Why Is Resting Membrane Potential Negative?

Every living cell carries a small voltage across its outer membrane, and in almost every case that voltage is negative on the inside relative to the outside. In a typical neuron at rest, this sits around −60 to −80 millivolts. The negativity comes from an unequal distribution of charged particles (ions) across the membrane, combined with the membrane’s preference for letting some ions pass more easily than others. Potassium is the central character in this story, but it takes a supporting cast of pumps, leaked sodium, trapped proteins, and a significant slice of your energy budget to keep the inside of every cell quietly negative.

Potassium Leak Sets the Baseline

The single biggest reason the resting membrane potential is negative is that cell membranes are far more permeable to potassium than to anything else. Potassium ions are concentrated inside the cell (roughly 140 millimoles per liter inside versus about 5 outside), and at rest, potassium channels sit open, allowing potassium to drift outward down its concentration gradient. Each potassium ion that leaves carries a positive charge with it, making the inside progressively more negative. That growing negativity eventually pulls potassium back in, and an equilibrium develops where the electrical pull inward balances the concentration-driven push outward. The voltage at which that balance occurs for potassium alone is around −90 mV.

The actual resting potential of most cells is a bit less negative than −90 mV because the membrane is not perfectly selective. A small amount of sodium leaks inward, nudging the voltage in a positive direction. Measurements in cultured parasympathetic neurons found that sodium permeability was about 12 percent of potassium permeability, enough to shift the resting potential away from the pure potassium equilibrium but not enough to dominate it.¹ Chloride also plays a role, particularly in skeletal muscle, where its permeability is high enough to matter. In mammalian muscle fibers, experimental work showed that chloride distribution can drift out of equilibrium with the resting potential, meaning chloride is not simply a passive bystander but actively influences where the voltage settles.²

The Sodium-Potassium Pump Keeps the Gradient Alive

If potassium kept leaking out and sodium kept leaking in, the concentration differences would eventually collapse, and the resting potential would vanish. The sodium-potassium pump prevents that. This protein, embedded in the membrane of virtually every animal cell, uses one molecule of ATP to push three sodium ions out of the cell while pulling two potassium ions back in.³ Because it moves three positive charges out for every two it brings in, the pump itself generates a small net outward current, contributing directly to the negativity inside. That direct electrical contribution is modest, though. The pump’s bigger job is maintaining the steep potassium and sodium gradients that the leak channels then convert into voltage.

The fuel cost is not trivial. In the brain’s grey matter, the ATP consumed just to reverse resting ion leaks and keep the membrane potential stable accounts for roughly 15 percent of total energy use. In white matter, where action potentials are less frequent but axons are long, that figure rises to about 44 percent.⁴ More than 90 percent of the ATP powering the pump comes from oxidative metabolism in mitochondria rather than from anaerobic pathways.⁵ This helps explain why the brain is so sensitive to oxygen deprivation: cut off the oxygen, and the pumps fail within seconds.

Trapped Proteins and the Donnan Effect

There is a separate physical reason that contributes a small negative bias inside cells: the cytoplasm is full of large, negatively charged molecules, mostly proteins and nucleic acids, that are too big to cross the membrane. These trapped anions attract cations inward and repel smaller anions outward, creating what physicists call a Donnan equilibrium. On its own, this effect would generate a transmembrane voltage. However, the resting potential of nerve and muscle cells is not a Donnan potential. The active pumping and selective permeability described above dominate, and the Donnan contribution is secondary.⁶ Still, those trapped proteins matter for cell volume: without the pump counteracting the Donnan-driven water entry, cells would swell and burst.

Why the Exact Number Varies Between Cell Types

Not every cell rests at −70 mV. Neurons can sit anywhere from about −60 to −80 mV, while some non-excitable cells hover around −40 mV.⁷ The exact voltage depends on which channels are open at rest and how many pumps are running. A cell that expresses more resting potassium channels will sit closer to the potassium equilibrium and be more negative. A cell that allows more sodium leak, or that has fewer pumps, will be less negative.

Chloride concentration also shifts the number. In mature neurons bathed in conditions mimicking the fluid around the brain, internal chloride concentration is typically in the range of 6 to 13 millimoles per liter, kept low by chloride transporters.⁷ During early development, immature neurons have higher internal chloride, which makes them less negative at rest and changes how they respond to the neurotransmitter GABA. This is one reason the developing brain behaves differently from the adult brain.

How Scientists First Measured It

The resting potential was not directly measured until researchers developed microelectrodes thin enough to pierce a cell without destroying it. The squid giant axon, sometimes close to a millimeter in diameter, was the ideal test subject. In vivo recordings from intact squid estimated the absolute resting potential at about 77 mV (inside negative), which was close to the voltage predicted by the potassium concentration ratio between squid blood and axoplasm.⁸ When axons were isolated and handled in the lab, the measured potential was typically a bit lower, suggesting that even gentle dissection introduced a small leakage conductance that let extra ions through.

In small mammalian cells, getting an accurate reading is harder. A study comparing sharp-microelectrode recordings with whole-cell patch clamp measurements in human monocytes found that the microelectrode reading was, on average, 6 mV less negative than the true resting potential, because puncturing the membrane with a glass tip introduces a small leak.⁹ For tiny cells, the patch clamp method gives a more faithful reading.

Temperature Changes the Voltage

If you warm a cell, the resting potential tends to become more negative (hyperpolarize). Part of this is straightforward thermodynamics: the voltage predicted by ion concentration ratios changes with temperature. But the measured shift is often larger than that prediction alone would suggest. In barnacle muscle fibers, the resting potential changed by 1 to 2 millivolts per degree Celsius, roughly five to ten times more than the theoretical minimum.¹⁰ The extra shift comes from temperature-dependent changes in how permeable the membrane is to different ions. Warming tends to increase potassium permeability relative to sodium, pulling the voltage closer to the potassium equilibrium and making it more negative.

A related effect occurs through the pump itself. When temperature rises, sodium leaks into the cell faster, and the pump speeds up to compensate. Since the pump moves net positive charge outward, this adds an extra hyperpolarizing push. In optic nerve fibers, the pump’s direct contribution to resting potential at room temperature was estimated at about −4 mV, and warming amplified this because the pump rate has a strong temperature dependence.¹¹ Fever, hypothermia, and deliberate cooling during surgery all alter membrane potentials in clinically meaningful ways, which is one reason cardiac surgeons cool the heart during certain procedures.

What Happens When the Resting Potential Goes Wrong

The clinical consequences of a disrupted resting potential are serious and fast. The most common everyday example is hyperkalemia, an excess of potassium in the blood. When extracellular potassium rises, the concentration gradient that drives potassium out of cells becomes less steep. The resting potential moves toward zero (depolarizes), and cardiac cells are especially vulnerable. Mild elevations cause subtle changes on an electrocardiogram; severe hyperkalemia can trigger deadly arrhythmias.¹²

Magnesium offers a partial defense. In both human and canine heart cells, elevated magnesium reduced the depolarization caused by high potassium by more than half. Ventricular cells depolarized by only about 9 mV in high-magnesium conditions compared with 21 mV at normal magnesium levels, and the drop in conduction velocity that usually accompanies hyperkalemia did not occur at all when magnesium was high.¹³ This is why clinicians sometimes administer magnesium alongside other treatments for dangerous hyperkalemia.

Stroke provides an even more dramatic illustration. Within about two minutes of blood flow stopping in a brain region, the sodium-potassium pumps fail and neurons undergo a sudden, massive depolarization called anoxic depolarization. The membrane essentially loses its ability to keep ions separated, and the resulting energy crisis is what kills neurons in the ischemic core.¹⁴ Blocking that depolarization in lab experiments has protected brain slices from damage, which is why it remains a target of stroke research.

Genetic Conditions That Affect the Resting Potential

Some diseases trace directly to mutations in the channels responsible for maintaining the resting potential. Inwardly rectifying potassium channels, a family of channels that help hold cells near their resting voltage, are linked to at least two major human disorders. Persistent hyperinsulinemic hypoglycemia of infancy involves pancreatic beta cells that cannot properly regulate their membrane potential, leading to uncontrolled insulin release. Bartter’s syndrome, characterized by potassium wasting, alkalosis, and elevated hormone levels, is also connected to mutations in these channels or their associated proteins.¹⁵ In mice, mutations in the same channel family cause the weaver phenotype, a neurological disorder affecting movement and brain development.

Astrocytes Clean Up After Neurons

When neurons fire, they dump potassium into the narrow spaces between cells. If that extracellular potassium builds up, it depolarizes neighboring neurons, potentially causing runaway excitation. Astrocytes, the star-shaped glial cells that outnumber neurons in many brain regions, handle the cleanup. Their fine processes wrap around synapses and express pumps and channels dedicated to soaking up excess potassium. Modeling work has shown that astrocytic sodium-potassium pumps and another transporter called NKCC drive potassium uptake so aggressively that they can overshoot, temporarily pulling extracellular potassium below its baseline and causing a brief “potassium undershoot.” Inwardly rectifying potassium channels on the astrocyte then help balance this excess uptake.¹⁶ This buffering system keeps the resting potential of surrounding neurons stable even during intense activity.

Plants and Fungi Use a Different Engine

Animal cells rely on the sodium-potassium pump, but plants and fungi took a different evolutionary path. Their plasma membranes use proton pumps instead, actively pushing hydrogen ions (protons) out of the cell. This creates a proton gradient that serves the same basic function: it establishes a negative interior voltage and provides the driving force for nutrient transport across the membrane.¹⁷ The sodium-potassium pump appears to be a distinctly animal invention, tied to the animal cell’s particular challenges of volume regulation and fast electrical signaling.

Evolutionary analysis suggests that the earliest cells faced a survival problem: ions and water constantly entered down osmotic gradients, threatening to burst the cell. A proton extrusion system likely evolved first to handle this, and only later did animal lineages develop the sodium-potassium ATPase as a specialized tool for managing a sodium-based economy that powers nutrient uptake, volume control, and action potentials.¹⁸ The negative resting potential, in other words, is not an accident of chemistry. It is an ancient solution to the problem of keeping a bag of salty water from destroying itself.

Membrane Voltage as a Signal for Cell Fate

The resting potential is not just housekeeping. Increasingly, researchers have found that cells use their voltage as a developmental signal. Human mesenchymal stem cells become more negative as they differentiate into bone or fat cells, and deliberately preventing that hyperpolarization blocks differentiation. Conversely, artificially hyperpolarizing stem cells with drugs pushed them toward bone-forming pathways.¹⁹

The connection extends to the very beginning of an organism’s life. In frog embryos, a potassium channel called kcnh6 polarizes cells during early development, and this polarization appears necessary for cells to exit their pluripotent state and commit to becoming specific tissue types. When kcnh6 was depleted, cells retained the ability to become any tissue far longer than they should have, suggesting that the shift toward a more negative resting potential is one of the triggers that tells a cell to stop being a blank slate and start specializing.²⁰ Cancer cells, which often revert to a less differentiated state, tend to be more depolarized than their healthy counterparts, and some researchers suspect that restoring normal membrane voltage could be a future therapeutic strategy.