Neurons are the cells that carry information throughout your body. They process everything you think, feel, sense, and do by sending rapid electrical signals to one another and converting those signals into chemical messages at the junctions between them. Your brain alone contains roughly 86 billion neurons, and they work together in networks that let you pull your hand off a hot stove, recall a childhood memory, or read this sentence.
How a Neuron Is Built
A neuron has three main parts, each with a distinct job. The cell body (soma) houses the nucleus and the machinery that keeps the cell alive and functioning. Branching off the cell body are dendrites, tree-like extensions that receive incoming signals from other neurons. Dendrites don’t just passively collect information. They can also make proteins and send their own signals to neighboring cells.
Extending from the other side of the cell body is the axon, a long cable-like fiber that carries outgoing signals away from the cell. Axons can be extremely short or stretch more than a meter, like the ones running from your spinal cord down to your feet. At the far end, the axon splits into terminals that release chemical messengers to communicate with the next cell in the chain.
Electrical Signals Inside the Neuron
Neurons communicate internally through rapid voltage changes called action potentials. At rest, a neuron holds a slight negative charge inside its membrane relative to the outside. When the cell receives enough stimulation, channels in the membrane snap open and let positively charged sodium ions rush in. This surge flips the voltage from negative to positive in about one millisecond, and it’s an all-or-nothing event: the neuron either fires at full strength or doesn’t fire at all.
Almost immediately, a second set of channels opens to let potassium ions flow out, dragging the voltage back down. These potassium channels are slower to close, so the voltage briefly dips below its normal resting level before the cell resets. A molecular pump then shuffles sodium back out and potassium back in, restoring the original balance so the neuron is ready to fire again. This entire cycle happens in just a few milliseconds, which is why neurons can fire hundreds of times per second.
Chemical Signals Between Neurons
When an electrical signal reaches the end of an axon, it can’t jump directly to the next cell. Instead, the neuron converts it into a chemical message. The axon terminal releases tiny packets of chemical messengers (neurotransmitters) into the narrow gap between cells, called the synapse. These molecules drift across the gap and latch onto specific receptors on the receiving cell’s membrane, like a key fitting into a lock. If enough neurotransmitter binds, the receiving cell generates its own electrical signal, and the chain continues.
This chemical step is what makes the nervous system so flexible. Different neurotransmitters produce different effects. Some excite the next neuron, pushing it closer to firing. Others inhibit it, making it less likely to fire. The balance between these excitatory and inhibitory inputs is how your brain fine-tunes everything from muscle coordination to mood.
How Neurons Decide Whether to Fire
A single neuron can receive thousands of incoming signals at once, some telling it to fire and others telling it to stay quiet. The cell adds all of these up through a process called integration. Signals arriving at different spots on the dendrites combine spatially, while signals arriving in quick succession from the same spot combine over time. Inputs that land closer to the base of the axon carry more weight than those arriving at distant dendritic tips.
If the combined total pushes the neuron’s voltage past a critical threshold, it fires. If not, the signals fade without producing an action potential. This constant voting process, happening across billions of neurons simultaneously, is the foundation of everything your nervous system does.
Three Types of Neurons, Three Jobs
Not all neurons do the same thing. They fall into three broad categories based on the direction they carry information.
- Sensory neurons detect what’s happening in and around your body. When you touch a hot surface, sensory neurons in your fingertips fire and relay that information to your spinal cord and brain.
- Motor neurons carry commands from the brain and spinal cord outward to muscles and glands. They control every voluntary movement, from blinking to sprinting, and also drive involuntary actions like digestion.
- Interneurons sit between the other two types, forming complex circuits that process and relay information. Most of the neurons in your brain are interneurons, and they’re responsible for the layered processing behind thought, decision-making, and coordination.
How Fast Signals Travel
Signal speed varies enormously depending on the neuron. The fastest neurons transmit at up to 200 meters per second, roughly 450 miles per hour. The slowest crawl along at less than 0.1 meters per second. The biggest factor is whether the axon is wrapped in myelin, a fatty insulating layer produced by specialized support cells called oligodendrocytes. Myelin forces the electrical signal to hop between exposed gaps along the axon rather than traveling continuously, which dramatically increases speed. Axon diameter matters too: thicker axons conduct faster than thin ones.
This range exists because not every signal needs to be fast. The sharp, immediate pain when you stub your toe travels along fast myelinated fibers, while the dull, throbbing ache that follows uses slower unmyelinated ones.
How Neurons Support Learning and Memory
Neurons don’t just transmit signals. They change over time in response to experience, a property called plasticity. When two connected neurons fire together repeatedly, the synapse between them strengthens, making the receiving neuron more responsive to future signals from that same partner. This process, called long-term potentiation, has been studied extensively in the hippocampus, a brain region central to forming new memories.
Strengthening is specific to the synapses being used. If one connection on a neuron is repeatedly activated while a neighboring connection is idle, only the active one gets stronger. This selectivity allows the brain to store precise patterns of information rather than crudely boosting everything at once. There’s also an associative property: a weak connection can be strengthened if it fires at the same time as a strong one on the same cell, which mirrors the way we learn by association. Timing matters, too. The pre- and post-synaptic cells need to be active within about 100 milliseconds of each other for the strengthening to take hold.
The Support Cells That Keep Neurons Working
Neurons don’t operate alone. They’re outnumbered and supported by glial cells, which handle the maintenance work that neurons can’t do for themselves. Astrocytes, the most abundant type, perform a long list of jobs: they form the blood-brain barrier, regulate blood flow to active brain regions, supply neurons with nutrients and antioxidants, recycle used neurotransmitters, and help regulate how synapses form and function. Oligodendrocytes produce the myelin insulation that speeds up signal transmission and also provide metabolic support to the axons they wrap.
Without these support cells, neurons would quickly run out of energy, drown in leftover neurotransmitters, and lose the insulation that makes fast signaling possible. The nervous system is very much a team effort.