A message travels through a neuron as a brief, self-regenerating wave of electrical charge called an action potential, which sweeps from one end of the cell to the other in roughly a thousandth of a second. The process starts when incoming signals nudge the neuron past an electrical tipping point, triggering voltage-sensitive channels to snap open in sequence along the length of the cell’s long output fiber, the axon. When the wave reaches the far end, it triggers the release of chemical messengers that carry the signal across a tiny gap to the next cell. That basic sequence, reception to electrical spike to chemical handoff, underlies everything from reflexes to conscious thought, but the details at each stage are more intricate than a simple relay.
The Neuron at Rest Is Already Working
Before any message arrives, a neuron maintains a small voltage difference across its outer membrane, roughly negative seventy millivolts on the inside relative to the outside. This baseline, called the resting membrane potential, exists because the cell actively sorts its ions. A protein embedded in the membrane known as the sodium-potassium pump continuously shuttles three sodium ions out of the cell for every two potassium ions it pulls in, both against their natural concentration gradients. That lopsided exchange keeps the interior slightly negative and chemically primed, like a spring held under tension.1PubMed Central. Na+/K+-pump and neurotransmitter membrane receptors The energy for this pump comes from ATP, the cell’s universal fuel molecule. Without it, the voltage difference would gradually collapse and the neuron would lose its ability to fire.
Receiving the Signal at the Dendrites
Most neurons have a bushy tree of branching extensions called dendrites, which serve as the main receiving surface. When a chemical messenger from an upstream neuron lands on a receptor on a dendrite, it opens a small ion channel that lets charged particles flow in or out. This produces a tiny, local voltage change, either a small positive shift (excitatory) or a small negative one (inhibitory). A single one of these inputs is remarkably weak. Recordings from live cortical neurons show that an isolated synaptic input is strongly attenuated by the time it reaches the cell body, contributing very little to somatic depolarization on its own.2PubMed Central. Highly attenuated dendritic propagation of isolated synaptic potentials in vivo
What matters is timing and clustering. When multiple inputs arrive within a narrow time window, their small voltage bumps add together. The same research indicates that coincident inputs, those arriving nearly simultaneously from different synapses, are far more effective at pushing the cell body toward the firing threshold than inputs spread out over time.2PubMed Central. Highly attenuated dendritic propagation of isolated synaptic potentials in vivo This means a neuron is not simply tallying up all incoming signals; it is acting more like a coincidence detector, preferentially responding when many of its inputs are active at once.
Distance also plays a role. Signals arriving at dendrite tips far from the cell body lose more voltage as they travel down the branch than those arriving closer in. But the neuron partially compensates: synapses on distant dendrites tend to show stronger short-term facilitation, meaning their signal gets amplified with repeated use compared to closer synapses.3Neuron. Presynaptic Short-Term Plasticity Creates a Distance-Dependent Gradient of Facilitation in CA1 Pyramidal Neurons This gradient of facilitation helps level the playing field so that far-flung inputs still have a voice at the cell body.
Where the Action Potential Begins
If the combined input pushes the neuron past its threshold, the action potential does not start at the cell body itself. It ignites at a small, specialized stretch of the axon right next to the cell body called the axon initial segment, or AIS.4PubMed Central. Axon initial segments: structure, function, and disease The AIS earns this role because it is packed with an extraordinarily high density of voltage-gated sodium channels. Estimates from cortical neurons put the sodium channel density at the AIS at roughly fifty times that of the nearby dendrites, with these channels tightly anchored to the cell’s internal scaffolding.5Nature Neuroscience. Action potential generation requires a high sodium channel density in the axon initial segment That concentration means the AIS reaches the electrical tipping point more easily than any other part of the neuron, making it the natural ignition site.
Think of it as a bottleneck designed to be sensitive. All the graded, messy electrical input from thousands of synapses funnels toward this one spot, and if the voltage there crosses the threshold, the neuron fires. If it doesn’t, no action potential is generated. It is an all-or-nothing decision point.
The Action Potential Itself
Once the threshold is crossed at the AIS, the action potential unfolds in a rapid, stereotyped sequence that takes roughly one to two milliseconds.
In the rising phase, voltage-gated sodium channels snap open and sodium ions rush into the cell, driven by both their concentration gradient and the electrical pull of the negative interior. This inward flood of positive charge swings the local membrane voltage sharply positive, a shift from about negative seventy millivolts to roughly positive thirty or forty millivolts. The sodium channels open because the membrane has depolarized past a critical voltage, and the inrush of sodium depolarizes the membrane further, which opens more channels nearby. This self-reinforcing loop is what makes the signal propagate: the positive voltage at one patch of membrane is enough to push the neighboring patch past threshold, opening sodium channels there in turn, and the wave moves forward.
The falling phase follows almost immediately. The sodium channels have a built-in timer; within a fraction of a millisecond they swing shut and enter an inactive state where they cannot reopen, no matter how strong the voltage. Simultaneously, voltage-gated potassium channels open. Potassium ions flow outward, carrying positive charge out of the cell and dragging the voltage back down. Research on hippocampal and brainstem neurons has shown that multiple distinct potassium channel types work together to repolarize the membrane, including calcium-activated potassium channels and purely voltage-dependent ones.6PubMed Central. Potassium conductances underlying repolarization and after-hyperpolarization in rat CA1 hippocampal interneurones7PubMed. Potassium currents contributing to action potential repolarization and the afterhyperpolarization in rat vagal motoneurons
The voltage typically dips slightly below the resting level before settling back, a brief undershoot called the afterhyperpolarization. During this period the neuron is harder to fire again, which enforces a minimum spacing between action potentials and ensures the signal only travels forward (the membrane behind the advancing wave is temporarily refractory).
How Myelin Speeds Things Up
In many neurons, the axon is wrapped in segments of an insulating sheath called myelin, produced by specialized support cells. Between these insulated segments sit tiny exposed gaps called nodes of Ranvier, where ion channels are concentrated. Rather than creeping along every micrometer of the axon surface, the action potential effectively jumps from one node to the next, a process called saltatory conduction.8PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit This dramatically increases speed. A myelinated axon can conduct signals at well over 100 meters per second in some motor neurons, while an unmyelinated axon of the same diameter might manage only a fraction of that.
Recent work has revealed that the electrical picture under the myelin sheath is more complex than the textbook sketch. Patch-clamp recordings and high-speed optical voltage imaging of myelinated cortical axons show that a narrow, conductive fluid-filled space exists between the axon surface and the inner face of the myelin. This periaxonal space is incompletely sealed at the edges of each myelin segment, creating a “double cable” in which fast nodal potentials travel ahead of slower, attenuated waves in the internodal membrane beneath the myelin.9Cell. A Nanoscale Periaxonal Space Provides Electrical Compartmentalization in Myelinated Axons The upshot is that saltation involves both jumping across space and jumping across time, with the signal at a downstream node arriving before the slower wave under the previous internode has fully played out.
Axon diameter matters too, especially in unmyelinated fibers. In unmyelinated hippocampal axons, conduction speed depends heavily on how wide the axon shaft is, and that width can change with use. High-frequency firing causes axon shafts to widen, which sustainably increases conduction speed, while synaptic boutons (the swellings where signals are handed off) transiently enlarge and briefly slow things down.10PubMed Central. Superresolution imaging reveals activity-dependent plasticity of axon morphology linked to changes in action potential conduction velocity This means the speed of neural signaling is not fixed. It adapts to the neuron’s recent activity.
Handing Off the Signal at the Synapse
When the action potential reaches the axon terminal, the electrical message must be converted into a chemical one to cross the gap, typically about twenty nanometers wide, between one neuron and the next. The arriving spike opens voltage-gated calcium channels in the terminal membrane, and the resulting rush of calcium ions into the terminal is the critical trigger for neurotransmitter release.11PubMed Central. Ca(2+) influx and neurotransmitter release at ribbon synapses Calcium interacts with specialized docking and fusion proteins to cause tiny vesicles filled with neurotransmitter molecules to merge with the terminal membrane and spill their contents into the synaptic gap.12PubMed. Interactions of presynaptic Ca2+ channels and snare proteins in neurotransmitter release
On the receiving side, the neurotransmitter molecules bind to receptors that come in two broad flavors. Fast-acting receptors are themselves ion channels; the neurotransmitter physically opens the pore, letting ions flow within less than a millisecond, which produces the quick voltage changes that feed into the integration process described earlier.13Neuron. Holistic View of GluR Crosstalk: A Network Perspective Slower-acting receptors work indirectly, activating internal signaling cascades that can modulate the cell’s behavior over seconds to minutes.14PubMed Central. Ionotropic and metabotropic mechanisms in chemoreception: ‘chance or design’? Most synapses use a mix of both types, giving the receiving neuron both a fast initial kick and a slower, longer-lasting influence.
The Energy Bill
All of this ion shuffling is metabolically expensive. Every action potential leaves behind a small surplus of sodium inside the cell and a deficit of potassium. The sodium-potassium pump must burn ATP to restore the original gradients before the neuron can fire again reliably. Generating and conducting action potentials requires large quantities of energy devoted to this restoration work.15Nature Neuroscience. State and location dependence of action potential metabolic cost in cortical pyramidal neurons The brain as a whole consumes roughly twenty percent of the body’s resting energy despite making up only about two percent of body weight, and a substantial share of that energy budget goes to maintaining and restoring ion gradients in neurons.
Interestingly, the cost is not uniform across the neuron. Computational models of thalamocortical relay neurons have been used to estimate how much ATP is needed to reverse the sodium influx at different locations along the cell, finding that the metabolic price of a spike varies depending on where it is generated and how far it propagates.16PubMed. Energy Cost of Action Potential Generation and Propagation in Thalamocortical Relay Neurons During Deep Brain Stimulation Myelination helps on this front too: by limiting ion exchange to the nodes of Ranvier, it reduces the total number of ions that cross the membrane per action potential and therefore cuts the energy cost of long-distance signaling.
When the Message Gets Disrupted
Because the system depends on precise insulation and channel function, damage at any point can derail signaling. Demyelinating diseases like multiple sclerosis strip the myelin sheath from patches of axon, and the consequences go beyond simple slowing. When myelin is lost, action potentials can fail entirely as they try to cross the bare region. Computer simulations dating back decades showed that propagation past a single demyelinated segment continues only as long as some minimal myelin thickness remains; below roughly three percent of normal thickness, the signal stalls.17PubMed Central. A computer simulation of conduction in demyelinated nerve fibres More recent modeling has clarified that the failure tends to happen when the action potential exits the demyelinated zone, not when it enters, because of the electrical mismatch between the bare and insulated segments.18PubMed Central. Linking demyelination to compound action potential dispersion with a spike-diffuse-spike approach
Even when signals do get through a demyelinated patch, they slow unevenly across different axons in a nerve bundle. This differential slowing causes signals that were originally synchronized to arrive at their destination at scattered times, a phenomenon called temporal dispersion.19PubMed Central. Physiological Dynamics in Demyelinating Diseases: Unraveling Complex Relationships through Computer Modeling The result can be weakness, numbness, or scrambled sensory information, not because individual signals are weaker but because the precise timing the brain relies on is lost.
Toxins That Target the Machinery
The voltage-gated sodium channel is such a critical bottleneck that evolution has produced toxins aimed squarely at it. Tetrodotoxin, the famous poison found in pufferfish, binds to the outer mouth of the sodium channel’s pore and physically blocks sodium ions from flowing through. The binding is highly selective; tetrodotoxin does not affect other receptor or channel systems.20PubMed Central. Tetrodotoxin: a brief history. By preventing sodium entry, it stops action potentials from being generated or propagated, which can lead to paralysis and death from respiratory failure as the nerves controlling breathing go silent.21Toxicology Letters. 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 similar principle but from the inside of the channel, and they are designed to be reversible and locally applied. The shared logic is the same: block sodium channels, and you block the action potential. That is why a dentist’s injection numbs a region completely, silencing both pain and touch signals until the drug wears off and channels reopen.
Signals That Skip the Standard Route
The chemical synapse, where vesicles release neurotransmitter into a gap, is the dominant way neurons talk to each other. But it is not the only way. Some neurons are connected by gap junctions, physical protein pores that bridge the membranes of two adjacent cells and allow ions and small molecules to pass directly from one cell’s interior to the next. These electrical synapses transmit signals almost instantaneously and bidirectionally, without the delay of vesicle release and receptor binding.22Current Biology. Neuronal Gap Junctions They are especially common in circuits that require very tight synchronization, such as those coordinating rhythmic breathing or rapid escape reflexes.
Neurons can also influence each other without any direct connection at all. When an action potential fires, the ion currents produce small voltage changes in the fluid surrounding the axon. If another axon runs close by, these extracellular fields can nudge its membrane voltage, a phenomenon called ephaptic coupling. The effect is tiny: modeling suggests the shift at a neighboring axon about a hundred micrometers away is on the order of a hundredth of a millivolt from a single node’s ion flow.23PubMed Central. Possible mechanism of action potential propagation mediated by static electric field: A novel assumption of understanding nerve interaction and ephaptic coupling That is far too small to fire a resting neuron on its own, but when many axons in a bundle fire synchronously, the cumulative effect could subtly bias the timing of action potentials in their neighbors.24Frontiers in Computational Neuroscience. Extracellular stimulation and ephaptic coupling of neurons in a fully coupled finite element-based Extracellular—Membrane—Intracellular (EMI) model
Signals That Travel Backward
The textbook picture has information flowing in one direction: dendrites receive, the axon transmits, the terminal releases. But neurons also send messages in reverse. The best-studied example involves endocannabinoids, small fat-soluble molecules produced by the receiving (postsynaptic) cell. When a postsynaptic neuron is strongly activated, it synthesizes endocannabinoids that drift backward across the synapse and bind to receptors on the presynaptic terminal, dialing down neurotransmitter release. This retrograde signaling is the principal mode by which endocannabinoids shape both short-term and long-term changes in synaptic strength at excitatory and inhibitory synapses alike.25PubMed Central. Endocannabinoid signaling and synaptic function It acts as a kind of volume knob, letting the receiving neuron tell its own inputs to quiet down when activity gets too intense. The discovery of retrograde signaling was one of the reasons neuroscientists stopped thinking of synapses as one-way valves and began treating them as dynamic, bidirectional conversation points.