine behind it is a protein called the sodium-potassium pump, or Na+/K+-ATPase, embedded in the cell membrane. For every molecule of ATP it burns, the pump shoves three sodium ions out of the cell and pulls two potassium ions in. Both moves go against the ions’ natural tendency to spread out evenly, so the pump is constantly working uphill, like bailing water out of a boat that’s always taking some back in.

That uneven distribution is the whole point. A typical mammalian neuron keeps potassium concentrated inside, around 140 millimolar inside versus about 5 millimolar outside, and sodium concentrated outside, around 145 millimolar outside versus 12 millimolar inside. Chloride and calcium have their own gradients, also maintained against the grain. None of this happens by accident, and none of it is free. Estimates from neuroscience textbooks put the sodium-potassium pump’s energy cost at somewhere around a fifth to a third of a resting neuron’s total ATP budget, just to hold a voltage that isn’t doing anything in that instant except being ready.

The resting potential isn’t rest at all. It’s a battery paying an electric bill every second.

Once the gradients exist, the membrane’s job is to let some ions leak through selectively, and that leak is what actually generates the voltage you can measure. At rest, the membrane is far more permeable to potassium than to sodium, thanks to potassium “leak” channels that stay open even when the cell isn’t being stimulated. Potassium drifts out along its concentration gradient, taking positive charge with it, until the electrical pull dragging it back in balances the chemical push driving it out. That balance point is described by the Nernst equation, developed by physical chemist Walther Nernst in the 1880s, long before anyone applied it to cell membranes. For potassium alone, that equilibrium point lands around negative 90 millivolts in a typical neuron.

But real cells don’t sit exactly at potassium’s equilibrium, because the membrane leaks a small amount of sodium too, and that sodium pulls the voltage slightly less negative. The equation that accounts for multiple ions and their different permeabilities is the Goldman-Hodgkin-Katz equation, formalized by David Goldman in 1943 and refined by Hodgkin and Katz afterward. It’s the reason the textbook number for a neuron’s resting potential lands around negative 70 millivolts rather than negative 90. The gap between those two numbers is small on paper and enormous in consequence: it’s the difference between a cell that’s electrically stable and one perched right at the edge of firing.

Where the simple story breaks down

The clean version, potassium leaks out, sodium leaks in a little, the pump restores the balance, is the one you’ll find in most introductory biology courses, and it’s correct as far as it goes. Where it gets honestly more complicated is that “the resting potential” isn’t one fixed number shared by all cells. It varies enormously by cell type. A typical neuron rests around negative 70 millivolts. Skeletal muscle cells rest closer to negative 90. Red blood cells and many other non-excitable cells sit much closer to zero, sometimes only negative 10 to negative 20 millivolts, because they express a different mix of channels and don’t need to stay primed for a rapid electrical spike.

That variability is a real limitation in how the concept gets taught. Students often walk away thinking negative 70 millivolts is a universal biological constant, like body temperature. It isn’t. It’s an emergent property of whatever combination of pumps, channels, and gradients a particular cell happens to run, tuned by evolution to whatever job that cell does. A cardiac pacemaker cell in the sinoatrial node doesn’t even hold a stable resting potential at all. It drifts steadily upward after each heartbeat in a slow, spontaneous depolarization that’s the literal source of your heart’s rhythm, a feature, not a malfunction, of a cell that was never built to sit still.

A cell that stops spending energy on its ion pumps is a cell that’s already dying.

This is also where the resting potential’s fragility becomes clear. Cut off a cell’s oxygen supply, and ATP production collapses within minutes. Without ATP, the sodium-potassium pump stalls. Without the pump running, the gradients it maintains start to collapse toward zero, ions leak down their concentration gradients instead of being pumped back up them, and the membrane potential drifts toward zero along with them. That drift is a large part of what happens during a stroke or cardiac arrest at the cellular level: cells lose their voltage because they lose the energy to pay for it, and once neurons depolarize enough, they can dump excess calcium and glutamate in a cascade called excitotoxicity that damages neighboring cells too. The resting potential, in other words, isn’t just a setup for signaling. Its collapse is one of the actual mechanisms of cell injury.

Why a “boring” cell keeps its voltage anyway

You might reasonably ask why a skin cell or a liver cell bothers maintaining any membrane potential at all, given that it’s never going to fire an action potential. Part of the answer is basic housekeeping: the voltage helps regulate cell volume, since without active ion pumping, water would follow ion gradients into the cell and the cell would swell and potentially burst, a phenomenon called the Donnan effect. Part of the answer is that many everyday cellular processes, including nutrient transport and pH regulation, are wired directly to the ion gradients the membrane potential reflects.

But there’s a newer, stranger answer that’s reshaped how some researchers think about bioelectricity outside the nervous system entirely. Michael Levin’s lab at Tufts University has spent years documenting how non-neural cells use small, slow changes in resting potential as a kind of communication layer during development and regeneration, essentially a bioelectric code that helps tell a cluster of cells where a limb boundary should be, or whether a group of cells should stay part of a normal tissue pattern or start behaving abnormally. Levin’s group has shown that artificially shifting the resting potential of cells in a frog embryo, using ion channel drugs rather than any genetic edit, can trigger the growth of an extra limb or reshape how a tadpole’s face forms. None of that involves a nerve impulse. It’s the same basic machinery, pumps, channels, gradients, just running a much slower, quieter kind of signal.

Notice what the table actually shows: no single ion’s equilibrium potential matches the neuron’s real resting potential of about negative 70 millivolts. That’s the honest catch in the whole topic. The resting potential isn’t any one ion’s Nernst potential. It’s a weighted compromise across several ions, weighted by how permeable the membrane is to each one at that moment, which is exactly what the Goldman equation calculates and exactly why potassium, being the most permeable ion at rest, pulls the compromise so much closer to its own equilibrium than sodium’s.

The signal was always there, waiting

What makes the resting potential worth understanding, beyond the exam-question version of it, is that it’s the raw material every other piece of bioelectricity depends on. An action potential is just a brief, controlled violation of the resting state, sodium channels snapping open, the membrane swinging positive for a millisecond, then potassium channels restoring order. A pacemaker cell’s rhythm is a resting potential that refuses to hold still. A wound-healing electric field, the kind that guides skin cells toward an injury site during tissue repair, arises because a break in the epithelium creates a rift between two regions sitting at different potentials, generating a current that flows through the wound itself. None of it works without the baseline first: a membrane, a set of pumps, and a voltage that costs energy to hold and would vanish within minutes without that spending.

Every cell keeps a voltage, whether it ever fires a signal or not.

The gap in the popular understanding is usually not that people think the resting potential is fake. It’s that people assume it’s static, a fixed electrical fact about a cell the way its DNA sequence is a fixed genetic fact. It isn’t. It’s closer to a savings account that a cell is continuously funding through metabolism, available to be spent in a spike, a contraction, or a slow developmental instruction, and instantly vulnerable the moment the funding stops. That instability is not a flaw in the system. It’s the entire reason a resting cell can become an active one in under a millisecond, and it’s a fair bet that as tools for reading and writing bioelectric state get better, in labs like Levin’s and in the broader push toward bioelectronic medicine, the line between “resting” and “signaling” will look less like two categories and more like one continuous, metered voltage that never actually turns off.

Credit: Steve A Johnson on Unsplash