A nerve impulse is a rapid, self-propagating wave of electrical activity that travels along a neuron, carrying information from one part of the body to another. It lasts roughly a millisecond at any given point along the nerve fiber, relies on charged particles (ions) rushing in and out of the cell through specialized protein gates, and can travel at speeds ranging from under a meter per second in the thinnest fibers to well over 100 meters per second in the thickest myelinated ones. The phenomenon is more nuanced than a simple on-off electrical signal, though, because the way a nerve cell prepares for the impulse, fires it, and hands it off to the next cell involves a chain of physical and chemical events that scientists are still refining their understanding of.
How a Neuron Stays Charged at Rest
Before a nerve impulse can happen, the neuron has to be in a state of readiness, and that state is electrical. A resting neuron carries a voltage difference across its outer membrane, with the inside sitting at roughly negative 60 to negative 80 millivolts compared to the outside. This “resting potential” exists because the cell actively maintains an uneven distribution of sodium and potassium ions. A protein pump embedded in the membrane, called the sodium-potassium pump, continuously shuttles three sodium ions out of the cell and two potassium ions in for every molecule of ATP it burns.1PubMed. On the concept of resting potential–pumping ratio of the Na⁺/K⁺ pump and concentration ratios of potassium ions outside and inside the cell to sodium ions inside and outside the cell This imbalance leaves the interior of the cell more negative than the exterior.
Potassium ions can leak back out through channels that are open at rest, and that leak is actually the dominant electrical current near the resting potential. The pump itself adds a small extra amount of negativity, around 10 millivolts, because it moves more positive charges out than it brings in.2PubMed. On the resting potential of isolated frog sympathetic neurons The result is a neuron that acts like a tiny charged battery, primed and waiting for a signal. Without that stored voltage difference, no impulse could ever fire.
What Triggers the Impulse
The nerve impulse itself is technically called an action potential, and it begins when something pushes the membrane voltage past a critical threshold, usually around negative 55 millivolts. That “something” could be a sensory stimulus, a chemical signal from another neuron, or an artificial electrical current applied in a lab. Once the voltage crosses that threshold, the event becomes all-or-nothing: it either fires fully or not at all.
What happens at the threshold is that voltage-sensitive sodium channels snap open. Sodium ions, which are far more concentrated outside the cell, rush inward. This flood of positive charge drives the membrane voltage sharply upward, from its negative resting value to roughly positive 30 or 40 millivolts in about half a millisecond. The process is self-amplifying: the more sodium channels open, the more the voltage rises, which opens still more sodium channels.
The spike does not last, though. Sodium channels automatically inactivate within a fraction of a millisecond, and potassium channels open in response to the depolarization, letting potassium ions flow out. This outward potassium current drives the voltage back down and often briefly overshoots, dipping below the resting potential for a short period known as the refractory period. During that window, the neuron is resistant to firing again, which prevents the impulse from doubling back on itself. Potassium channels play a direct role in shaping how long the spike lasts and how quickly the neuron can fire again.3PubMed. Different voltage-dependent potassium conductances regulate action potential repolarization and excitability in frog myelinated axon
How the Signal Travels Along the Nerve
An action potential at one spot on the membrane does not simply appear at the far end. It has to propagate, and the way it does so depends on whether the nerve fiber is myelinated or not.
In unmyelinated fibers, the impulse spreads continuously. The inward sodium current at one point depolarizes the membrane immediately adjacent, pushing it past threshold, which opens the next batch of sodium channels, and so on down the line. This works, but it is slow because every stretch of membrane has to cycle through the full action potential.
Most of the body’s long-distance signaling fibers are wrapped in myelin, a fatty insulation produced by glial cells. Myelin covers the axon in segments, with tiny gaps called nodes of Ranvier spaced between them. Sodium channels are clustered at these nodes. When an action potential fires at one node, the electrical current spreads passively through the insulated internode and depolarizes the next node, which then fires its own action potential. This process, called saltatory conduction, effectively makes the impulse jump from node to node, dramatically increasing speed while using less energy.4PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit
The clustering of sodium channels at nodes, combined with the insulating effect of myelin between them, reduces both the metabolic cost and the physical space needed for fast signaling.5Current Biology. Saltatory Conduction: Jumping to New Conclusions Both voltage-gated potassium channels and a family of leak potassium channels at the nodes also contribute to maintaining the speed and reliability of saltatory conduction, particularly when the neuron is firing rapidly.6PubMed Central. Role of Voltage-Gated K(+) Channels and K2P Channels in Intrinsic Electrophysiological Properties and Saltatory Conduction at Nodes of Ranvier of Rat Lumbar Spinal Ventral Nerves
What Determines How Fast the Impulse Travels
The presence or absence of myelin is the single biggest factor, but it is not the only one. For myelinated fibers that share similar proportions, conduction velocity is roughly proportional to the fiber’s diameter: thicker axons conduct faster.7PubMed. Determinants of conduction velocity in myelinated nerve fibers Temperature matters too. Cooling a nerve slows conduction measurably, which is why local anesthetic injections sometimes feel cold and why extremely cold weather can make your fingers feel sluggish before they go numb. The chemical environment around the axon, including ion concentrations and pH, also influences how readily channels open and close.
In the human body, the fastest signals travel in large myelinated motor and sensory fibers at around 70 to 120 meters per second. Pain carried by thin, unmyelinated C-fibers creeps along at under 2 meters per second. That difference in speed is why you feel the sharp, localized pain from stubbing your toe before the dull, throbbing ache arrives a moment later: the initial sensation travels on fast myelinated fibers, and the slower burning feeling follows on unmyelinated ones.
What Happens When the Impulse Reaches Another Cell
Nerve impulses do not leap directly from one neuron to the next. At the junction between neurons, called a synapse, the signal usually converts from electrical to chemical and back again. When the action potential arrives at the axon terminal, it opens voltage-gated calcium channels. Calcium ions flood into the terminal within microseconds, and this calcium influx triggers the release of chemical messengers called neurotransmitters into the narrow gap between the two cells.8PubMed Central. Calcium control of neurotransmitter release
The calcium channels begin activating during the rising phase of the action potential and continue opening during the falling phase. This means the precise shape of the action potential waveform, including the exact voltage at which it peaks and how fast it comes back down, influences how much neurotransmitter gets released.9Frontiers in Synaptic Neuroscience. Presynaptic Calcium Channel Open Probability and Changes in Calcium Influx Throughout the Action Potential Determined Using AP-Waveforms The shape of the voltage dip after the spike also affects calcium entry: a slower return to rest allows more calcium to flow in and can actually produce a larger release of neurotransmitter, even though the peak calcium current is smaller.10Journal of Neuroscience. Neurotransmitter Release Can Be Stabilized by a Mechanism That Prevents Voltage Changes Near the End of Action Potentials from Affecting Calcium Currents
Once released, neurotransmitters bind to receptors on the receiving neuron, which can either excite or inhibit it. Excitatory signals push the receiving cell closer to its firing threshold; inhibitory ones push it further away. A single neuron typically receives thousands of these inputs simultaneously and sums them up. The timing of those inputs matters as much as their strength: under certain conditions, the maximum combined effect comes from slightly staggered inputs rather than perfectly simultaneous ones.11PubMed Central. Synaptic integration mechanisms. Theoretical and experimental investigation of temporal postsynaptic interactions between excitatory and inhibitory inputs
Chemical synapses are the dominant form in the vertebrate nervous system, but electrical synapses also exist. At an electrical synapse, tiny protein channels called gap junctions connect the interiors of two adjacent cells directly, letting ions and small molecules pass between them. These two forms of communication are not independent systems; they interact with and modulate each other during development and in the mature brain.12PubMed Central. Electrical synapses and their functional interactions with chemical synapses
The Energy Cost of Firing
Every action potential comes with a metabolic bill. The sodium-potassium pump has to reverse the ion movements that produced the spike, and that requires ATP. At a single node of Ranvier, the minimum cost has been calculated at around a million ATP molecules. For a full action potential spreading across a larger region of membrane, the estimate rises to trillions of ATP molecules per square centimeter of membrane surface.13PubMed. The cost of an action potential This cost is a major reason why the brain, despite being only about 2% of body weight, consumes roughly 20% of the body’s resting energy.
Myelination dramatically reduces this bill. Because ion exchange is confined to the nodes rather than occurring along the full length of the axon, far fewer ions need to be pumped back. The energy savings compound across the billions of impulses the nervous system generates every second. Even so, the ATP demand of maintaining neural signaling is substantial enough that the brain is one of the first organs to suffer when blood supply is interrupted, which is why strokes cause damage within minutes.14PubMed. Energy Cost of Action Potential Generation and Propagation in Thalamocortical Relay Neurons During Deep Brain Stimulation
When Nerve Impulses Fail
Several things can disrupt the normal flow of nerve impulses, and they tend to target specific steps in the process.
Toxins often go after the sodium channels directly. Tetrodotoxin, the poison found in pufferfish, binds to voltage-gated sodium channels from the outside and physically blocks the pore that sodium ions pass through. It does not alter how the channel opens or closes; it simply plugs the hole.15PubMed Central. Tetrodotoxin: a brief history Without sodium flowing in, the action potential cannot fire, which leads to paralysis of muscles (including the diaphragm) and can be fatal from respiratory failure.16PubMed. Mixture effects of tetrodotoxin (TTX) and drugs targeting voltage-gated sodium channels on spontaneous neuronal activity in vitro Local anesthetics like lidocaine work on a conceptually similar principle: they block sodium channels, though from the inside and in a reversible, dose-controlled way.
Diseases that damage myelin, such as multiple sclerosis, attack the insulation rather than the channels themselves. When myelin is stripped away, the current that normally jumps efficiently between nodes leaks out through the bare membrane instead. Conduction can slow down severely, become unreliable, or fail entirely in the damaged segment. Even when impulses still get through, the variable slowing across different axons can desynchronize signals that normally arrive together, scrambling the information the brain receives.17PubMed Central. Physiological Dynamics in Demyelinating Diseases: Unraveling Complex Relationships through Computer Modeling This desynchronization helps explain why symptoms of demyelinating diseases can be so varied and unpredictable, ranging from vision problems to muscle weakness to cognitive fog, depending on which tracts are affected.
Signals That Skip the Synapse
The textbook picture of neural signaling goes: action potential travels down the axon, neurotransmitter crosses the synapse, next neuron responds. But there is a less well-known route called ephaptic coupling, where the electrical field generated by one nerve fiber directly influences its neighbors without any chemical messenger involved.
When an action potential propagates along an axon, the movement of ions creates a small extracellular electric field. In bundles of tightly packed nerve fibers, this field is strong enough to slightly shift the membrane voltage of adjacent axons. Modeling studies show that when two impulses travel on neighboring fibers with a small time lag, ephaptic interaction can cause them to attract or repel each other, eventually locking them into synchrony or pushing them apart.18PubMed Central. A mathematical model of ephaptic interactions in neuronal fiber pathways: Could there be more than transmission along the tracts?
Recent experimental work has added another wrinkle. When an action potential reaches the end of an axon and annihilates (ceases to propagate because there is nowhere left to go), it produces a local electrical discharge. This discharge generates a field strong enough to influence nearby target neurons immediately, creating a form of coupling that is distinct from both chemical and gap-junction transmission.19eLife. Annihilation of action potentials induces electrical coupling between neurons The functional significance of ephaptic coupling is still debated, but it may help explain how groups of neurons synchronize their activity during tasks like sensory processing and rhythmic oscillations.
How Glial Cells Shape Nerve Impulses
Neurons get most of the attention, but glial cells are not passive bystanders. Beyond producing myelin, a type of glial cell called an astrocyte actively modulates how excitable neurons are by managing the concentration of potassium ions in the extracellular fluid around them.20PubMed. Astrocytic modulation of neuronal excitability through K(+) spatial buffering
Remember that potassium ions flow out of the neuron during every action potential. If that potassium accumulates in the narrow extracellular space, it shifts the resting potential of nearby neurons closer to threshold, making them more likely to fire. Astrocytes soak up the excess potassium and redistribute it to areas where concentrations are lower, a process called spatial buffering. This keeps the environment stable enough for neurons to fire reliably without accidentally triggering their neighbors. When astrocyte function is impaired, as happens in certain types of epilepsy, the unchecked buildup of extracellular potassium can make entire populations of neurons fire uncontrollably.
Sodium Channels Before Nervous Systems Existed
Voltage-gated sodium channels are so central to nerve impulses that it is natural to assume they evolved alongside nervous systems. They did not. Genome studies have identified genes for sodium channel relatives in choanoflagellates, single-celled organisms that are the closest living relatives of animals and have no neurons at all. Similar channels also appear in placozoans, tiny multicellular animals that lack any form of nervous system. Both of these organisms express the channel genes, meaning they actually produce the proteins, not just carry the DNA.21PubMed Central. Evolution of sodium channels predates the origin of nervous systems in animals
The choanoflagellate version has a selectivity filter that sits between what you would expect for a calcium channel and what you would expect for a sodium channel, suggesting that modern sodium channels evolved from an ancestral calcium-permeable channel. In other words, the molecular machinery that makes nerve impulses possible was already present hundreds of millions of years before animals developed neurons to use it. What those single-celled organisms do with their proto-sodium channels is still an open question, but the finding underscores that evolution frequently repurposes existing tools rather than building from scratch.
Alternative Theories of the Nerve Impulse
The standard model of the nerve impulse, dating back to the landmark work of Hodgkin and Huxley in 1952, describes it as a purely electrical event driven by ion flows through membrane channels.22PubMed Central. A brief historical perspective: Hodgkin and Huxley That framework has been extraordinarily successful and remains the foundation of neuroscience. But it does not account for everything that happens during an action potential. Careful measurements have shown that nerves also undergo tiny mechanical and thermal changes as impulses pass: the fiber briefly thickens, and there are small, transient changes in temperature.
These observations have inspired an alternative proposal known as the soliton model, which treats the nerve impulse as a thermodynamic and mechanical wave in the membrane rather than a purely electrical phenomenon. In this picture, the pulse is an electromechanical event where voltage, temperature, pressure, and the physical state of the membrane lipids are all coupled together.23PubMed. The thermodynamic soliton theory of the nervous impulse and possible medical implications Proponents argue that the Hodgkin-Huxley model, which focuses on ion currents through channels, cannot fully explain all observed phenomena, and that a more complete picture needs to include the physical properties of the membrane itself.24PubMed. Breathing pulses in the damped-soliton model for nerves
The soliton model remains a minority view and has not displaced the standard framework, but it has pushed researchers to take the mechanical and thermal accompaniments of the action potential more seriously. Whether the soliton picture will eventually merge with or remain separate from the ion-channel model is one of the genuinely unresolved questions in biophysics.
Recording Nerve Impulses With Modern Technology
For most of the history of neuroscience, measuring a single nerve impulse required inserting a fine electrode into or next to a nerve fiber. Modern microelectrode arrays have turned that into a massively parallel operation. These arrays contain dozens to thousands of tiny recording sites that can simultaneously capture the action potentials of many individual neurons in a living brain.
A single action potential, as picked up by an extracellular electrode, appears as a brief voltage blip of a few hundred microvolts lasting about a millisecond, with signal energy concentrated in a frequency band between roughly 300 and 10,000 hertz.25PubMed Central. In vivo microelectrode arrays for neuroscience Separating these tiny, fast signals from the slower, broader fluctuations of the surrounding tissue requires careful filtering and amplification. The same arrays can also deliver small electrical currents to stimulate neurons, making it possible to both listen to and talk to the nervous system through the same device.
Newer designs have pushed into even more intimate recording. Volcano-shaped microelectrodes, barely larger than a cell, can gain access to the interior of individual cells and record full intracellular action potentials with amplitudes as high as 62 millivolts. These recordings can be repeated on demand over consecutive days without destroying the cells, opening the door to long-term monitoring of how individual neurons behave over time.26PubMed Central. Nanovolcano microelectrode arrays: toward long-term on-demand registration of transmembrane action potentials by controlled electroporation Technologies like these are already being adapted for brain-computer interfaces, where reading and writing nerve impulses in real time lets paralyzed patients control cursors or robotic limbs with their thoughts.