When a neuron fires, it sends a brief electrical pulse along its length and then converts that pulse into a chemical message at the junction with the next cell. The whole event takes just a millisecond or two, but it involves a tightly choreographed sequence of ion channels opening and closing, calcium flooding into nerve terminals, and tiny packets of signaling molecules launching across the gap between cells. This chain reaction is the basic unit of everything your brain does, from keeping your heart beating to composing a sentence, and the details of how it works reveal why the brain is both remarkably efficient and surprisingly fragile.
The Resting Neuron and What It Takes to Set One Off
Before a neuron fires, it sits in a charged-and-waiting state. The inside of the cell is slightly negative compared to the outside, a voltage difference maintained by the constant leak and pumping of charged particles (mainly sodium and potassium ions) across the cell membrane. This resting voltage hovers around negative 70 millivolts. Think of it like a spring held under tension: the neuron has already done the work of separating charges, so when the right signal arrives, all it has to do is let go.
The “right signal” is usually a combination of chemical inputs from other neurons that push the voltage at a critical zone of the cell past a threshold. Once that threshold is crossed, what follows is not gradual. It is an all-or-nothing explosion of voltage called an action potential. The neuron either fires fully or not at all; there is no half-strength version.
Where Firing Begins
Not every part of a neuron is equally primed to fire. The action potential almost always starts at a small stretch of the axon right where it branches off from the cell body, called the axon initial segment. This region is packed with voltage-gated sodium channels at a density roughly 50 times greater than in nearby parts of the cell, anchored tightly to the internal skeleton of the neuron so they stay concentrated in place.1Nature Neuroscience. Action potential generation requires a high sodium channel density in the axon initial segment That dense cluster of sodium channels is what makes the initial segment the trigger zone: it takes less incoming voltage to push this spot past the firing threshold than it would anywhere else on the neuron.2PubMed Central. Axon initial segments: structure, function, and disease
When the threshold is crossed, sodium channels at the initial segment snap open, and positively charged sodium ions rush into the cell. This sudden influx flips the local voltage from negative to positive in well under a millisecond. Almost immediately afterward, potassium channels open, potassium flows out, and the voltage swings back down. That rapid up-and-down voltage spike is the action potential itself. It is brief, stereotyped, and essentially identical every time a given neuron fires.
How the Signal Travels Down the Axon
Once ignited at the initial segment, the action potential needs to travel, sometimes a very long way. Motor neurons running from your spinal cord to your foot can stretch over a meter. If the signal simply crept along the membrane like a lit fuse, it would be far too slow for useful communication. The brain’s solution is myelin, a fatty insulating sheath wrapped around most axons by specialized glial cells. Myelin is interrupted at regular gaps called nodes of Ranvier, where sodium channels cluster again.
The electrical signal effectively leaps from one node to the next. When one node fires an action potential, the current flows through the insulated stretch of axon beneath the myelin and arrives at the next node with enough strength to trigger a fresh action potential there. Researchers have shown that this jumping pattern, known as saltatory conduction, arises because the space between the axon surface and the inner face of the myelin sheath acts as a tiny cable, feeding depolarization forward from one node to the next in a repeating cycle.3PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit The result is fast, energy-efficient transmission. In thickly myelinated axons, signals can travel at over 100 meters per second.
What Happens at the Synapse
When the action potential reaches the end of the axon, the electrical signal has to become a chemical one. The axon terminal does not physically touch the next neuron; a tiny gap called the synaptic cleft separates them. Converting the signal across that gap is one of the most tightly regulated steps in the whole process.
The arriving action potential opens voltage-gated calcium channels in the terminal membrane. Calcium ions rush in, and that calcium influx is the direct trigger for neurotransmitter release.4PubMed Central. Ca(2+) influx and neurotransmitter release at ribbon synapses Tiny membrane-bound sacs called synaptic vesicles, pre-loaded with neurotransmitter molecules, fuse with the terminal membrane and dump their contents into the cleft. The more calcium that enters, the more vesicles fuse and the more neurotransmitter gets released. Experimental work has shown that both the probability of vesicle release and the size of the readily available pool of vesicles scale steeply with calcium concentration, which gives the synapse a built-in volume knob.5Journal of Neuroscience. Presynaptic Calcium Influx Controls Neurotransmitter Release in Part by Regulating the Effective Size of the Readily Releasable Pool
The neurotransmitter molecules drift across the cleft and bind to receptor proteins on the surface of the receiving neuron. Depending on the type of neurotransmitter and receptor, this can either push the receiving cell closer to its own firing threshold (excitation) or pull it further away (inhibition). Glutamate is the brain’s main excitatory neurotransmitter; GABA is the main inhibitory one. Most neurons receive thousands of excitatory and inhibitory inputs simultaneously, and whether the receiving neuron fires depends on whether the combined effect tips its voltage past threshold at the axon initial segment.
The Energy Bill
All of this ion-shuffling is expensive. The brain accounts for roughly 20 percent of the body’s energy consumption despite being only about 2 percent of its weight, and the bulk of that cost comes from the very mechanisms that underlie neuronal firing. An influential energy budget estimated that action potentials alone consume around 47 percent of signaling energy in cortical gray matter, with the postsynaptic effects of glutamate accounting for another 34 percent.6PubMed. An energy budget for signaling in the grey matter of the brain A separate analysis found that pre- and postsynaptic transmission mechanisms together eat up about 55 percent of the total ATP devoted to action potentials, synaptic transmission, and resting potentials.7Neuron. How Synapses Provide, Themselves, and Consume Energy Support Their Information Processing
To meet that demand, active brain regions need a rapid increase in blood flow. Astrocytes, star-shaped glial cells that wrap around both synapses and blood vessels, play a key role in coupling neural activity to blood delivery. They also absorb and recycle neurotransmitters from the synaptic cleft, helping to reset the synapse for the next round of signaling.8PubMed. The role(s) of astrocytes and astrocyte activity in neurometabolism, neurovascular coupling, and the production of functional neuroimaging signals This coupling between neural activity, astrocyte metabolism, and blood flow is also the basis of functional MRI: the scanner detects changes in local blood oxygenation that track where neurons are busy firing.
The Refractory Period and Speed Limits
A neuron cannot fire as fast as you might imagine. Immediately after an action potential, the sodium channels that just opened enter an inactive state and cannot reopen for a brief window, typically one to two milliseconds. During this absolute refractory period, no stimulus, no matter how strong, can trigger another spike. That sets a hard ceiling on firing rate, generally somewhere in the range of 500 to 1,000 spikes per second for the fastest neurons, though most fire far more slowly in practice.
Following the absolute refractory period comes a relative refractory period, during which the neuron can fire again but requires a stronger-than-normal stimulus to do so. Modeling work has shown that these refractory dynamics are not just a limitation; they actually sharpen the temporal precision of a neuron’s output. Because the membrane’s recovery from a spike follows a predictable time course, the refractory period constrains when the next spike can occur, reducing jitter and making the timing of spike trains more reliable.9PubMed Central. Refractoriness and neural precision
How Patterns of Firing Carry Information
If every action potential is essentially the same voltage spike, how does the brain encode the difference between a whisper and a shout, or between the color red and the color blue? The answer lies not in the shape of individual spikes but in their patterns. The simplest code is rate: a neuron that fires 80 times per second in response to a bright light and 10 times per second in dim light is using its firing rate to represent brightness. But rate is not the whole story.
Neurons also encode information in the precise timing of their spikes, a strategy called temporal coding. Recent work recording from the visual cortex found that temporal codes operating on timescales of tens of milliseconds produced more stable representations of visual stimuli over time than rate codes measured over hundreds of milliseconds, particularly for neurons whose responses were less individually reliable.10PubMed Central. Temporal coding carries more stable cortical visual representations than firing rate over time In practice, the brain appears to use both strategies simultaneously. Rate changes in individual neurons encode specific stimulus features, while the timing relationships between spikes across groups of neurons help bind those features into a coherent perception.11Neuron. Mechanisms of Assembly Generation in Cortical Networks
This dual coding scheme explains why the brain can extract so much information from relatively sparse activity. Most cortical neurons fire at low average rates, just a few spikes per second. But because the exact timing of those sparse spikes carries additional information, a small number of active neurons can collectively represent a rich set of details about the outside world.
Firing That Rewires
One of the most consequential things that happens when neurons fire is that the connections between them change strength. This is the cellular basis of learning and memory. When a presynaptic neuron repeatedly drives a postsynaptic neuron to fire, the synapse between them can become stronger, a phenomenon called long-term potentiation. The process depends on a specific type of glutamate receptor, the NMDA receptor, which opens only when the postsynaptic cell is already depolarized. Once open, it lets calcium flow into the postsynaptic neuron, triggering a cascade of molecular changes that make that synapse more effective in the future.12Methods in Neurosciences. Long-Term Potentiation
The flip side exists too. Patterns of firing that are poorly correlated, or that arrive at the wrong timing, can weaken synapses through long-term depression. The combination of strengthening and weakening allows neural circuits to sculpt themselves based on experience, gradually tuning which signals get amplified and which get suppressed. This is why practicing a skill makes it easier: the relevant circuits become more efficient at passing their signals along.
When Firing Goes Wrong
The same mechanisms that make neuronal communication possible can cause severe damage when they are dysregulated. Glutamate, the brain’s most abundant excitatory neurotransmitter, is also its most dangerous when present in excess. If extracellular glutamate concentrations rise to abnormal levels, the resulting over-activation of glutamate receptors floods postsynaptic neurons with calcium. That excessive calcium influx can trigger cell death pathways, a process called excitotoxicity.13PubMed Central. Role of glutamate excitotoxicity and glutamate transporter EAAT2 in epilepsy: Opportunities for novel therapeutics development
Excitotoxicity plays a role in multiple neurological conditions. In epilepsy, synchronized bursts of hyperexcitable neurons generate seizures, and prolonged seizure activity can cause brain damage through exactly this glutamate-mediated mechanism.14PubMed. Excitotoxic mechanisms of epileptic brain damage The acute damage has a characteristic appearance under a microscope: swollen dendrites and vacuolar degeneration of cell bodies, with axons initially left intact. Both NMDA and non-NMDA glutamate receptors contribute to the calcium overload that drives this injury.15PubMed. Pathophysiological mechanisms of brain damage from status epilepticus Similar excitotoxic processes contribute to cell death after stroke, traumatic brain injury, and in some neurodegenerative diseases.
Interestingly, research into Huntington’s disease has found that cortical neurons from a mouse model of the disease show significantly greater calcium influx at presynaptic terminals during stimulation compared to normal neurons.16Frontiers in Molecular Neuroscience. Altered Synaptic Vesicle Release and Ca2+ Influx at Single Presynaptic Terminals of Cortical Neurons in a Knock-in Mouse Model of Huntington’s Disease That excess calcium entry could contribute to the increased excitatory activity seen in Huntington’s, illustrating how even a subtle shift in the calcium machinery at the synapse can have outsized consequences.
Communication Beyond the Synapse
The neat picture of one neuron firing, releasing neurotransmitter across a synapse, and stimulating the next cell is the dominant mode of brain communication, but it is not the only one. Neurons can also send chemical messages through the extracellular space without forming traditional point-to-point synapses. In this mode, neurotransmitters or neuromodulators diffuse outward from release sites and act on any nearby cells equipped with the right high-affinity receptors.17PubMed Central. Non-synaptic receptors and transporters involved in brain functions and targets of drug treatment
This diffuse signaling is especially important for neuromodulators like dopamine, serotonin, and norepinephrine. Rather than toggling a single downstream neuron on or off, these molecules shift the overall operating state of whole brain regions. They can change how sensitive neurons are to incoming signals, adjust the dynamics of ion channels, or alter synaptic strength across multiple timescales, from rapid moment-to-moment tuning to slow persistent changes. When you feel more alert after a cup of coffee or more relaxed after exercise, it is partly because neuromodulatory systems have adjusted the gain across large populations of neurons, changing how readily those neurons fire without directly triggering any individual action potential.
How the Brain’s Firing Rules Change During Development
The rules governing what happens when neurons fire are not fixed from birth. One of the most striking developmental changes involves GABA, the brain’s primary inhibitory neurotransmitter in adults. Early in brain development, GABA actually excites neurons rather than inhibiting them. The reason is that immature neurons have a different balance of chloride transporters, which makes the reversal potential for GABA-activated currents more depolarized. In young neurons, GABA receptor activation moves the membrane voltage toward roughly negative 45 millivolts, which is close to or above the firing threshold. In mature neurons, the same receptors push voltage toward about negative 61 millivolts, firmly below threshold and therefore inhibitory.18Neuron. GABA Promotes the Switch of GABAA Receptor-Mediated Transmission from Developmentally Immature to Mature Phenotype
This excitatory-to-inhibitory switch happens as the expression of specific chloride transporters shifts during maturation. In cultured wild-type neurons, the switch occurs at a characteristic time point that correlates with increasing overall network firing rates and the maturation of excitatory glutamate synapses.19Scientific Reports. Developmental excitatory-to-inhibitory GABA-polarity switch is disrupted in 22q11.2 deletion syndrome: a potential target for clinical therapeutics When this switch is disrupted, as researchers have observed in a genetic model of 22q11.2 deletion syndrome, the balance of excitation and inhibition in neural circuits goes awry. The existence of this developmental transition means that the consequences of neuronal firing are literally different in the infant brain compared to the adult brain, which has implications for how we think about early-life seizures, anesthesia in newborns, and brain development disorders.
Blocking the Fire
Understanding exactly how neurons fire has made it possible to deliberately prevent them from firing when that is medically useful. Local anesthetics, the drugs a dentist uses to numb your jaw, work by physically entering sodium channels from the inside of the nerve membrane and blocking the sodium current that is essential for action potential generation.20PubMed. Molecular mechanisms of nerve block by local anesthetics The anesthetic molecules bind preferentially to channels that have recently been open, which means they accumulate in nerves that are actively firing. This use-dependent blocking is why local anesthetics are so effective at silencing pain-transmitting nerve fibers, which tend to fire rapidly in response to injury, while largely sparing thicker motor fibers that fire less frequently at rest.
The same principle, targeting specific ion channels at specific points in the firing cycle, runs through much of modern neuropharmacology. Antiepileptic drugs often work by stabilizing sodium channels in their inactive state, making it harder for neurons to fire in the rapid, synchronized bursts that characterize seizures. General anesthetics typically enhance inhibitory GABA signaling or dampen excitatory glutamate signaling, shifting the overall excitation-inhibition balance of the brain toward silence. Even the caffeine keeping you awake acts by blocking adenosine receptors that would otherwise dampen neuronal excitability. In each case, the drug’s mechanism maps directly onto one or more steps in the firing-and-signaling sequence.
How Scientists Watch Neurons Fire
Much of what we know about neuronal firing comes from increasingly powerful recording technologies. The classic method is electrophysiology, inserting a fine electrode near or inside a neuron and measuring voltage changes directly. Modern versions use multi-electrode arrays like Neuropixels probes, which can record from hundreds of neurons simultaneously with high temporal precision. An alternative approach uses two-photon calcium imaging, which takes advantage of the calcium influx that accompanies each action potential: neurons are loaded with or genetically engineered to express a calcium-sensitive fluorescent indicator, and a laser microscope scans through brain tissue to detect which cells light up when they fire.
Each method has strengths and blind spots. Recent work comparing the two in the mouse visual cortex found that neurons in different cortical layers responded differently to visual versus movement-related signals, with superficial neurons more strongly driven by visual stimuli and deeper neurons more strongly modulated by the animal’s own movement.10PubMed Central. Temporal coding carries more stable cortical visual representations than firing rate over time Electrophysiology captures fast spike timing with sub-millisecond resolution but samples relatively blindly from whatever neurons happen to be near the electrode tip. Calcium imaging can target genetically defined cell types and image large populations, but its temporal resolution is slower, since calcium signals rise and fall over tens of milliseconds rather than the sub-millisecond timescale of the electrical spike itself. The field is converging on using both approaches together, each compensating for the other’s weaknesses, to build a more complete picture of how firing patterns across thousands of neurons give rise to perception, decision-making, and behavior.