When a neuron is at rest, it carries a small electrical charge of about -70 millivolts (mV) across its membrane, with the inside negative relative to the outside. This baseline voltage, called the resting membrane potential, typically falls between -70 and -80 mV depending on the neuron type. Far from being inactive, a resting neuron is actively maintaining this charge difference, spending significant energy to stay ready for its next signal.
What “At Rest” Actually Means
A neuron is considered “at rest” when it is not currently sending or receiving a signal. It’s not firing an electrical impulse, and no incoming message is pushing it toward or away from firing. But “rest” is misleading. The neuron is doing constant behind-the-scenes work to maintain the precise electrical imbalance across its membrane. Think of it like a coiled spring: stable, but loaded with potential energy that can be released in milliseconds.
That stored energy comes from the uneven distribution of charged particles (ions) on either side of the neuron’s outer membrane. The specific arrangement of these ions is what creates the -70 mV charge, and the neuron burns a surprising amount of fuel to keep it that way. In the brain’s gray matter, maintaining the resting potential accounts for roughly 15% of total energy use. In white matter, that figure climbs to about 44%.
The Ion Setup That Creates the Charge
Two ions do most of the heavy lifting: sodium and potassium. At rest, their concentrations are almost mirror images of each other across the membrane. Potassium is heavily concentrated inside the cell, around 140 millimoles (mM), while only about 5 mM sits outside. Sodium is the opposite: roughly 145 mM outside and only 5 to 15 mM inside.
This lopsided arrangement matters because ions carry electrical charge. Potassium and sodium are both positively charged. With far more potassium packed inside and far more sodium packed outside, the stage is set for both ions to want to move down their concentration gradients, like water wanting to flow downhill. But the membrane controls which ions can actually cross, and that selective barrier is what produces the resting voltage.
Large negatively charged molecules also sit inside the cell, including proteins and organic phosphates. These molecules are too big to cross the membrane, so they’re permanently trapped inside. Their negative charge contributes to the overall negative interior.
Why Potassium Sets the Voltage
The neuron’s membrane at rest is far more permeable to potassium than to sodium. Small channels in the membrane, called leak channels, stay open all the time and allow potassium to drift out of the cell down its concentration gradient. Sodium has its own leak channels, but far fewer are open at rest.
Because potassium flows out much more freely, it dominates the resting voltage. If only potassium mattered, the membrane potential would settle at about -90 mV, which is potassium’s equilibrium potential (the voltage at which its concentration gradient pulling it out is perfectly balanced by the electrical gradient pulling it back in). But a small, steady trickle of sodium leaking into the cell nudges the voltage slightly more positive, landing it around -70 mV. Sodium’s equilibrium potential is about +65 mV, so even a small sodium leak has a noticeable effect.
This is why the resting potential sits much closer to potassium’s equilibrium (-90 mV) than sodium’s (+65 mV). The membrane’s favoritism toward potassium at rest is the single biggest factor determining the baseline voltage.
How the Neuron Maintains This Balance
Left alone, the steady leak of potassium out and sodium in would gradually erase the concentration differences. Potassium would slowly drain from the cell, sodium would slowly build up inside, and the resting potential would collapse. To prevent this, the neuron runs a molecular pump embedded in the membrane that actively reverses the leaks.
This pump uses one molecule of ATP (the cell’s energy currency) to push 3 sodium ions out of the cell while pulling 2 potassium ions back in. The 3-for-2 exchange does two things. First, it restores the concentration gradients by moving each ion back to the side where it’s already concentrated. Second, because it exports more positive charges than it imports, the pump itself adds a small extra negative tilt to the interior. This direct contribution to the voltage is modest compared to the effect of the concentration gradients the pump maintains, but it’s not negligible.
The pump runs continuously for as long as the neuron is alive. It’s the reason maintaining the resting potential is so energy-expensive.
Why the Resting Potential Matters
The resting potential is essentially the neuron’s ready state. Without it, a neuron cannot fire. When the neuron receives a signal, sodium channels snap open and sodium floods inward, rapidly driving the voltage from -70 mV toward positive values. This sudden reversal is the action potential, the electrical impulse that carries information through the nervous system. But that reversal is only possible because the resting state established the negative starting point and the steep sodium gradient waiting to be unleashed.
The specific value of the resting potential also determines how excitable a neuron is. If something shifts the resting voltage closer to the firing threshold (typically around -55 mV), the neuron becomes easier to trigger. If the resting potential becomes more negative than usual, the neuron needs a stronger signal to fire. Potassium levels in the fluid surrounding neurons are a key variable here. Because the resting voltage depends so heavily on the potassium gradient, even small changes in extracellular potassium concentration can shift the resting potential and alter how readily neurons fire. This is one reason why abnormal blood potassium levels can cause muscle weakness, cramping, or dangerous heart rhythm changes, since the same electrical principles apply to muscle cells.
Resting vs. Dead: The Difference Is Active Work
A dead cell has no membrane potential. The ions equalize across the membrane, and the voltage drops to zero. A resting neuron, by contrast, is spending energy every second to keep its ions separated and its voltage stable. The resting state is not passive. It’s an active, energy-consuming process that keeps the neuron poised to respond the instant a signal arrives.