How Neurons Work

Your thoughts are a very large number of tiny cells passing electrical sparks and chemical notes to one another.

By Greg Goodwin · Published October 17, 2026 · Updated October 24, 2026

Cheat Sheet

Count: an adult male brain holds about 86.1 billion neurons (plus or minus 8.1 billion) and about 84.6 billion non-neuronal cells; the old figure of 100 billion was never directly measured.
Location: only about 19% of the brain's neurons are in the cerebral cortex, even though the cortex makes up about 82% of brain mass.
All or none: an action potential fires fully or not at all; textbooks put the threshold near -55 mV and the peak near +30 to +40 mV.
Speed: large myelinated axons conduct at roughly 100 to 120 meters per second; published figures vary by source.
1906: Camillo Golgi and Santiago Ramon y Cajal shared the Nobel Prize 'for their work on the structure of the nervous system.'
1921: Otto Loewi's frog-heart experiment showed nerves can act chemically; the substance was later identified as acetylcholine (Nobel Prize, 1936, shared with Henry Dale).

The 60-Second Version

A neuron is a nerve cell built to carry signals. Most have a cell body holding the nucleus, branching dendrites that collect incoming messages, and a long axon that sends the signal onward. At rest, a neuron keeps a small electrical charge across its membrane, roughly 70 millivolts negative inside relative to outside. When enough input arrives, ion channels open, sodium rushes in, and a brief voltage spike called an action potential races down the axon. At the far end, the signal reaches a synapse, a microscopic gap, where the neuron usually releases chemical messengers that cross over and either excite or quiet the next cell. Fatty myelin insulation speeds the signal in many axons. The human brain contains tens of billions of neurons, and learning works by strengthening or weakening the connections between them.

The Long Version

The Anatomy of a Signaling Cell

Britannica describes the neuron as the basic cell of the nervous system, with a cell body containing a nucleus and two or more long fibers. In the typical picture, dendrites branch like a tree to receive input, the cell body integrates it, and a single axon carries the output. Axons can be many times longer than the cell body itself, and many are wrapped in segments of myelin, with small uninsulated gaps between them called nodes of Ranvier. Neurons come in functional families: sensory neurons carry information in from the body, motor neurons carry commands out to muscles, and interneurons connect them to each other inside the brain and spinal cord.

Charged at Rest

A resting neuron is like a battery. Its membrane is studded with channels and with pumps, including the sodium-potassium pump, that keep different concentrations of ions inside and outside the cell. Because potassium leaks out more readily than other ions leak in, the inside ends up negative, typically around -70 millivolts, though textbooks give a range and the exact value differs between cell types. That stored difference is a form of potential energy, waiting to be released when a signal arrives.

The Spike

If incoming signals push the membrane to a threshold, around -55 millivolts in textbook examples, voltage-gated sodium channels snap open. Sodium floods in and flips the inside of the cell positive, to a peak of roughly +30 to +40 millivolts. Then the sodium channels close and potassium channels open, potassium flows out, and the cell resets. This sequence is the action potential, and it is all or none: it does not get bigger for a stronger stimulus, but a stronger stimulus can produce more spikes per second. Alan Hodgkin and Andrew Huxley worked this out in squid giant axons. By replacing sodium in the surrounding fluid with choline, they showed that the early inward current is carried by sodium and the later outward current by potassium. Their model, published in 1952, earned them a share of the 1963 Nobel Prize in Physiology or Medicine with John Eccles. In myelinated axons the spike effectively jumps from one node of Ranvier to the next, which is why those axons are so much faster.

Crossing the Gap

For a long time, scientists argued over whether the nervous system was a continuous web or made of separate cells. Santiago Ramon y Cajal, using Camillo Golgi's silver-staining method, showed that neurons are individual cells that communicate through contacts later called synapses; the two shared the 1906 Nobel Prize. A second debate was whether the signal crossing a synapse is electrical or chemical. In 1921 Otto Loewi stimulated the vagus nerve of one frog heart, slowing it, then transferred fluid from that heart to a second one and slowed it too, proving a soluble chemical carried the message. That substance, later identified as acetylcholine, was an early neurotransmitter. Neurotransmitters can excite or inhibit; glutamate and GABA are generally described as the brain's main excitatory and inhibitory messengers.

Counting and Rewiring

For decades, textbooks said the brain had 100 billion neurons. A 2009 study using the isotropic fractionator method counted about 86 billion in adult male brains, along with a similar number of non-neuronal cells, and noted that the 100 billion figure had never been directly measured. Only about 19% of those neurons are in the cerebral cortex. Counting cells is just the start, though. Experience reshapes the strength of synapses, a property called neuroplasticity, and when axons lose their myelin, as in multiple sclerosis, signals slow and falter. This is general science, not medical advice.

Why People Care

Every sensation, movement, memory, and mood runs through neurons, so understanding them is the foundation for neuroscience, brain medicine, and even artificial neural networks. It also frames how learning, memory, and many brain disorders are studied, since so much of it comes down to how neurons connect and signal.

Glossary

Action potential: A brief, self-propagating electrical spike that travels along a neuron's axon.
Axon: The long fiber that carries a neuron's signal away from the cell body toward other cells.
Synapse: The tiny junction where one neuron passes a signal to another cell.
Neurotransmitter: A chemical released at a synapse that excites or inhibits the receiving cell.
Myelin: A fatty insulating sheath around many axons that speeds up signal conduction.

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