Neurons are the core signaling cells of the nervous system, responsible for carrying information between your brain, spinal cord, and every other part of your body. The human brain alone contains somewhere between 61 and 99 billion of them, with a commonly cited estimate of 86 billion. Each neuron communicates through a combination of electrical impulses and chemical signals, forming vast networks that control everything from breathing and heart rate to thought, memory, and emotion.
How a Neuron Is Built
A neuron has three main parts, each with a distinct job. Dendrites are branch-like extensions that receive incoming signals from other neurons. They can also synthesize proteins and communicate independently with neighboring cells. The cell body, or soma, houses the nucleus and the machinery that keeps the neuron alive and functioning. The axon is a long, cable-like projection that carries electrical impulses away from the cell body and ends at a terminal where chemical messengers are released to pass the signal along to the next cell.
This structure creates a one-directional flow: signals arrive through the dendrites, get processed in the cell body, travel down the axon, and leave through the terminal. That basic architecture is shared by neurons throughout the body, whether they sit in the brain or stretch from your spinal cord to your toes.
Electrical Signals Inside the Neuron
At rest, a neuron holds a small electrical charge of roughly negative 70 to negative 80 millivolts across its outer membrane. This “resting potential” exists because of an uneven distribution of charged particles (ions) on either side of the membrane. The inside of the cell is more negative than the outside, creating a kind of loaded spring.
When a neuron receives enough stimulation from its neighbors, the voltage at a specific trigger zone crosses a threshold. At that point, channels in the membrane snap open, positive ions rush in, and the voltage flips sharply positive. This rapid voltage swing, called an action potential, races down the length of the axon like a pulse traveling along a wire. It’s an all-or-nothing event: the signal either fires at full strength or doesn’t fire at all.
Chemical Signals Between Neurons
Neurons don’t physically touch each other. A tiny gap called a synapse separates one neuron’s axon terminal from the next neuron’s dendrite. When an electrical impulse reaches the end of the axon, it triggers calcium to flow into the terminal. That calcium influx causes small packets of chemical messengers, called neurotransmitters, to spill into the gap.
Those neurotransmitter molecules drift across the synapse and lock onto receptors on the receiving neuron. Depending on the type of neurotransmitter involved, the effect on the next neuron falls into one of three categories:
- Excitatory: The signal encourages the next neuron to fire. Glutamate is the most common excitatory neurotransmitter in the nervous system.
- Inhibitory: The signal discourages the next neuron from firing, effectively putting the brakes on. GABA is the most common inhibitory neurotransmitter in the brain, while glycine plays that role in the spinal cord.
- Modulatory: The signal adjusts how strongly other neurotransmitters affect the receiving cell, fine-tuning the overall response rather than simply switching it on or off.
Other well-known neurotransmitters include serotonin (generally inhibitory), norepinephrine and epinephrine (excitatory, involved in alertness and the stress response), and acetylcholine (excitatory, important for muscle control and memory). The balance between excitatory and inhibitory signaling across billions of synapses is what allows the nervous system to produce coordinated, appropriate responses rather than chaotic activity.
Three Types of Neurons, Three Jobs
Not all neurons do the same work. They fall into three broad functional categories based on the direction they carry information.
Sensory neurons detect stimuli from the outside world or from inside the body, such as pressure, temperature, pain, and the position of your limbs, and relay that information toward the brain and spinal cord. In the spinal cord, different types of sensory processing cells are organized in layers: those handling pain, heat, and itch tend to sit in the outermost layers, while those handling touch and body position are located deeper.
Motor neurons carry instructions outward from the brain and spinal cord to muscles and glands, translating decisions into actions. Every voluntary movement you make, from typing to walking, depends on motor neurons delivering precisely timed signals to the right muscle fibers.
Interneurons sit between sensory and motor neurons and make up the vast majority of neurons in the brain. They process, interpret, and route information. In the spinal cord, for example, interneurons can relay pain signals up to the brain while simultaneously triggering motor circuits that pull your hand away from a hot surface. Inhibitory interneurons in the spinal cord also regulate how intensely you feel pain and itch through gate-control mechanisms, and these circuits are a major target for opioid pain medications.
How Myelin Speeds Up Signals
Many axons are wrapped in a fatty insulating layer called myelin, produced by specialized support cells. Myelin doesn’t cover the axon continuously. Instead, it leaves small exposed gaps spaced about a millimeter apart. Because the insulation prevents electrical current from leaking out along the covered sections, the signal effectively jumps from one gap to the next. This jumping pattern, called saltatory conduction, dramatically increases speed while conserving energy.
The difference is striking. Unmyelinated nerve fibers conduct signals at roughly 0.5 to 10 meters per second. Myelinated fibers can reach up to 150 meters per second, fast enough to send a signal from your spinal cord to your foot and back in a fraction of a second. This speed is critical for reflexes, coordination, and any task that requires rapid communication across long distances in the body.
How Neurons Strengthen Connections
Neurons are not static wiring. The connections between them change with experience, a property known as synaptic plasticity. When two connected neurons are repeatedly activated together within a narrow time window (around 100 milliseconds), the synapse between them becomes stronger, producing a larger response in the receiving cell. This phenomenon, called long-term potentiation, was first demonstrated in the early 1970s when researchers found that a few seconds of rapid stimulation could enhance signaling at a synapse for days or even weeks.
Long-term potentiation has several features that make it a likely foundation for learning and memory. It requires the sending and receiving neurons to be active at nearly the same time, matching a long-standing theoretical prediction about how the brain encodes associations. It also displays a property called associativity: a weak connection that wouldn’t normally strengthen on its own can be boosted if a neighboring, stronger pathway to the same neuron fires at the same time. This means neurons can link related pieces of information simply because they arrive together.
New Neurons in the Adult Brain
For most of the twentieth century, scientists believed adults could not grow new neurons. That view has changed. In animals, new neurons are generated in at least two brain regions: a zone lining the brain’s fluid-filled cavities (the subventricular zone) and a part of the hippocampus, a structure central to memory. In rodents, neurons born in the subventricular zone migrate to the olfactory bulb and integrate into smell-related circuits.
In humans, the picture is more limited. Subventricular neurogenesis appears to be rudimentary compared to other animals, contributing only modestly to olfactory circuits. The stronger evidence points to the hippocampus, where new neurons generated in a region called the subgranular zone are thought to support memory, learning, and resilience to stress. Some research has also detected signs of new neuron growth in the striatum, a region involved in movement and reward, though those findings haven’t been independently replicated. Adult neurogenesis in humans remains an active and debated area of science, but its existence in at least some brain regions is now well established.