How does the brain work?

Your brain weighs about 1.3 to 1.4 kg, roughly as much as a small pineapple, and it is soft, about like firm tofu. The biggest part is the cerebrum: two halves, the left and right hemispheres, split by a deep groove down the middle. We are looking at the patient’s left side, with the forehead on your left.

Each hemisphere has four lobes. The frontal lobe plans and decides, the parietal lobe feels touch and knows where you are, the temporal lobe hears and helps you remember, and the occipital lobe at the back sees. The central sulcus separates frontal from parietal, and the lateral fissure tucks the temporal lobe underneath.

The surface is folded into ridges, gyri, and grooves, sulci. The outer layer, the cortex, is grey matter only 2 to 4 mm thick. Folding packs a sheet about the size of a large pillowcase into your skull. Under it lies white matter: billions of wires wrapped in fatty insulation.

Under the back sits the cerebellum (“little brain”), which keeps you balanced. The brainstem (midbrain, pons and medulla) runs your breathing and heartbeat and joins the spinal cord. Deep inside, turn on X-ray to find the thalamus, hypothalamus, hippocampus, amygdala, basal ganglia, the corpus callosum bridge, and fluid-filled ventricles.

Your brain has about 86 billion neurons, nerve cells that talk with electricity and chemicals. A neuron has branching dendrites that collect messages, a cell body (soma) that adds them up, and one long axon that sends the answer, sometimes over a metre, to its axon terminals.

At rest, the inside of a neuron is about −70 mV compared with the outside, like a tiny battery. Pumps keep extra sodium (Na⁺) outside and potassium (K⁺) inside. When inputs nudge the voltage up to the threshold, about −55 mV, sodium gates snap open, Na⁺ floods in and the voltage shoots to about +35 mV. Then potassium gates open, K⁺ flows out, and the cell resets. That 1 to 2 millisecond blip is an action potential, or spike. For how voltage and current work, see OhmsLawClear.

Spikes are all-or-nothing: a weak push that misses threshold does nothing, and a strong push gives the same-sized spike. So how does a neuron say “strong”? It fires more often. That is rate coding.

Many axons are wrapped in myelin, a fatty insulation with small gaps called nodes of Ranvier. The spike jumps from node to node (saltatory conduction), reaching up to about 120 m/s. Bare axons manage about 1 m/s. In multiple sclerosis, the immune system damages myelin, which is why signals slow or fail.

Neurons don’t quite touch. Where an axon terminal meets the next cell there is a gap about 20 nanometres wide, the synaptic cleft. The whole junction is a synapse. Estimates put about 100 trillion of them in your brain, though the true number is uncertain by a factor of several.

When a spike arrives, calcium (Ca²⁺) gates open and calcium rushes in. That makes tiny bubbles, vesicles, fuse with the membrane and spill neurotransmitter into the gap. The molecules drift across in well under a millisecond and fit into receptors on the other side, which open to let ions through.

Glutamate is the main “go” signal: it lets Na⁺ in and nudges the next cell towards threshold (an EPSP). GABA is the main “stop” signal: it lets Cl⁻ in and holds the cell back (an IPSP). One synapse gives well under 1 mV, so the receiving neuron fires only when many inputs add up, arriving together (spatial summation) or in quick succession (temporal summation).

Learning changes synapses. When one cell repeatedly helps fire another, their synapse gets stronger, often by adding receptors. This long-term potentiation (LTP) is summed up as “cells that fire together wire together”. Many drugs act here too: caffeine blocks receptors for adenosine, a chemical that builds up and makes you sleepy; nicotine mimics acetylcholine; many antidepressants slow the clean-up of serotonin; alcohol boosts GABA’s braking effect. These are facts, not advice: medicines are for a doctor to decide.

Different patches of cortex have different main jobs. At the back, the visual cortex in the occipital lobe handles what you see (see EyeClear). On top of the temporal lobe, the auditory cortex handles sound (see EarClear). The prefrontal cortex behind your forehead plans, decides and holds back impulses. The cerebellum smooths every movement, and the hippocampus helps you remember.

Just in front of the central sulcus runs the motor strip, which moves the body, and just behind it the touch strip. Each has a map of the body, the homunculus (“little man”). Hands, lips and tongue get huge areas because they are so skilled and sensitive. The maps are crossed: your left hemisphere moves and feels your right side.

In most people, language leans left: Broca’s area helps produce speech and Wernicke’s area helps understand it. But “left-brained” logical and “right-brained” creative personalities are a myth. A 2013 study of over 1,000 brain scans found no sign of people being more left- or right-brained overall. Both halves work together all the time.

And the “we only use 10% of our brain” myth? Also false. Brain scans show activity all over the brain, and over a day every region gets busy. Damage to almost any small area causes problems. The brain is far too hungry for energy to carry 90% dead weight. Real scans, like fMRI (see MRIClear), show the extra activity a task adds on top of a brain that is always working.

Short-term memory holds only a handful of things, about four to seven, for seconds, like a phone number you are about to dial. Long-term memory can last a lifetime. Getting from one to the other depends on the hippocampus, a curved strip deep in each temporal lobe.

We know this partly thanks to one man. In 1953, Henry Molaison, known for decades only as “H.M.”, had surgery to treat severe epilepsy. Surgeons removed much of both hippocampi. His seizures eased, but he could no longer form new long-term memories of events and facts. He could still chat, remember his childhood and even learn new skills, such as drawing in a mirror, without remembering the lessons. The psychologist Brenda Milner and others studied him with his cooperation for over 50 years. His case showed that memory has separate systems, and that the hippocampus is key to making new ones.

Much of this work happens while you sleep. A night runs in cycles of about 90 minutes: light N1 and N2, deep slow-wave N3, then dreaming REM sleep. Deep sleep comes mostly early in the night and REM mostly late. During N3 and the sleep spindles of N2, the hippocampus replays the day and the cortex files it away. That is why a good night’s sleep after studying helps you remember.

Your brain is about 2% of your body weight but uses about 20% of its energy at rest, roughly 20 watts, like a dim light bulb (see EnergyClear). It burns mostly glucose and oxygen, delivered by about 750 mL of blood a minute. It stores almost none, so even a few minutes without blood flow causes damage.

Brain blood vessels are lined with tightly sealed cells, the blood–brain barrier. It lets in oxygen, glucose and a few other things and keeps out most germs and toxins, which also makes many medicines hard to get into the brain.

A stroke happens when an artery to the brain is blocked by a clot (most strokes) or bursts and bleeds. The cells it feeds start dying within minutes: in a typical large stroke, about 1.9 million neurons a minute. So doctors say “time is brain”. Learn BE FAST: Balance, Eyes, Face, Arms, Speech, Time. If you see these signs, call emergency services straight away. Treatments that dissolve or remove a clot work best in the first hours.

A hard knock can shake the brain against the skull: a concussion. Headaches, confusion or memory gaps after a knock need a doctor’s check. Helmets spread the stop over a longer distance, cutting the force (see MotorcycleClear).

The brain keeps rewiring all your life: neuroplasticity. After an injury, other areas can sometimes take over lost jobs with practice. Dementia, most often Alzheimer’s disease, slowly damages memory and thinking, mostly in older age. Sleep, physical activity, not smoking, and treating high blood pressure are linked with healthier brains. This box gives general facts only: for any worry about your own health, see a doctor.