How the Brain Works: Neurons, Synapses, Networks

The three words to know

Almost everything on this page rests on three terms. Fix these and the rest follows naturally.

What a neuron is

A neuron is a cell built for communication. At its centre is the cell body; reaching out from it are dendrites, fine branches that gather signals from other cells, and a single longer fibre, the axon, that carries the neuron's own signal onward. Some axons are microscopically short, linking neighbours; others stretch a remarkable distance, reaching from the brain down the spinal cord. Many are wrapped in myelin, the fatty insulation that lets a signal travel faster, and it is bundles of these insulated axons that form the brain's white matter.

What makes a neuron special is not any one part but its job: it takes in many inputs, weighs them, and decides whether to pass a signal on. Multiply that simple act by tens of billions of cells, each wired to thousands of others, and you begin to see how a lump of tissue can compute. The magic is not in the individual cell, which is fairly simple, but in the scale and pattern of the wiring.

Neurons are also not alone in there. They are outnumbered, or at least matched, by supporting cells called glia, which were long dismissed as mere packing but are now known to insulate axons, tidy away chemical debris, and help regulate how signals pass. The old picture of the brain as neurons alone has given way to one in which several cell types cooperate. For understanding signalling, though, the neuron remains the star, so the rest of this page keeps the spotlight on it while remembering that it never works in isolation.

How a single signal travels

The clearest way to grasp neural signalling is to follow one signal from arrival to hand-off. The journey has a small number of steps, and it repeats, in various forms, billions of times a second across the brain.

1. Inputs arrive

   Signals from many other neurons land on the cell's dendrites. Some are excitatory, nudging the cell towards firing; others are inhibitory, holding it back. The neuron is constantly adding these pushes and pulls together.

2. A threshold is crossed

   If the combined push is strong enough to cross a certain threshold, the neuron fires. Below that line, nothing happens. This all-or-nothing rule is why neural signalling is often compared to a switch rather than a dimmer.

3. An electrical pulse fires

   Firing sends a brief electrical spike, the action potential, racing down the axon. It is regenerated along the way, so it arrives at the far end just as strong as it began, and myelin lets it skip ahead faster still.

4. The synapse converts the signal

   At the axon's tip the electrical pulse triggers the release of chemical messengers, neurotransmitters, into the narrow synaptic gap. The signal has now switched from electrical to chemical to cross the space between cells.

5. The next neuron receives it

   Those messengers land on the next neuron's dendrites, nudging it towards or away from firing. That cell now begins the same weighing process, and the cycle continues onward through the network.

Two features of this process matter enormously. First, synapses are not fixed: their strength changes with use, which is the physical basis of learning and memory. Connections that fire together repeatedly tend to strengthen, so experience literally reshapes the wiring. Second, because every neuron sums both excitatory and inhibitory inputs, the brain is as much about suppressing signals as sending them. Well-timed inhibition is what keeps activity from spreading into a useless storm.

From cells to networks

Here is the step that changes how you understand the whole brain. A neuron on its own can do almost nothing recognisable. It cannot see a face or recall a name. Those abilities appear only when huge numbers of neurons, wired into a specific pattern and firing in coordination, act together. The function lives in the pattern of connection and activity, not in any component. This is what neuroscientists mean when they call the brain a network, and it is why the same cells can contribute to quite different tasks depending on what they are linked with at the moment.

This network view resolves an apparent puzzle from the previous pages. If the hippocampus is for memory and the occipital lobe for vision, how can any complex act, which needs many faculties at once, ever get done? The answer is that these regions are not sealed departments but hubs joined by dense white-matter cables, constantly exchanging signals. A single memory of a face might weave together visual regions, memory structures, and emotional ones into one momentary network. Take any node away and the whole pattern shifts.

It also explains the sheer scale involved. With roughly 86 billion neurons, each forming thousands of synapses, the number of connections runs into the hundreds of trillions, a web of a complexity that is genuinely hard to picture. That vastness is not idle: it is what lets the same set of regions form countless different momentary patterns, one for each thing you might perceive, recall, or do. When neuroscientists say the brain is more like a weather system than a filing cabinet, this is what they mean. The parts are fixed, but the patterns of activity flowing across them are fluid, ever-changing, and never quite the same twice.

Where to go next

With the cellular picture in place, the natural next question is how all this activity connects to mind and action. That is the subject of brain and behaviour. To revisit which regions host these networks, return to the major brain regions. And the research page lays out how confident we can be about all of it.