Depolarization is the rapid shift in a neuron’s electrical charge that makes the inside of the cell temporarily less negative than it is at rest. Neurons maintain a baseline voltage of roughly −70 millivolts, kept in place by the steady leak of potassium ions outward through the membrane. When a stimulus arrives and sodium ions rush inward through specialized channels, that negative charge climbs toward zero and beyond, generating the electrical signals the nervous system uses for everything from reflexes to abstract thought. The process is fast, tightly regulated, and fundamental to how your brain and body communicate.
Why Neurons Are Electrically Charged in the First Place
Before depolarization makes sense, you need to know that a neuron at rest is not electrically neutral. The inside of the cell sits at about −70 millivolts relative to the outside, a difference called the resting membrane potential. That voltage exists because the cell membrane is studded with channels that passively leak potassium ions outward. These “leak” channels, particularly a family called two-pore domain potassium channels, keep the interior slightly negative by letting positively charged potassium drift out along its concentration gradient.1PubMed. Role of leak potassium channels in pain signaling At the same time, a pump called the sodium-potassium ATPase actively pushes three sodium ions out for every two potassium ions it pulls in, adding a small extra nudge of negativity. The result is a cell that sits quietly at a stable negative voltage, loaded with potential energy like a compressed spring, waiting for the right trigger to release it.
What Happens During Depolarization
When a stimulus reaches a neuron, whether from another nerve cell, a sensory receptor, or an electrical probe in a lab, it can cause the membrane voltage to rise. If it rises enough, voltage-gated sodium channels in the membrane snap open. These channels are selective: they allow sodium ions, which are concentrated outside the cell, to flood inward. That inward rush of positive charge is the depolarizing current.2PubMed Central. Voltage-gated sodium channel-associated proteins and alternative mechanisms of inactivation and block The channels open very quickly, and the sodium current they produce is responsible for the sharp upward spike of the action potential in both nerve and muscle fibers.3Journal of Medicinal Chemistry. Voltage-Gated Sodium Channels: Structure, Function, Pharmacology, and Clinical Indications
The voltage swings from around −70 millivolts up past zero, often reaching about +30 to +40 millivolts at its peak. That entire excursion takes roughly a millisecond. Once the sodium channels have been open for a fraction of that time, they automatically inactivate, a built-in shutoff that closes the channel even while the membrane is still depolarized. Potassium channels, which are slower to open, then take over, letting potassium flow outward and dragging the voltage back down. This falling phase is called repolarization, and it returns the neuron to its resting state, sometimes briefly overshooting into slightly more negative territory before settling back to baseline.
The Threshold and Where It Happens
Not every small depolarization produces a full action potential. The membrane has to reach a critical voltage, typically around −55 millivolts, before enough sodium channels open to create a self-reinforcing cascade. Below that threshold, the depolarization fizzles out. Above it, the process becomes all-or-nothing: the spike fires at full strength regardless of how far above threshold the stimulus was.
The place on the neuron where action potentials usually begin is a specialized region called the axon initial segment, located where the cell body transitions into the long cable-like axon. This spot has an unusually high density of ion channels, making it the most excitable part of the cell.4PubMed Central. Axon initial segments: structure, function, and disease The sodium channels found there are tuned to open at slightly lower voltages than those on the cell body, and they activate and inactivate faster, which helps the axon initial segment fire before anywhere else on the cell.5Neuron. The Axon Initial Segment and the Properties of the Axon Initial Segment Think of it as a hair trigger attached to the beginning of the axon, ready to convert incoming signals into outgoing spikes.
Graded Potentials and How They Add Up
The signals that arrive at a neuron’s dendrites and cell body are not all-or-nothing. They are graded potentials: small, variable changes in voltage that fade with distance. A single excitatory input from another neuron might only nudge the membrane a few millivolts toward threshold, nowhere near enough on its own. But if multiple inputs arrive in quick succession or from many synapses at once, their depolarizations can add together, a process called summation.
Temporal summation, where inputs arrive rapidly one after another at the same synapse, is especially sensitive to the membrane’s electrical properties. Certain potassium channels that are active at rest can dampen the buildup. When those channels are reduced, incoming excitatory signals stack up more effectively and push the membrane closer to threshold.6PubMed. M1 muscarinic receptor modulation of Kir2 channels enhances temporal summation of excitatory synaptic potentials in prefrontal cortex pyramidal neurons This tuning mechanism matters because it means the neuron is not just passively collecting inputs; it actively adjusts how sensitive it is to patterns of incoming activity.
Graded potentials also weaken as they travel along the dendrites toward the cell body. A synapse far out on a thin dendrite produces a local depolarization that shrinks considerably by the time it reaches the soma, because the signal leaks across the membrane along the way.7PubMed Central. The attenuation of passively propagating dendritic potentials in a motoneurone cable model Synapses closer to the axon initial segment therefore tend to have more influence on whether the neuron fires, simply because their signals lose less voltage in transit.
How Depolarization Travels Along an Axon
Once an action potential ignites at the axon initial segment, it needs to travel the full length of the axon, which can be a meter or more in motor neurons running from the spinal cord to the foot. In unmyelinated axons, the depolarization spreads continuously: each patch of membrane depolarizes the next, triggering new sodium channel openings in a wave that moves steadily along. This works, but it is relatively slow.
In myelinated axons, the process is dramatically faster. The myelin sheath, formed by glial cells that wrap around the axon in segments, acts as electrical insulation. Between the myelin segments are tiny gaps called nodes of Ranvier, where sodium channels are concentrated. The depolarization at one node generates enough current to jump across the insulated segment and depolarize the next node. This “jumping” pattern is called saltatory conduction. Recent work using high-speed recordings of myelinated axons has shown that the mechanism involves a conducting nanoscale space between the axon and the myelin, with attenuating waves propagating beneath the sheath between nodes and fresh, full-strength action potentials regenerating at each node.8PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit
Several physical features affect how fast the signal moves. Axon diameter matters: wider axons conduct faster because the internal resistance to current flow is lower. In myelinated axons, conduction speed increases in a straightforward linear relationship with diameter.9Nature Communications. Importin 13-dependent axon diameter growth regulates conduction speeds along myelinated CNS axons Myelin thickness and the distance between nodes also scale with diameter, and both contribute to speed. Additionally, the density and type of sodium channels at the nodes influence how quickly each node fires, adding another layer of regulation.10PubMed Central. Regulation of Conduction Time along Axons
The Refractory Period
After a neuron fires, it cannot immediately fire again. For a brief window, the sodium channels that just opened are inactivated, physically stuck in a closed state that no amount of depolarization can reopen. This absolute refractory period lasts about a millisecond and puts a hard cap on how fast a neuron can fire, typically a few hundred spikes per second at most. Following that is a relative refractory period, during which firing is possible but requires a stronger-than-usual stimulus because the membrane is temporarily hyperpolarized and some sodium channels have not yet recovered.
Whether sodium channel inactivation is complete or incomplete during the falling phase of the spike influences the length and character of the refractory period.11Neuron. Sodium Influx during Action Potentials of Mammalian Central Neurons: Incomplete Inactivation in Fast-Spiking Neurons In fast-spiking neurons, like certain inhibitory interneurons that can fire at very high rates, inactivation tends to be incomplete, meaning some channels recover and are ready to reopen almost immediately. In slower-firing neurons, inactivation is more thorough and recovery takes longer. The refractory period also ensures that the action potential moves in one direction along the axon: the region just behind the spike is temporarily unable to fire, so the signal cannot loop back on itself.
What Happens When Depolarization Reaches a Synapse
When the action potential arrives at the axon terminal, depolarization does not jump directly to the next neuron. Instead, it triggers a different set of voltage-gated channels: calcium channels. These open in response to the depolarized membrane and allow calcium ions to flood into the terminal.12PubMed Central. Functions of Presynaptic Voltage-gated Calcium Channels The calcium influx causes tiny vesicles loaded with neurotransmitter molecules to fuse with the membrane and release their contents into the synaptic gap. The neurotransmitter then drifts across to the next cell and binds to receptors on its surface.
On the receiving side, the type of receptor determines what happens next. At excitatory synapses that use glutamate, the neurotransmitter opens AMPA receptors, which let sodium in and produce a fast, small depolarization in the postsynaptic cell. That local depolarization can then activate a second type of receptor, the NMDA receptor, which is both voltage-sensitive and neurotransmitter-gated. NMDA receptors are normally blocked by a magnesium ion sitting in their pore; the depolarization from AMPA current pushes the magnesium out and lets calcium and sodium through.13PubMed Central. A model of cooperative effect of AMPA and NMDA receptors in glutamatergic synapses This two-step process is important for learning and memory, because the NMDA receptor serves as a coincidence detector: it opens fully only when the sending neuron fires and the receiving neuron is already partially depolarized.
Depolarization Block
More depolarization is not always better. When a neuron is held at a depolarized voltage for too long or pushed too strongly, it can stop firing altogether, a state called depolarization block. Under sustained, increasing input current, neurons eventually cease spiking because enough sodium channels enter a slow inactivated state that the cell cannot generate new action potentials.14PubMed. On the mechanisms underlying the depolarization block in the spiking dynamics of CA1 pyramidal neurons
This phenomenon has been studied closely in dopamine neurons of the midbrain. In living animals, these neurons fire quick high-frequency bursts that carry signals about reward and motivation. But in brain slices in the lab, the same neurons cannot sustain firing above roughly 10 spikes per second before entering depolarization block. Modeling work has shown that a slow component of sodium channel inactivation is responsible: it allows a few spikes during a strong depolarization before gradually shutting down the neuron’s ability to fire. The block occurs near or even below the normal spike threshold, at membrane potentials between about −45 and −30 millivolts.15PubMed Central. Mathematical analysis of depolarization block mediated by slow inactivation of fast sodium channels in midbrain dopamine neurons Depolarization block is not simply an interesting lab curiosity; some antipsychotic medications are thought to work in part by driving dopamine neurons into depolarization block, reducing excessive dopamine signaling.
Spreading Depolarization in Disease
Depolarization can also go wrong on a much larger scale. In spreading depolarization, a wave of massive, near-complete depolarization sweeps across the surface of the brain. Unlike a normal action potential, which is brief and localized to one neuron at a time, spreading depolarization involves both neurons and glial cells over large regions and can last for minutes. It is the neuronal event underlying migraine aura, the visual disturbances some people experience before a migraine headache, and it is also a reliable consequence of acute brain injuries such as stroke and traumatic brain injury.16PubMed Central. A Single Episode of Cortical Spreading Depolarization Increases mRNA Levels of Proinflammatory Cytokines, Calcitonin Gene-Related Peptide and Pannexin-1 Channels in the Cerebral Cortex17PubMed. Cell type-specific contribution to the initiation of cortical spreading depolarization in the mouse
Interestingly, the trigger for cortical spreading depolarization in migraine may not be what researchers long assumed. The traditional view centered on excessive glutamate-driven excitation. But recent work has shown that hyperactivity of inhibitory interneurons, the cells that normally dampen excitation, can be sufficient to ignite spreading depolarization. When these interneurons fire excessively, they dump enough potassium into the surrounding space to overwhelm nearby cells and start the depolarization wave.18PubMed Central. Initiation of migraine-related cortical spreading depolarization by hyperactivity of GABAergic neurons and NaV1.1 channels This finding has implications for understanding familial hemiplegic migraine, a genetic form of the condition linked to mutations in the sodium channel subtype primarily expressed in those interneurons.
Toxins That Block Depolarization
Some of the deadliest natural toxins work by directly interfering with the depolarization process. Tetrodotoxin, found in pufferfish, blue-ringed octopuses, and certain newts, binds to the outer mouth of voltage-gated sodium channels and physically prevents sodium ions from flowing through.19PubMed Central. Tetrodotoxin: a brief history Without sodium influx, there is no depolarization, no action potential, and no signal propagation. The result is progressive paralysis: sensory numbness comes first, followed by loss of motor control, and in severe cases, death from respiratory failure as the muscles of breathing stop working.20PubMed. Mixture effects of tetrodotoxin (TTX) and drugs targeting voltage-gated sodium channels on spontaneous neuronal activity in vitro
Tetrodotoxin’s extreme specificity for sodium channels has made it one of the most useful tools in neuroscience. Researchers apply it to silence specific populations of neurons and tease apart the contributions of different cell types to a circuit. The toxin binds only from the outside of the membrane, plugging the channel’s selectivity filter without needing to enter the cell.21PubMed. Interaction between voltage-gated sodium channels and the neurotoxin, tetrodotoxin Local anesthetics such as lidocaine work on a similar principle, though they approach the sodium channel from the inside and are far less potent. Both demonstrate the same lesson: block the sodium channel, block depolarization, block the signal.
The Energy Bill for Every Spike
Depolarization is not free. Every action potential leaves behind a small mess of displaced ions: sodium that leaked in, potassium that leaked out. To restore the original gradients, the sodium-potassium pump must burn ATP, the cell’s energy currency, to push those ions back where they belong.22PubMed. Energy Cost of Action Potential Generation and Propagation in Thalamocortical Relay Neurons During Deep Brain Stimulation The cost per spike is tiny in absolute terms, but the brain contains tens of billions of neurons firing thousands of times per day. The cumulative energy demand is enormous: the brain accounts for roughly 20 percent of the body’s total energy consumption despite being only about 2 percent of body weight, and a large share of that goes to maintaining and restoring ion gradients.
Efficiency varies across neuron types and across different parts of the same neuron. During each spike, some sodium ions flow in while potassium ions are simultaneously flowing out, and the overlap between those two currents is wasted energy because the pump has to undo both. Neurons with less overlap between sodium and potassium currents are more metabolically efficient per spike. Computational models of neurons receiving deep brain stimulation have been used to estimate the ATP expenditure at different points along the cell, showing that the energy cost is not uniform: it varies with local channel density, axon geometry, and the frequency at which the cell is driven.
Measuring Depolarization in Living Tissue
For decades, the gold standard for measuring depolarization was the patch-clamp technique, a descendant of the voltage-clamp method that Hodgkin and Huxley used to first dissect the ionic currents underlying the action potential in squid giant axons in the late 1940s and early 1950s.23PubMed Central. A brief historical perspective: Hodgkin and Huxley A glass pipette with a tip smaller than a micrometer is pressed against a neuron’s membrane, forming a seal tight enough to record the currents through individual ion channels. This approach is exquisitely precise, but it can only record from one or a handful of cells at a time, and it is impractical in a freely behaving animal.
Newer tools have started to change that. Genetically encoded voltage indicators are fluorescent proteins engineered to change their brightness when the membrane voltage changes. By expressing these proteins in specific neuron types using genetic tools, researchers can watch depolarization sweep through entire brain regions in real time under a microscope. In zebrafish, for example, one such indicator called ASAP1 was used to detect depolarization across the cerebellum and optic tectum during electrical stimulation, revealing large-scale patterns of activity that would be invisible to an electrode touching a single cell.24PubMed Central. Optical interrogation of neuronal circuitry in zebrafish using genetically encoded voltage indicators These optical methods are still being refined, with ongoing work to improve their speed and sensitivity enough to capture individual action potentials across thousands of neurons simultaneously. The goal is to move from asking “what does one neuron do?” to “what does an entire circuit do?”—and depolarization, the voltage change that starts it all, remains the signal everyone is trying to see.