Sodium and potassium channels generate nerve impulses by passing ions across the cell membrane in a tightly choreographed sequence. At rest, the membrane is negatively charged inside. When a stimulus arrives, sodium channels snap open and let positive sodium ions rush in, rapidly flipping the voltage positive. Milliseconds later, potassium channels open and let positive potassium ions flow out, dragging the voltage back down. That brief voltage spike, repeated along the length of a nerve fiber, is the action potential, and it is the fundamental unit of electrical signaling in the nervous system.
Setting the Stage With the Resting Potential
Before any impulse can fire, the nerve cell has to be electrically primed. That priming comes from an uneven distribution of ions. The inside of the cell sits at roughly −70 millivolts relative to the outside, a state called the resting membrane potential. Two things maintain it. First, a protein called the sodium-potassium pump continuously moves three sodium ions out of the cell for every two potassium ions it brings in, spending one molecule of ATP per cycle. Because it exports more positive charge than it imports, the pump itself makes the interior slightly more negative.
1PubMed Central. Na+/K+-pump and neurotransmitter membrane receptorsSecond, even at rest, certain potassium channels are open. These “leak” channels allow potassium ions to drift out of the cell down their concentration gradient. Because potassium carries a positive charge, every ion that leaks out leaves the inside a bit more negative. Together, the pump and the leak channels establish and hold the resting voltage, keeping the neuron ready to fire.
2PubMed. Gating the pore of potassium leak channelsHow Sodium Channels Fire the Impulse
Voltage-gated sodium channels are the triggers. Each channel has a voltage-sensing region built from a helical segment called S4, studded with positively charged amino acids. At resting voltage, those charges are pulled inward by the negative interior. When the membrane starts to depolarize, the electric field changes and the S4 helix slides outward by roughly 6 to 8 angstroms, rotating and tilting as it moves. That mechanical shift pulls on a linker that connects the sensor to the channel’s central pore, physically prying the gate open.
3PubMed Central. Structural basis for gating charge movement in the voltage sensor of a sodium channelOnce open, sodium ions flood inward. Because sodium is far more concentrated outside the cell, and because the interior is negative, the driving force is enormous. The membrane voltage rockets from −70 millivolts toward +30 or +40 millivolts in less than a millisecond. This is the rising phase of the action potential. Recent structural work supports the “sliding helix” model of this process, showing how the sensor movement is directly and rigidly coupled to the opening of the pore.
4PubMed Central. Resting-State Structure and Gating Mechanism of a Voltage-Gated Sodium ChannelThe Hinged Lid That Shuts Sodium Channels Down
If sodium channels simply stayed open, the nerve would depolarize and stay depolarized. Instead, within about a millisecond of opening, each sodium channel inactivates itself. A short loop of protein on the inner face of the channel swings over like a hinged lid and plugs the pore from the inside. This process is called fast inactivation, and it is built into the channel protein itself: no external signal is needed.
The critical part of that lid is a cluster of three amino acids, isoleucine, phenylalanine, and methionine, that act as a hydrophobic latch. Mutating any of these residues disrupts inactivation, but phenylalanine at position 1489 is the most important. Removing it alone nearly abolishes fast inactivation entirely.
5PubMed. A cluster of hydrophobic amino acid residues required for fast Na(+)-channel inactivation Structural studies have confirmed that these three residues form the hydrophobic latch that holds the lid in place once the channel has been plugged.6PubMed. Solution structure of the sodium channel inactivation gate
Fast inactivation is what makes the action potential a brief spike rather than a sustained plateau. It also sets up the refractory period, which we will get to shortly.
Potassium Channels Restore the Resting Voltage
While sodium channels are snapping shut, voltage-gated potassium channels are just getting going. They respond to the same depolarization that opened the sodium channels, but they open more slowly. By the time they are fully conducting, the sodium channels have already inactivated. Potassium ions then pour out of the cell, carrying positive charge with them and dragging the membrane voltage back toward its resting level. This is repolarization.
Potassium channels often overshoot slightly, pulling the voltage a few millivolts more negative than the resting potential before closing. This dip, called the undershoot or afterhyperpolarization, is a normal part of the action potential waveform. In heart muscle cells, specific types of delayed-rectifier potassium channels contribute directly to shaping the repolarization of each beat.
7PubMed Central. Characterization of an ultrarapid delayed rectifier potassium channel involved in canine atrial repolarizationHow Each Channel Tells Its Ion Apart
Sodium and potassium ions are both small, positively charged metal atoms. Potassium is slightly larger. Yet each channel is remarkably selective, letting one through while blocking the other. The mechanisms are different for the two families.
Potassium channels achieve selectivity through the number of oxygen atoms lining the pore. As a potassium ion enters the selectivity filter, it sheds its water coat and is cradled by a specific number of carbonyl oxygens from the protein backbone. That number of oxygens happens to fit potassium perfectly. Sodium ions, being smaller, would sit loosely in the same arrangement, making the interaction energetically unfavorable. Computational studies have shown that this coordination number, not the physical diameter of the pore, is the dominant factor in keeping sodium out.
8PubMed Central. The predominant role of coordination number in potassium channel selectivitySodium channels use a different trick. In bacterial sodium channels, four glutamate side chains form a high-field site at the outer mouth of the pore. This site holds three hydrated sodium ions in single file. Because three positively charged ions packed closely together repel each other strongly, that electrostatic repulsion is enough to overcome the energy penalty of moving ions through the narrow filter and drive rapid conduction.
9PubMed Central. Three in a row-how sodium ions cross the channelThe Refractory Period and One-Way Travel
After an action potential passes a given spot on a nerve fiber, the sodium channels at that spot are inactivated. They cannot reopen until the membrane has repolarized and they have had time to reset, a process called recovery from inactivation. During this absolute refractory period, no stimulus can trigger another spike at that location. The practical result is that the action potential can only move forward along the axon, because the membrane behind it is temporarily unable to fire again.
Following the absolute refractory period there is a relative refractory period, during which a stronger-than-normal stimulus can fire a spike, but the threshold is raised. Modeling studies of mammalian nerve fibers show that the recovery cycle depends on afterpotentials generated by both persistent sodium channel activity and slow potassium channel activation.
10PubMed. Modeling the excitability of mammalian nerve fibers: influence of afterpotentials on the recovery cycleWhen refractory periods are disrupted, things go wrong. Computer simulations of demyelinated axons show that the absolute refractory period lengthens as myelin is lost, because the action potential at each node repolarizes more slowly, delaying the recovery of inactivated sodium channels.
11PubMed. Action potential refractory period in axonal demyelination: a computer simulationWhere the Impulse Begins
Not all parts of a neuron are equally excitable. The action potential typically starts at a specialized region near the cell body called the axon initial segment. This zone has a much higher density of sodium channels than the rest of the cell membrane, which means it reaches threshold more easily than any other part of the neuron. Early computational models predicted this would be necessary, and binding experiments in cultured neurons later confirmed that sodium channels are indeed concentrated there.
12Neuron. Axon Initial Segment: The Master Zone of Neuronal ExcitabilitySpeeding Things Up With Myelin
In many axons, glial cells wrap layers of fatty membrane, myelin, around the fiber. The myelin acts as an insulator, preventing ion flow across the membrane everywhere it covers. Sodium channels are clustered at the tiny gaps between myelin segments, called nodes of Ranvier.
13PubMed Central. Mechanisms of sodium channel clustering and its influence on axonal impulse conductionBecause current can only cross the membrane at these nodes, the action potential effectively jumps from one node to the next, a process called saltatory conduction. This is much faster than continuous conduction along an unmyelinated fiber of the same diameter. Even in unmyelinated axons, though, simulations have revealed a related phenomenon: random spontaneous openings of sodium channels ahead of the advancing spike can pre-depolarize a stretch of membrane, causing the wavefront to leap forward by several hundred micrometers. Researchers have called this “stochastic microsaltatory conduction,” and it resembles true saltatory conduction but arises from the random behavior of individual channels rather than from specialized anatomy.
14PLoS Computational Biology. Stochastic Simulations on the Reliability of Action Potential Propagation in Thin AxonsToxins and Drugs That Hijack the Channels
Because sodium and potassium channels are so central to nerve signaling, they are prime targets for both natural toxins and pharmaceutical drugs. Tetrodotoxin, the poison found in pufferfish, blocks sodium channels by binding to the selectivity filter from the outside of the membrane. It physically prevents sodium ions from passing through, but it does not interfere with the channel’s gating machinery. The channel still tries to open and close on schedule; it just cannot conduct.
15PubMed Central. Tetrodotoxin: a brief history Structural studies show that the toxin molecule wedges into the outer mouth of the pore and forms a network of hydrogen bonds that seal it shut.16PubMed. Mechanism of tetrodotoxin block and resistance in sodium channels
Local anesthetics like lidocaine take a different approach. Rather than plugging the pore from outside, they enter the channel from the intracellular side and bind preferentially when the channel is in its open or inactivated state. The result is use-dependent block: the more frequently a nerve fires, the more drug molecules get trapped inside channels, and the greater the suppression of the sodium current.
17Biophysical Journal. Guarded Receptor Formulation of Local Anesthetic-Sodium Channel Interaction This is why a dentist’s injection numbs the area: the drug slows the recovery of sodium channels from inactivation, prolonging the time between one action potential and the next until the nerve effectively stops firing.18Molecular Pharmacology. Mechanism of Modification, by Lidocaine, of Fast and Slow Recovery from Inactivation of Voltage-Gated Na+ Channels
When Channel Mutations Cause Disease
Mutations in the genes encoding voltage-gated sodium channels can throw neural signaling into disarray. In epilepsy, for example, genetic variants in several sodium channel subtypes alter the kinetics of activation, inactivation, or recovery from inactivation. A channel that inactivates too slowly lets too much sodium in, pushing the neuron past threshold repeatedly. A channel that recovers too quickly fires again before it should. Either way, the result is abnormal, synchronized bursts of electrical activity that manifest as seizures.
19PubMed Central. Epilepsy-Related Voltage-Gated Sodium Channelopathies: A ReviewPotassium channel mutations cause their own family of disorders. Because potassium channels are responsible for repolarization, defects in these channels can prolong the action potential in heart muscle cells, leading to dangerous cardiac arrhythmias. The logic is the same as in neurons: the balance between sodium entry and potassium exit determines the shape and timing of every electrical signal, and disturbing either side of that balance has consequences.
How the Nervous System Tunes Its Own Channels
Ion channels are not fixed components. The nervous system adjusts their behavior constantly through chemical modifications, the most common being phosphorylation, where enzymes attach phosphate groups to specific sites on the channel protein. This can change how much current flows through, how fast the channel opens and closes, and at what voltage it responds.
In voltage-gated potassium channels of the Kv1 family, for instance, phosphorylation by the enzyme Src can suppress current amplitude by more than 95 percent and alter inactivation kinetics. Different tyrosine residues on the same channel protein control different aspects of behavior, meaning the cell can independently tune the size of the current and the speed of inactivation.
20PubMed. Tyrosine phosphorylation modulates current amplitude and kinetics of a neuronal voltage-gated potassium channel Another enzyme, AMP-activated protein kinase, phosphorylates the Kv2.1 channel and shifts its activation voltage in the hyperpolarizing direction, effectively making the channel easier to open and the cell more excitable.21PubMed Central. Phosphorylation of the voltage-gated potassium channel Kv2.1 by AMP-activated protein kinase regulates membrane excitability
This kind of modulation is how neurons adapt to changing demands. A cell that has been firing a lot can dial down its sodium or potassium conductances to become less excitable, while a cell that needs to respond more sensitively can do the opposite. The channels are not simply on-off switches; they are tunable instruments whose properties shift in real time.
The Energy Bill for Every Impulse
Every action potential runs up a small metabolic tab. The sodium that rushed in during the spike and the potassium that flowed out during repolarization both need to be moved back to their original sides of the membrane. That job falls to the sodium-potassium pump, which spends ATP to do it. In computational models of thalamocortical relay neurons, researchers have estimated the ATP cost of each action potential by calculating how much sodium enters during the spike and how much pump activity is needed to reverse it.
22PubMed. Energy Cost of Action Potential Generation and Propagation in Thalamocortical Relay Neurons During Deep Brain StimulationThe brain as a whole consumes a disproportionate share of the body’s energy, and a large fraction of that goes to maintaining ion gradients. One measure of how efficiently a neuron uses its fuel is the overlap between sodium and potassium currents during the spike. If both channel types are open at the same time, sodium flows in while potassium flows out, and the two currents partially cancel each other, wasting energy. Evolution has tuned the timing of channel opening and inactivation to minimize this overlap, but it can never be eliminated entirely.
Sodium Channels Before Nervous Systems Existed
Voltage-gated sodium channels are not an invention of complex brains. Genomic searches have found sodium-channel homologs in choanoflagellates, single-celled organisms that are the closest living relatives of animals. These ancestral channels have selectivity filters that look intermediate between calcium and sodium channels, suggesting that sodium selectivity evolved gradually from calcium-permeable ancestors.
23PubMed Central. Evolution of sodium channels predates the origin of nervous systems in animalsEven more striking, homologous channels exist in placozoans, tiny flat animals that lack any nervous system at all. The presence of sodium-channel genes in organisms without neurons implies that these channels originally served other functions, possibly related to calcium signaling or cellular sensing, and were later co-opted for electrical signaling as nervous systems evolved. Bacteria also have their own voltage-gated sodium channels, which have become valuable tools for structural biologists trying to understand how the channel pore works, since bacterial channels are simpler and easier to crystallize than their animal counterparts.
24PubMed. Voltage-Gated Sodium Channels: Evolutionary History and Distinctive Sequence FeaturesHow Scientists Watch Single Channels at Work
Much of what we know about sodium and potassium channels comes from a technique called patch clamping, developed in the late 1970s and early 1980s. The method involves pressing a very fine glass pipette against a cell membrane to form a tight seal, then recording the tiny electrical currents that flow through one or a few channels in that patch. The currents are on the order of a single picoampere, a trillionth of an amp, but the seal is tight enough to resolve them clearly in real time.
25Annual Reviews. Patch clamp techniques for studying ionic channels in excitable membranesPatch clamping made it possible to observe channels flickering open and shut individually, revealing that the smooth macroscopic currents recorded from whole nerves are actually the summed activity of thousands of channels each making discrete, probabilistic transitions. That insight transformed the field. It showed that nerve impulses are not analog waves flowing through a continuous medium but statistical events built from the behavior of individual protein molecules, each obeying its own voltage-dependent probability of being open or closed. The Hodgkin-Huxley equations, the foundational mathematical model of the action potential, describe these averaged behaviors with four differential equations whose parameters come directly from experimental measurements of channel conductance.
26PubMed Central. Effects of maximal sodium and potassium conductance on the stability of Hodgkin-Huxley model