Brain 101

Wiggle your toes. Before they moved, a signal left your brain and traveled the length of your body to your feet, in a fraction of a second.

86 billion nerve cells

Your brain has about 86 billion nerve cells, called neurons, and roughly as many support cells, called glia. You may have read that there are ten support cells for every neuron. That old figure doesn’t hold up: counted carefully, the ratio across the whole brain is closer to one to one, and it changes from one area to another.

Glia aren’t just packing material, either. Some make the fatty coating that speeds signals up, some clear away used messengers and help control blood flow, and some are the brain’s own immune cells, which also prune away connections that aren’t needed.

How a signal travels

A neuron has branches that collect messages, a cell body that adds them up, and one long cable, called an axon, that sends its own message on. Each neuron listens to thousands of others, some saying “go” and some saying “wait”. When enough “go” arrives, the neuron fires: the electrical charge across its outer skin flips for about a thousandth of a second, and that pulse races down the cable.

A pulse is all or nothing. A neuron can’t fire harder to say something louder. Instead it fires more often, and more neurons join in. The speed depends on the cable. Thin bare fibers carry a pulse at about a meter a second, thin fibers with a light coat of fatty myelin at roughly 5 to 30 meters a second, and thick, heavily wrapped fibers at up to about 100 meters a second. That mix of speeds is why a burn can hurt twice, as part five shows.

Crossing the gap

Neurons don’t quite touch. At the end of the cable there’s a meeting point called a synapse, with a tiny gap in it, and the message crosses the gap as a chemical messenger, which drifts across in less than a thousandth of a second and fits into receptors on the next cell. How that works, and what drugs change about it, is drawn step by step in our Drugs 101 article.

Nobody has counted the synapses in a human brain. Estimates run to a hundred trillion or more, which is a sense of scale rather than a measurement.

The main messengers

The brain uses dozens of chemical messengers. These are the ones you’ll hear about most:

• Glutamate is the main “go” signal, behind most of the brain’s fast traffic.
• GABA is the main “slow down” signal.
• Dopamine helps you want things, learn what’s worth having and get moving. It isn’t just a pleasure chemical, as part three explains.
• Serotonin has many jobs, including mood, sleep and appetite, and most of the body’s serotonin is in the gut.
• Noradrenaline, also called norepinephrine, makes you alert and ready to act.
• Acetylcholine helps with attention and learning, and it’s the signal that tells your muscles to move.
• Endorphins are the brain’s own painkillers.

Each one does more than its line here, and most jobs use several of them together.

How the brain learns

When you practice something, the connections you use get stronger and the ones you don’t use weaken. You may have heard this summed up as “neurons that fire together wire together”. It’s a catchy version of an idea from the 1940s, and it’s roughly right, with two catches. Timing matters: a connection tends to strengthen when one cell fires just before the next, and to weaken when it fires just after. And learning is about losing connections as well as gaining them. Think of a path across a lawn: walk it often and it wears in, and leave it alone and the grass grows back.

An expensive organ

Your brain is about 2% of your body weight and uses about 20% of the energy your body burns at rest, around 20 watts, which is about as much power as a dim light bulb. Most of that goes on keeping its signaling running all the time. Thinking hard adds surprisingly little, only a few percent on top, even though it can leave you feeling wiped out.

The barrier and the coat

All that energy arrives in the blood, and blood carries more than fuel. It also carries hormones, traces of whatever you ate or drank, and now and then a germ or a toxin. So the tiny vessels that thread through nervous tissue are built differently from those in the rest of your body. Their walls are made of cells sealed tightly edge to edge, with support cells pressed against the outside, and together they form the blood-brain barrier.

The barrier is selective. Oxygen and carbon dioxide pass through freely, and so do small molecules that dissolve in fat, which is why caffeine and alcohol reach your nerve cells so quickly. Glucose, the main fuel, is let in by dedicated carriers. Most germs and large molecules are kept out, and so are many medicines, which is one reason drugs for the nervous system are hard to design. A few small patches have no barrier at all. One of them, the area that triggers nausea, samples the blood and can set off vomiting when it detects a toxin.

Then there is the coat. Myelin is wrapped on in short stretches, with tiny bare gaps between them. The pulse jumps from gap to gap instead of crawling along the whole cable, a bit like crossing a stream on stepping stones. That jump makes coated cables fast, and it saves energy too, because only the gaps have to reset their charge after each pulse.

When the coat is damaged, signals slow down or stall. That’s what happens in multiple sclerosis, where the immune system attacks myelin in the brain and spinal cord, and depending on where the damage falls, it can affect vision, movement and feeling.

Put your hand flat on top of your head. Under it, past skin and bone, is a sheet of brain tissue only a few millimeters thick, folded so tightly that it fits inside your skull.

A crumpled sheet

The cortex is the brain’s wrinkled outer layer, about 2 to 4 millimeters thick, holding around 16 billion neurons. It comes in two halves, left and right, and each half has four main areas, called lobes. Every lobe is known for certain jobs, and every lobe joins in on plenty of others.

Tap any part of the brain map below to see what it does. Switch to the middle view to see the deep parts that part three is about.

The frontal lobe

Behind your forehead, the frontal lobe helps you plan, decide, speak and move on purpose. Right at the front is the prefrontal cortex, which keeps a goal in mind and nudges the rest of the brain toward it: remembering the question you meant to ask while someone else is still talking, and not blurting it out. It’s often called the brain’s boss, which misses how it works. It’s tied into the same loops as feeling and valuing, and it can’t do its job without them.

At the back of the frontal lobe, running from ear to ear like a headband, is the motor strip, which sends commands to your muscles. It’s laid out roughly as a map of your body, with extra space for the hands, lips and tongue, which do the finest work.

The parietal lobe

On top and toward the back, the parietal lobe makes sense of touch and of where your body is, works out where things are around you and helps steer your attention. It’s how you reach for your mug without looking, or find your keys in a bag by feel.

The temporal lobe

Down at the sides, near your ears, the temporal lobes handle hearing, recognizing faces, voices and objects, and, mostly on the left in most people, understanding language. It’s how you know a friend from three words of a voicemail. Tucked on their inner side are two of the deep parts from part three, the hippocampus and the amygdala.

The occipital lobe

At the back of your head, the occipital lobe takes the first steps of seeing. Seeing itself uses much more of the brain than this, and what you see is your brain’s best interpretation rather than a recording. That’s why one photo of a dress could look blue and black to some people and white and gold to others.

Two folds tucked inside

Two more areas of cortex are hidden from the outside. The insula, folded deep inside each side of the brain, reads signals from inside your body, such as your heartbeat, breathing and stomach, and helps those signals become feelings, like the butterflies before a talk. It’s sometimes called the disgust center, but it’s just as busy with pain, craving and body awareness.

The cingulate curves around the middle of the brain, just above the bridge between the two halves. Its front part is involved in noticing that something’s off, in how unpleasant pain feels and in effort. It’s at work in the “wait, that’s wrong” moment when you spot a typo (usually one second after you hit send).

Somewhere in the middle of your brain, right now, a part about the size of a sugar cube is keeping track of whether you’re warm enough, hungry or thirsty, and you’ll never notice it doing so.

Under the cortex sit smaller parts, each with jobs of its own. Switch the brain map in part two to the middle view to see where they are.

The hippocampus

Curled inside each temporal lobe, the hippocampus is needed to form new memories of what happens to you, like where you parked this morning. In 1953, a man known as H.M. had much of both of his removed, along with nearby tissue, in an operation for epilepsy. Afterward he could no longer form new everyday memories, though he kept many older ones and could still learn new movement skills.

The hippocampus doesn’t store your memories like a hard drive, though. Long-term memories are spread across the cortex, and the hippocampus helps tie each one together.

The amygdala

Next to the hippocampus sits the amygdala, a pair of small clusters named after the Greek word for almond, for their shape. It helps flag what’s important, new or unclear, and helps you learn what to fear and what to go toward. It’s famous as the brain’s fear center, and that’s an oversimplification.

The amygdala responds to good surprises as well as threats. And in one study, a woman with damage to both of hers felt no fear of snakes, spiders or a haunted house, yet she and two other people with the same damage felt sudden panic when they breathed air with extra carbon dioxide in it. Fear needs more of the brain than one spot, and scientists still argue about exactly what the amygdala adds. What stress does to it, and to the hippocampus and prefrontal cortex, is in our article on stress and the brain.

The thalamus

In the very center sits the thalamus, a pair of egg-shaped clusters that almost every sense, except smell, passes through on its way to the cortex. It’s often called a relay station, but it shapes and filters what gets through, and different areas of the cortex also talk to each other through it. It helps control sleep and waking, too.

The hypothalamus

Just below the thalamus, the hypothalamus looks after the body’s basics: temperature, hunger, thirst, the timing of sleep and the hormones released by the pituitary gland, which hangs just beneath it. It’s also where the body’s stress hormone chain starts, as part six explains. When you shiver, or get thirsty after a salty meal, the hypothalamus is involved.

The basal ganglia

On either side of the thalamus sit the basal ganglia, a group of structures that help choose which action to take, start and stop movements and turn repeated actions into habits. They’re part of why you can drive the usual route home on autopilot. In Parkinson’s disease, the cells that supply them with dopamine are lost, which is why movements become hard to start.

At their front sits the nucleus accumbens, a hub of the brain’s reward circuit. It helps turn “that was better than I expected” into wanting more. Wanting isn’t the same as enjoying, though. You can want one more episode of a show you didn’t much enjoy (autoplay is counting on it), and that gap is why dopamine isn’t really the pleasure chemical. It’s more about wanting, learning and moving. Our article on dopamine explains how wanting and enjoying come apart.

The cerebellum

At the back, tucked under the cortex, sits the cerebellum. It’s about a tenth of the brain’s size and holds around four in five of its neurons. It fine-tunes movement, balance and timing and learns from small mistakes, which is how you catch a ball or type without looking. It also plays a part in attention, language and steadying emotions, and damage to it can affect all three.

The brainstem

The brainstem connects the brain to the spinal cord. It keeps you breathing, helps control your heart rate and blood pressure, lets you swallow and cough, and houses the systems that keep you awake and alert. Damage to a few small areas here can cause a coma, which shows how much this narrow stalk does.

The bridge between the halves

The corpus callosum is a thick band of hundreds of millions of nerve fibers that links the left and right halves of the brain, so they can share what each one knows. When surgeons cut it to treat very severe epilepsy, experiments showed that each half could know things the other didn’t, and still the people seemed like one person in everyday life. Those experiments helped spark the popular idea of “left-brained” and “right-brained” people, and it doesn’t hold up: some jobs lean to one side, but a study of over 1,000 people found no sign that anyone’s brain as a whole leans left or right.

Is there a lizard brain?

You may have heard that your brain is built in layers: a reptile brain at the bottom for survival, an emotional mammal brain around it, and a thinking human brain on top. It’s a story from the 1960s, and it’s wrong. Reptiles, birds and mammals all have the same basic brain parts, evolution didn’t stack new layers on top of old ones, and the parts work as one system. Far from primitive, your brainstem keeps you alive every minute of the day.

You’re in the shower, your mind wanders to a conversation from last week, and suddenly you know exactly what you should have said.

Why scientists think in teams

For a long time, brain science looked for the spot that does each job. Today researchers also look at which areas rise and fall in activity together, and they call those groups networks. Almost every job uses several areas, and almost every area takes part in several jobs.

Network maps are worked out statistically, their borders depend on the method, and everyone’s brain differs a little from the average, so there’s no single official list. Three networks come up again and again.

The wandering mind

The default mode network is most active when you’re not focused on the world outside: when you’re remembering, imagining the future, or thinking about yourself or other people. It was discovered when researchers noticed that some areas got busier while people rested between tasks. That doesn’t mean the brain is idle at rest, since its energy use barely changes between resting and working.

Spotting what matters

The salience network, centered on the insula and the front of the cingulate, notices what matters right now, whether it’s outside you or inside your body: a buzzing phone, a smell of smoke, a racing heart. Then it helps hand the brain over to whichever network is needed.

Getting it done

The central executive network, also called the frontoparietal control network, runs through the sides of the prefrontal cortex and the parietal lobe. It holds information in mind and keeps you on task, like following a recipe while someone talks to you, or filling in a form.

Taking turns

Put these together and you get a hand-off. Your mind drifts while you read a long email, your phone buzzes, the salience network flags it, the wandering network quiets down and the executive network steps up while you reply. Then your mind drifts again. One study found that the insula seems to help drive this switch.

It’s a useful picture and a simplified one. The wandering network doesn’t switch off during every task, since it helps with memory and with thinking about people, and the switching study was small. Try the hand-off for yourself below, and choose a team on the brain map in part two to see where each one sits.

Back to the room from the start of this article. Someone says your name, and less than a second later you’ve turned your head.

Your name across a room

The sound reaches your ears and turns into nerve signals within a few thousandths of a second. The signals pass through the brainstem and thalamus to the hearing areas of the temporal lobe in a few hundredths of a second. Over the next few tenths of a second, your brain recognizes the sound as a word and then as your name, the salience network flags it, your attention swings toward it, and your frontal lobe, motor strip and cerebellum turn your head. Many of those steps overlap rather than happen one after another.

In one classic experiment, people listened to two streams of speech, one in each ear, and were told to ignore one of them. When their own name was played in the ignored ear, about a third of them noticed. Your name is usually on your brain’s short list of things worth catching, which is part of why a softly spoken name can get through a loud room. Oddly, in a later version of the study, the people who noticed most often were the ones who were less good at holding information in mind, which suggests they were also less good at blocking out what they’d been told to ignore.

The hot pan

Touch a hot pan and the order changes. Nerve endings in your skin fire, and the signal reaches your spinal cord, which pulls your hand away before your brain has registered any pain at all. A copy of the signal travels on up to the thalamus, the parietal lobe works out where it hurts, and the insula and cingulate add how bad it feels. Pain often arrives twice: a sharp first jolt carried by faster, thinly coated fibers, then a slower, duller ache carried by thin bare ones.

Step through both moments below, as slowly as you like.

A brain that guesses first

Your brain doesn’t wait for the world to tell it everything, so it’s constantly predicting what it’ll see, hear and feel next, and checking those guesses against what arrives. That’s why a missing step in the dark gives you such a jolt: your foot expected a step. It’s why muffled speech becomes clear once you’ve read the caption. And it’s part of why that dress looked different colors to different people, since their brains made different guesses about the light.

Many scientists now see prediction as one of the brain’s main jobs, and some go further and describe the whole brain as a prediction machine. That bigger idea is a useful way to think, and it isn’t proven. Some findings fit it well, and others can be explained in other ways. One piece is well established: dopamine cells fire more when something turns out better than expected and less when it turns out worse, which is how the brain learns from its own wrong guesses.

Your heart is thumping before a job interview, and you start to feel nervous. Which came first?

News from the body

The brain doesn’t only send orders down to the body. It’s constantly listening to the heart, lungs, stomach and gut, and that information feeds into hunger, tiredness, mood and emotion. Researchers call this sense of the body from the inside interoception, and the insula is one of its main hubs. Some scientists think feelings are partly the brain’s predictions about what’s going on inside the body. That idea is new and still argued. If it’s right, “which came first?” has no clean answer, because the thumping heart and the nervous feeling feed each other.

The vagus nerve

The vagus nerve is a long two-way line that runs from the brainstem to the heart, lungs and gut. Most of its fibers carry news up to the brain rather than orders down. You’ll find plenty of tips online for “stimulating your vagus nerve”, and most of them have no good evidence behind them. Polyvagal theory, a popular account of the vagus nerve and feeling safe, has been seriously challenged by other researchers. Slow breathing does change your heart rhythm in ways that can be measured, and our article on slow breathing explains what it can and can’t do.

The stress chain

When something stressful happens, two systems kick in. A fast one works through your nerves and releases adrenaline within seconds: the racing heart and sweaty palms. A slower one is a chain of hormones that starts in the hypothalamus, passes through the pituitary gland and ends at the adrenal glands above your kidneys, which release cortisol. Cortisol helps you cope, and then it signals the chain to switch itself off. Stress that goes on for a long time keeps the chain busy, and that’s part of why it wears on both body and mind. If that’s where you are, a therapist can help you work out what’s keeping it going and what eases it.

Sleep and memory

Sleep isn’t the brain switching off. During deep sleep, the hippocampus and cortex seem to replay parts of the day, and many studies have found that sleeping after you learn something helps you remember it later. Exactly which stage of sleep does what is still being worked out. Our article on sleep covers what happens through the night.

To get a license, London taxi drivers have to learn tens of thousands of streets by heart, and it takes most of them years.

What practice does

Your brain changes with what you do. In one famous study, London taxi drivers had a larger back part of the hippocampus than other people, and the longer they’d been driving, the larger it tended to be. A later study followed trainees for several years and found the change in those who passed the test, which suggests the learning shaped the brain rather than the other way around.

Claims that a few weeks of training make parts of the adult brain grow are shakier. Many of those studies were small, and scan measurements can mislead. The brain changes with practice, mostly in what you practice.

Growing up

Different parts of the brain mature at different times. In brain charts built from more than 100,000 scans, the volume of gray matter, the tissue where most nerve cell bodies sit, peaked at about age 6, while white matter, the coated cables that link areas, kept growing until the late twenties. The prefrontal cortex is among the last areas to settle.

Is the brain finished at 25?

You’ve probably heard that the brain isn’t finished until 25. It’s a rough summary of those trends. There’s no single age when the brain is done. Different measures peak at different ages, people vary a lot, and the brain keeps changing all through adult life. It also doesn’t mean that someone under 25 can’t make good decisions.

Do adults grow new nerve cells?

For a long time, scientists believed adults don’t make new neurons. Then studies in animals found new neurons being made in adult brains, and the question became whether people do it too. This one is still open.

In 2018, one team looked at human brain tissue and found new neurons falling sharply through childhood, with none detected in adults. That same year, another team found new neurons in people up to their late seventies, and in 2019 a third found them in healthy people into their eighties, and far fewer in people with Alzheimer’s disease. Much of the disagreement comes down to methods, such as how the tissue was preserved and how new cells were recognized. In 2025, a study using a newer way of reading single cells found dividing cells in the adult hippocampus that are the starting point for new neurons, though only a few, and the number varied a lot from person to person.

What can be said safely is that if adults make new neurons, it’s mainly in the hippocampus, estimates of how many vary widely, and nobody has shown that it matters for memory or mood in people. Anything that promises to grow new brain cells is promising more than the science can show. Your brain keeps changing in other ways all the time, through its connections and its support cells. Exercise is one of the better-studied ways to look after your brain, and our article on exercise goes through what moving does for your mind.

Someone at a party tells you that we only use 10% of our brains. It sounds believable, and it’s wrong!

Myth, fact or still argued?

Some of these ideas have come up already, and some are new here. Before you read the answers, make a guess for each one.

What brain scans can and can’t show

Brain scan pictures are everywhere, with bright patches glowing on a gray brain. They’re useful, and they’re easy to misread.

• A scan doesn’t photograph thoughts. The most common kind, called fMRI, measures changes in blood flow and oxygen that follow brain activity by a few seconds. The colored patches are statistical maps of where a signal passed a cut-off, and changing the analysis can change the picture.
• A lit-up area doesn’t tell you what someone felt. Most areas respond during many different tasks, so “the insula lit up, so they felt disgust” doesn’t follow.
• Many famous findings came from small groups. Links between differences in people’s brain scans and differences in their behavior turn out to be small, and reliable ones usually need thousands of people.
• Brain words make claims sound stronger. In experiments, adding brain-science jargon made weak explanations seem more convincing to people who weren’t experts. It’s worth being a little wary of any headline that leans on it.
• A scan can’t diagnose depression, ADHD or anxiety in one person. Scans are very good at other things, such as finding a stroke or a tumor, or mapping where language sits before surgery.

If you’re worried about your own brain

If you’ve noticed changes in your memory, thinking or mood that worry you, especially if they’re new or getting worse, it’s worth talking to a doctor. Not everyone can easily see one, and waits can be long. If that’s you, a pharmacist or your local health information line can often tell you where to start.

Sudden weakness or numbness on one side of the body, a drooping face, trouble speaking or understanding, sudden confusion, sudden loss of vision or balance, or a sudden, severe headache needs emergency help straight away. Call your local emergency number.