A neuron works by converting incoming signals into a brief electrical impulse, then passing that impulse to the next cell using chemical messengers. This process, repeated billions of times per second across your nervous system, is the basis of every thought, movement, and sensation you experience. The entire sequence, from receiving a signal to passing it forward, takes just milliseconds.
Parts of a Neuron
A typical neuron has three main parts, each with a distinct job. Dendrites are branch-like extensions that receive incoming signals from other neurons. They funnel those signals toward the cell body, or soma, which contains the nucleus and the machinery that keeps the cell alive. If the combined incoming signals are strong enough, the soma triggers an electrical impulse that travels down the axon, a long cable-like projection that carries the signal away from the cell body toward its target.
At the far end of the axon are small bulb-shaped structures called axon terminals. This is where the electrical signal gets converted into a chemical one. The terminals release messenger molecules into a tiny gap between neurons, completing the handoff to the next cell in the chain.
The Resting Neuron
Even when a neuron isn’t firing, it’s doing important work. The cell actively maintains an electrical charge difference across its membrane, with the inside sitting at roughly -70 millivolts compared to the outside. This baseline voltage is called the resting potential, and it exists because the neuron carefully controls which charged particles (ions) sit on which side of its membrane.
Potassium ions are concentrated inside the cell (about 120 millimolar inside versus just 4 outside), while sodium ions are packed on the outside (140 millimolar outside versus 14 inside). The membrane is far more permeable to potassium at rest, which is why the resting voltage sits close to potassium’s natural balance point of -90 millivolts rather than sodium’s balance point of +65 millivolts. Think of this charge difference as a loaded spring. The neuron spends energy maintaining it so that when it’s time to fire, it can release that stored energy almost instantly.
How a Neuron Fires
A neuron fires when the signals arriving at its dendrites push the membrane voltage from its resting -70 millivolts up to about -50 millivolts. This value is the firing threshold. Once it’s reached, the response is all-or-nothing: the neuron either fires a full-strength impulse or doesn’t fire at all. There’s no half-strength signal.
The impulse itself, called an action potential, unfolds in a rapid sequence. First, sodium channels in the membrane snap open, and sodium ions rush into the cell. This flood of positive charge drives the voltage up sharply, from -50 millivolts to as high as +30 or +40 millivolts, in less than a millisecond. Almost immediately after, potassium channels open and potassium ions flow out, dragging the voltage back down. The membrane actually dips slightly below its resting level for a brief moment before settling back to -70 millivolts. This brief overshoot creates a short refractory period during which the neuron can’t fire again, which prevents signals from traveling backward.
This wave of charge change doesn’t stay in one spot. It ripples down the length of the axon like a chain of dominoes, each section of membrane triggering the next.
Why Some Signals Travel Faster
Not all neurons transmit at the same speed. The difference comes down to insulation. Many axons are wrapped in a fatty coating called myelin, produced by specialized support cells. In the brain and spinal cord, cells called oligodendrocytes do this work, with a single cell wrapping segments of multiple axons. In the rest of the body, Schwann cells handle the job, each one covering a segment of a single axon.
Myelin doesn’t cover the axon continuously. Small gaps between the wrapped segments force the electrical signal to jump from gap to gap rather than creeping along the entire length. This dramatically increases speed. A myelinated axon can carry signals at up to 120 meters per second (about 270 miles per hour), while an unmyelinated one tops out around 2 meters per second. In practical terms, large myelinated fibers carrying motor and sensory information in humans typically conduct at 50 to 70 meters per second.
Crossing the Gap Between Neurons
Neurons don’t physically touch each other. Between the axon terminal of one neuron and the dendrite of the next sits a tiny gap called the synaptic cleft, only about 20 to 24 nanometers wide (roughly a thousand times thinner than a human hair). The electrical impulse can’t jump this gap, so the neuron switches to chemical signaling.
When the action potential arrives at the axon terminal, it triggers voltage-sensitive calcium channels to open. Calcium ions flood in from outside the cell, and this calcium surge causes tiny packets (vesicles) filled with chemical messengers to fuse with the cell membrane and spill their contents into the cleft. These messenger molecules, called neurotransmitters, drift across the gap and lock onto receptor proteins on the receiving neuron, like a key fitting into a lock. That binding either nudges the receiving neuron closer to firing or pushes it further from firing, depending on the type of neurotransmitter and receptor involved.
Excitatory and Inhibitory Signals
Not every signal tells the next neuron to fire. Some push it toward firing (excitatory), and others hold it back (inhibitory). The brain’s most common excitatory neurotransmitter is glutamate. When glutamate binds to receptors on a receiving neuron, it opens channels that let positive ions flow in, pushing the membrane voltage up toward the firing threshold.
The brain’s main inhibitory neurotransmitter is GABA. When GABA binds to its receptors, it opens channels that let negatively charged chloride ions flow into the cell, pulling the voltage down and making it harder to reach the -50 millivolt threshold needed to fire. At any given moment, a neuron is receiving both excitatory and inhibitory inputs from hundreds or thousands of other neurons simultaneously. It fires only when the sum of all those inputs pushes it past threshold. This constant balancing act is how the brain fine-tunes its activity, amplifying some signals while suppressing others.
Clearing the Signal
Once a neurotransmitter has done its job, it needs to be removed from the synaptic cleft quickly. If it lingered, the receiving neuron would be stimulated (or inhibited) continuously, and the system would lose its ability to send distinct, timed signals. The brain uses several cleanup strategies. The most common is reuptake: specialized transporter proteins on the sending neuron (or nearby support cells) vacuum the neurotransmitter back up so it can be repackaged and used again. Some neurotransmitters are instead broken down by enzymes right there in the cleft. Many are cleared through a combination of both mechanisms, along with simple diffusion away from the synapse.
This recycling system is so central to brain function that many psychiatric medications work by altering it. Drugs that slow reuptake of certain neurotransmitters, for example, effectively increase the amount of messenger available in the cleft, boosting or prolonging its effect on the receiving neuron.
Support Cells That Keep Neurons Running
Neurons get a lot of attention, but they couldn’t function without glial cells, the support staff of the nervous system. Astrocytes, the most abundant cells in the brain, are star-shaped cells whose branching arms contact many neurons simultaneously. They supply neurons with the raw materials needed to produce energy, help regulate the chemical environment around synapses by absorbing excess ions and neurotransmitters, and form a key part of the blood-brain barrier that controls what substances can reach the brain from the bloodstream.
Microglia act as the brain’s immune cells, patrolling for damage or infection. Ependymal cells line the fluid-filled cavities of the brain and help circulate cerebrospinal fluid. Together with the myelin-producing oligodendrocytes and Schwann cells, these support cells outnumber neurons and are essential for keeping the signaling network healthy and functional.