Neurons exist to transmit information. They do this by converting incoming signals into brief electrical impulses, sending those impulses along their length, and then passing the message to other cells through chemical or electrical connections. This transmit-and-relay function underpins everything your brain does, from registering a stubbed toe to recalling a childhood memory. But the seemingly simple job description hides a remarkable amount of internal machinery, and neurons turn out to do far more than just pass signals down a wire.
Generating the Electrical Signal
A neuron at rest maintains a slight negative charge on its interior relative to the outside. When the cell receives enough stimulation, specialized protein channels in its membrane snap open and let positively charged ions rush in, briefly reversing that charge. This rapid flip is called an action potential, and it lasts only about a millisecond. The action potential is the neuron’s fundamental unit of communication: an all-or-nothing electrical event that either fires at full strength or doesn’t fire at all. Different types of ion channels fine-tune the signal. Even neurons that do not typically produce full-blown action potentials can possess fast sodium currents that sharpen how quickly they respond to input.1PubMed Central. Voltage- and calcium-gated ion channels of neurons in the vertebrate retina
Where the Signal Starts
Not every part of the neuron is equally excitable. The action potential typically begins at a short stretch of the axon right next to the cell body, called the axon initial segment. This region is packed with a high density of sodium channels, making it the most electrically sensitive spot on the cell.2PubMed Central. Spatial mismatch between the Na+ flux and spike initiation in axon initial segment Direct recordings from various types of brain neurons confirm that action potentials typically ignite roughly 20 to 40 micrometers from the cell body, in the far end of this segment.3Neuron. The Axon Initial Segment: The Station for Action Potential Initiation and Signal Processing Beyond just launching signals, this zone also shapes them: it influences how fast a neuron can fire in succession and how strongly the output reflects the inputs the cell body has gathered.4PubMed Central. The Axon Initial Segment: An Updated Viewpoint
How the Signal Travels Down the Axon
Once an action potential fires, it needs to reach the other end of the neuron, and some axons stretch surprisingly far. Motor neurons running from the spinal cord to your foot can be over a meter long. Speed matters here. In many neurons, glial cells wrap the axon in a fatty insulation called myelin. Rather than crawling continuously along the membrane, the electrical impulse jumps from one gap in the myelin to the next. This “saltatory” conduction dramatically accelerates signal travel.5PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit
Modeling work shows that adding just a few layers of myelin membrane produces a large speed gain, mostly because the wrapping lowers the electrical capacitance of the covered stretches. Full speed is reached at about 16 layers, with only modest improvement beyond that.6Cell. Biophysics of Action Potential Initiation in Myelinated Central Mammalian Axons The practical result is that myelinated axons can carry signals at speeds well over 100 meters per second, while unmyelinated ones may crawl at less than one meter per second. This speed difference is why diseases that strip myelin, like multiple sclerosis, can devastate motor control and sensation.
Handing Off the Message at Synapses
When the action potential arrives at the end of the axon, the neuron needs to pass its message to the next cell. At most synapses in the brain, this handoff is chemical. The incoming electrical signal opens calcium channels in the nerve terminal, and the sudden rise in calcium concentration triggers tiny membrane-bound packets of chemical messenger to fuse with the cell surface and spill their contents into the gap between the two cells.7PubMed Central. Calcium control of neurotransmitter release The entire process, from calcium entry to messenger release, takes only a few hundred microseconds.
The system is not a simple one-to-one relay. How much neurotransmitter gets released depends steeply on how much calcium enters, and the relationship is nonlinear. The pool of vesicles ready for immediate release itself changes size depending on calcium levels, adding another layer of regulation.8Journal of Neuroscience. Presynaptic Calcium Influx Controls Neurotransmitter Release in Part by Regulating the Effective Size of the Readily Releasable Pool Recent work has also clarified that just a handful of key proteins orchestrate the timing of release. Two calcium-sensing proteins compete for the same docking machinery: one drives fast release within a millisecond or so, while the other triggers a slower, delayed wave of release. This competition helps explain why a synapse’s behavior changes depending on how recently it was active.9PubMed Central. Minimal presynaptic protein machinery governing diverse kinetics of calcium-evoked neurotransmitter release
Receiving Signals and Adding Them Up
On the receiving end of a synapse, the released chemical messenger binds to receptor proteins on the next neuron. These receptors fall into two broad classes. Some are ion channels themselves that open immediately upon binding, producing a fast electrical jolt in the receiving cell. Others work indirectly through internal signaling cascades, producing slower and longer-lasting effects. A single neuron in the cortex can receive thousands of synaptic inputs, some pushing it toward firing and others pushing it away from firing. The cell body acts as a kind of ballot counter, continuously summing these excitatory and inhibitory votes. Only when the tally crosses a threshold does the axon initial segment launch a new action potential.
Interestingly, the dendrites (the branching input arms of the neuron) are not just passive receivers. Recordings from single dendritic branches show that the relationship between the number of active synapses and the resulting voltage is far from simple addition. Inputs arriving near the tips of a dendrite get amplified much more strongly than inputs arriving near the branch point: the gain of the input-output curve increases more than threefold from the base to the tip of a single branch.10Neuron. Dendritic Integration Gradients in Single Cortical Pyramidal Cell Dendrites The same study found that distant synapses are also better at summing up inputs that arrive slightly out of sync, with temporal summation nearly doubling from base to tip. In effect, each dendritic branch can perform its own local computation before passing a processed result to the cell body. This turns a single neuron into something far more powerful than a simple on-off switch.
The Internal Supply Chain
Because neurons can be extraordinarily long, they face a logistical challenge that most cells never deal with: getting the right materials to the right place. Synaptic vesicles, mitochondria, ion channels, and signaling molecules are assembled near the cell body and then actively shipped down the axon by molecular motor proteins that walk along internal tracks called microtubules. One family of motors, kinesins, carries cargo outward from the cell body toward distant nerve terminals. Another family, dyneins, hauls cargo back.11Neuron. Axonal Transport: Mechanisms and Regulation The distances involved can be remarkable: vesicles formed near the nucleus may travel more than a meter to reach a presynaptic terminal in a peripheral nerve.12PubMed Central. Axonal transport: Driving synaptic function
This transport system is not just about supplying fresh parts. Return-trip cargoes include aged organelles, protein clumps, and growth-factor signals that inform the cell body about conditions at distant terminals.13PubMed. Cytoplasmic dynein and microtubule transport in the axon: the action connection When axonal transport breaks down, misfolded proteins can accumulate and synapses starve of supplies. Defective transport has been linked to neurodegenerative conditions including ALS and Alzheimer’s disease, underscoring that a neuron’s ability to signal depends on a healthy logistics network, not just healthy ion channels.
Strengthening and Weakening Connections
If neurons only relayed fixed-strength messages, the brain would be incapable of learning anything new. Synapses can change their strength based on experience. When a synapse is repeatedly activated in a pattern that matters, its transmission efficiency can increase for hours, days, or longer. This phenomenon, called long-term potentiation, was recognized early on as a plausible mechanism for how the brain stores information.14PubMed. Half a century legacy of long-term potentiation The reverse process, long-term depression, weakens synapses that are consistently out of step with the neuron’s firing. Together, these bidirectional changes allow neural circuits to sculpt themselves around experience, keeping useful connections strong and pruning irrelevant ones.
The metabolic fuel for these changes involves an unexpected partner. Astrocytes, the star-shaped support cells that surround synapses, break down their stored glycogen into lactate and shuttle it to neurons as an energy source. Disrupting this lactate shuttle in the hippocampus, a brain region critical for memory, produces amnesia in animal studies. The memory loss can be rescued by supplying lactate directly, but only if the neuron’s own lactate importer is intact.15Cell. Astrocytic Glycogenolysis Provides Lactate for Long-Term Memory Formation So a neuron’s ability to learn is not purely its own achievement; it depends on metabolic cooperation with neighboring glial cells.
The Role of Support Cells in Neuronal Function
Astrocytes do more than supply fuel. A broader framework describes a metabolic partnership in which astrocytes produce abundant reactive oxygen species in their own mitochondria and offset the cost by exporting glycolytically derived metabolites to neurons, helping sustain their energy needs and protect them from oxidative stress.16PubMed. Astrocyte-neuron metabolic cooperation shapes brain activity Lactate from astrocytes does not merely serve as a backup fuel; a substantial body of evidence frames it as a routine energy substrate for active neurons.17PubMed Central. Brain energy homeostasis: the evolution of the astrocyte-neuron lactate shuttle hypothesis
This means that asking “what does a neuron do?” without considering its support cells gives an incomplete picture. Myelinating glia speed electrical conduction, astrocytes supply fuel and handle waste, and microglia prune unused synapses during development. The neuron is the star performer, but it plays on a stage built and maintained by these other cell types.
Electrical Synapses and Direct Coupling
Not every neuron-to-neuron connection involves releasing a chemical messenger. Some neurons are linked by gap junctions, protein channels that directly bridge the membranes of two adjacent cells and allow ionic current to flow between them. These electrical synapses are found across brain regions and across species, and their properties are simpler than chemical synapses in some ways: current flows in both directions and almost instantaneously.18PubMed Central. Synchrony and so much more: Diverse roles for electrical synapses in neural circuits
Despite that simplicity, electrical synapses play a surprisingly wide range of roles. They can synchronize groups of neurons so that they fire in lockstep, which is useful for generating rhythmic activity. But depending on circuit context, they can also desynchronize, inhibit, filter noise, or detect coincidences.19Neuron. Electrical Synapses and Gap Junctions in the Mammalian Brain The common textbook impression that electrical synapses are a primitive relic, less important than their chemical counterparts, undersells them considerably.
Inhibitory Neurons and Brain Rhythms
Roughly one in five cortical neurons uses the chemical messenger GABA, which inhibits rather than excites the receiving cell. These inhibitory interneurons are crucial for temporal precision in brain circuits. By periodically silencing groups of excitatory neurons, they create windows of opportunity for coordinated firing, giving rise to the oscillations (brain waves) that electroencephalograms detect on the scalp.20PubMed Central. Inhibitory Interneurons Regulate Temporal Precision and Correlations in Cortical Circuits Synchronized oscillations, particularly in the gamma-frequency range, depend on rhythmic inhibition delivered by specific subtypes of GABA neurons.21PubMed Central. GABA neurons and the mechanisms of network oscillations: implications for understanding cortical dysfunction in schizophrenia
Without inhibition, excitatory signaling would spread unchecked, and the brain would lose the ability to route information precisely. This is visible in epilepsy, where a breakdown in inhibitory control allows waves of uncontrolled excitation to sweep across cortical tissue. Inhibitory neurons, in other words, are not an afterthought. They are a core part of the neuron’s collective function: shaping when and where signals flow.
Neuromodulation Changes the Rules
Sitting alongside the fast excitatory and inhibitory neurotransmitters are slower-acting chemicals called neuromodulators. Dopamine, serotonin, and norepinephrine are the best-known examples. Rather than directly opening ion channels, these substances alter how neurons and circuits respond to their ongoing inputs. They can shift a whole network’s operating state, influencing alertness, mood, motivation, and movement.22PubMed. Monoamines: Dopamine, Norepinephrine, and Serotonin, Beyond Modulation, “Switches” That Alter the State of Target Networks When you feel suddenly more focused after a surprise, or sluggish on a gray afternoon, part of what has changed is the neuromodulatory tone bathing your neural circuits. This means the same neuron, receiving the same synaptic input, can produce a different output depending on its chemical environment.
When Signaling Goes Wrong
Glutamate is the most widely used excitatory neurotransmitter in the mammalian brain, and under normal conditions it is essential for everything from breathing to abstract thought. But excessive glutamate release, as happens during a stroke or traumatic brain injury, overstimulates receiving neurons to the point of damage. Calcium floods in through glutamate receptors, overwhelms the cell’s mitochondria, and triggers a cascade that kills the neuron.23PubMed Central. Excitotoxicity, calcium and mitochondria: a triad in synaptic neurodegeneration This process, called excitotoxicity, illustrates a central paradox of neuronal function: the very mechanism that makes signaling possible (calcium entry driving neurotransmitter release) becomes destructive when its volume knob is turned too high.
The vulnerability goes both ways. Too little signaling, as in neurodegenerative diseases that kill off dopamine-producing or acetylcholine-producing neurons, produces its own set of catastrophic failures: the tremor and rigidity of Parkinson’s disease, or the memory collapse of Alzheimer’s. The healthy brain depends on signaling that stays within a narrow window, maintained by a web of feedback mechanisms involving ion channels, transporters, enzymes, and support cells.
Sensory Neurons Turn the World into Electricity
One specialized class of neurons handles the very first step in perception: converting physical stimuli into electrical signals. Light, sound, pressure, heat, and chemical flavors are all different forms of energy, and each requires its own receptor machinery. In vertebrates, sensory detection generally relies on two categories of receptor proteins: ion channels that respond directly to mechanical force, temperature, or certain chemicals, and G-protein-coupled receptors that handle vision, smell, and some tastes through internal signaling cascades.24PubMed Central. Signaling by sensory receptors In either case, the end result is the same: a change in the neuron’s membrane voltage that, if strong enough, triggers action potentials headed for the brain. The diversity of sensory neurons is staggering. The retina alone contains dozens of ganglion cell types, each wired to extract a different feature of the visual scene, such as upward motion or contrast edges.25Neuron. Cell Types in the Brain and Nervous System
Neurons Did Not Always Look Like This
The elaborate, electrically independent neurons in our brains are not the only way an animal can wire a nervous system. Ctenophores, the comb jellies that rank among the earliest-branching animal lineages, have a nerve net in which neurons share a continuous membrane, forming a syncytium rather than discrete, individually sealed cells.26PubMed Central. Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems This arrangement suggests that the private, all-or-nothing action potential propagated by a single cell is not the only solution evolution has found for transmitting information across an animal body. It also raises the possibility that neurons as we know them evolved independently more than once, which would mean the function of information transmission was so valuable that biology converged on similar solutions from different starting points.