How does the brain work? A plain-words tour

The brain works by sending signals through a vast network of nerve cells, called neurons. Here is how it does that: each neuron gathers signals from others, and when enough arrive, it fires a brief electrical pulse that releases chemicals onto the next cells. The adult human brain has about 86 billion neurons and trillions of connections between them. Different regions specialize in seeing, moving, remembering or feeling, but nearly everything you do uses many of them at once.

In short

• A neuron's job is to add up its inputs and decide whether to fire; thought, feeling and movement are patterns of that firing across billions of cells.
• Regions specialize, such as the cerebellum for coordination and the hippocampus for new memories, but they work as a team, not as separate modules.
• The brain changes with use throughout life, and the popular claims that people use only 10% of it, or are "left-brained" or "right-brained", are myths.

What is the brain made of?

An adult brain weighs about 1.4 kilograms, roughly 3 pounds, and is about as soft as firm tofu. It is made of two main kinds of cells. Neurons carry signals. Glial cells support them: they supply nutrients, clean up, insulate nerve fibers and help tune the connections. For decades, textbooks said glia outnumbered neurons ten to one. When the neuroscientist Suzana Herculano-Houzel and colleagues actually counted the cells by dissolving brains into a countable soup, they found about 86 billion neurons and about 85 billion other cells, closer to one to one.

The brain is greedy. It is only 2% to 3% of body weight but uses about 20% of the body's energy at rest, according to Scientific American, mostly to keep neurons ready to fire.

How do neurons send signals?

A neuron has three main parts, as the Queensland Brain Institute describes them. Branching dendrites receive signals from other cells. The cell body adds them up. A long fiber called the axon carries the output onward, sometimes for a meter or more, as with the nerves that run from the spine to the toes.

The key event is the action potential: a brief electrical pulse that runs down the axon when a neuron's inputs push it past a threshold. It is all or nothing, like a camera flash rather than a dimmer switch. A neuron signals "more" by firing more often, not by firing harder. Many axons are wrapped in a fatty insulation called myelin, which makes the pulse jump along much faster.

At the end of the axon is a tiny gap, the synapse, between one neuron and the next. The electrical pulse can't cross it. Instead it makes the neuron release chemical messengers, called neurotransmitters, which drift across and fit into receptors on the next cell, nudging it toward firing or away from it. Glutamate is the main "go" signal and GABA the main "stop" signal. Others, such as dopamine, adjust how strongly whole circuits respond. Drugs work at this level: caffeine, for example, keeps you alert by blocking receptors for adenosine, a chemical that builds up and makes you sleepy.

Learning is largely a matter of synapses changing. Connections that are used together tend to strengthen, and unused ones weaken or are pruned. This capacity, called neuroplasticity, continues throughout life, which is why people can learn new skills at 70 and why the brain can sometimes reroute functions after an injury.

What do the main parts of the brain do?

The brain has three big divisions: the cerebrum on top, the cerebellum tucked behind and below it, and the brainstem connecting both to the spinal cord. The Cleveland Clinic's guide sets out what each part does, summarized here.

Part	Where it is	What it mainly does
Cerebral cortex	The wrinkled outer layer of the cerebrum	Perception, language, planning, reasoning, voluntary movement
Frontal lobe	Front of the cortex	Planning, decisions, self-control, starting movements
Parietal lobe	Top and back	Touch, body position, where things are in space
Temporal lobe	Sides, near the ears	Hearing, language, memory
Occipital lobe	Back of the head	Vision
Hippocampus	Deep in each temporal lobe	Forming new memories, and mapping places
Amygdala	Next to the hippocampus	Emotion, especially fear and spotting threats
Thalamus	The center of the brain	A relay station passing sensory signals to the cortex
Hypothalamus	Below the thalamus	Hunger, thirst, temperature, hormones, sleep cycles
Cerebellum	Back, under the cerebrum	Balance, timing and fine coordination of movement
Brainstem	Base, joining the spinal cord	Breathing, heart rate, swallowing, waking and sleeping

One surprise is the cerebellum. It is only about a tenth of the brain's mass but holds about 69 billion of its 86 billion neurons, around four in five. The cortex, which makes up most of the brain's mass, holds about 16 billion.

How do the parts work together? Catching a ball

No single region "does" an action. Take a friend tossing you a ball from 10 meters away at about 10 meters a second, so it reaches you in about one second.

1. Seeing it. Light from the ball hits the retina at the back of your eye, which turns it into electrical signals. These pass along the optic nerve to the thalamus and on to the occipital lobe, reaching the visual cortex within about a tenth of a second.
2. Working out where it is going. Visual areas split the job. One stream, running up into the parietal lobe, tracks where the ball is and how fast it moves. Another, running down into the temporal lobe, recognizes it as a ball.
3. Predicting. Here is the problem: a simple visual reaction takes around a fifth of a second, and in 0.2 seconds the ball travels 2 meters. If you reached for where you last saw it, you'd miss. So the brain predicts where the ball will be, using what it has learned about how thrown things fall.
4. Planning the catch. Frontal areas choose the action, and the motor cortex sends commands down the spinal cord to the muscles of your arm and hand.
5. Correcting on the fly. The cerebellum compares the movement you intended with what your eyes and joints report and adjusts the timing in fractions of a second. The brainstem steadies your posture so you don't topple as you reach.
6. Feeling about it. If the ball comes at your face, fast circuits through the brainstem and amygdala can trigger a flinch before you are fully aware of the threat. If you catch it, a burst of dopamine helps the brain learn what worked.

The whole sequence takes well under a second, uses every lobe, and almost none of it reaches conscious awareness.

Do people really use only 10% of their brains?

No. Brain scans show that over the course of a day, virtually every part of the brain is active, and even small acts such as pouring a cup of coffee light up many regions at once. Damage to almost any area causes some loss, which would not be true if 90% were idle. And an organ that burns a fifth of the body's energy would be a costly thing to carry around unused. The myth's origin is unclear; it may come from misread remarks by early psychologists.

The idea that people are "left-brained", logical, or "right-brained", creative, is also a myth. Some functions do lean to one side: language is mostly handled on the left in most people, and some aspects of attention and spatial processing lean right. But when researchers at the University of Utah scanned 1,011 people in 2013, they found no sign that individuals have a stronger left or right brain overall. Both halves work together all the time, linked by a thick bundle of fibers called the corpus callosum.

What is still unknown about the brain?

A great deal. Scientists can map which regions are active during a task and record single neurons, but how billions of firing cells add up to a thought, a memory or the feeling of being you remains unsolved. This is sometimes called the "hard problem" of consciousness. Why the brain needs sleep at all is only partly understood, as is what dreams are for. Artificial neural networks borrow the idea of simple units joined by adjustable connections, but they are far simpler than real neurons.

Studying the brain from the outside, through behavior and experience, is the work of psychology. Odd cases, such as synesthesia, where a sound can bring a color, show how much the brain shapes what people perceive. This note explains how the brain works in general; it isn't medical advice.

Check yourself

• Without looking back: how does a signal get from one neuron to the next, and why can't the electrical pulse simply jump the gap?
• Why can't the brain catch a ball just by reacting to where it sees it?
• What evidence shows that the "10% of the brain" claim is wrong?

Sources

• The Human Brain in Numbers: A Linearly Scaled-up Primate Brain — Suzana Herculano-Houzel, Frontiers in Human Neuroscience, 2009
• What is a neuron? — Queensland Brain Institute, University of Queensland
• Brain: Function and Anatomy — Cleveland Clinic
• Do people only use 10 percent of their brains? — Scientific American, 2008
• An Evaluation of the Left-Brain vs. Right-Brain Hypothesis with Resting State Functional Connectivity Magnetic Resonance Imaging — Nielsen et al., PLOS ONE, 2013