Cosmos

The Brain

A kilo and a half of soft tissue containing 86 billion neurons and more connections than there are stars in the galaxy. It is reading this, and it is the most complex object known.

By The Cosmos EditorsUpdated 1 November 2026 4 min read
The brain from the side. The wrinkled cerebrum dominates; the cerebellum sits below at the back; the brainstem connects to the spinal cord.
The brain from the side. The wrinkled cerebrum dominates; the cerebellum sits below at the back; the brainstem connects to the spinal cord. Dr. Johannes Sobotta, Public domain via Wikimedia Commons
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Hold your two fists together and you have roughly the size of it. It weighs about as much as a bag of sugar, has the texture of soft tofu, and is pinkish-grey. Nothing about its appearance suggests that it is the most complicated object in the known universe, or that it contains every memory, skill, fear and hope you have. The brain is the organ that produces you.

Cells that talk

The brain is built from about 86 billion nerve cells, neurons, supported by a similar number of helper cells. A neuron has a body, a tree of branching fibres called dendrites that receive signals, and a single long cable, the axon, that carries its own signal onward, sometimes for a metre or more. The signal is electrical, a pulse that travels down the axon at up to 120 metres per second. At the end, the axon meets the dendrites of other neurons at tiny gaps called synapses, and the signal crosses as a puff of chemical.

Each neuron connects to thousands of others, giving the brain something like a hundred trillion synapses. Thought, memory and sensation are patterns of activity across those connections, and learning is, physically, the strengthening of some synapses and the weakening of others. The brain you have tonight is not quite the brain you woke up with.

The parts

The largest part, the cerebrum, is the wrinkled dome that gives the brain its familiar look. Its surface layer, the cortex, is a sheet of neurons only a few millimetres thick but about the area of a large pizza, folded to fit inside the skull. Different regions have different jobs. The back processes vision; a strip across the top handles touch and, just in front of it, movement; the sides handle hearing and language; the front, the largest in humans, handles planning, decisions, personality and self-control. Each half of the cerebrum controls the opposite side of the body, and the two halves are connected by a thick bundle of 200 million fibres.

Beneath and behind sits the cerebellum, with more neurons than the rest of the brain combined, which coordinates movement and balance so smoothly that you can walk while talking without thinking about either. Deep in the centre lie older structures: the hippocampus, which forms new memories; the amygdala, which handles fear and emotion; the hypothalamus, which controls hunger, thirst, temperature and hormones. At the base, the brainstem runs breathing, heart rate and sleep, and connects everything to the spinal cord.

Memory

A memory is not stored in one place like a file but spread as a pattern across many neurons. Short-term memory holds a few items for seconds. Long-term memories are built in the hippocampus and gradually transferred to the cortex, especially during sleep, which is one reason sleep matters for learning. Memories are reconstructed each time they are recalled, and they change a little in the process; a vivid memory is not necessarily an accurate one. Skills, such as riding a bicycle, are stored separately from facts, which is why a person with amnesia can lose their past yet still play the piano.

Fuel and protection

The brain is two percent of body weight but burns about a fifth of the body's energy, almost all as glucose, delivered by a dense network of blood vessels. It cannot store fuel, so a few minutes without blood flow causes permanent damage; this is what a stroke does. It floats in fluid inside the skull, cushioned against knocks, and is shielded from most substances in the blood by a barrier that lets through only what it needs. The brain tissue itself has no pain sensors; headaches come from the surrounding blood vessels and membranes.

A changing organ

A baby's brain forms about a million synapses a second. Childhood prunes the surplus, keeping the connections that are used. The front of the brain, responsible for judgement and impulse control, is not fully mature until about 25. The adult brain keeps changing throughout life: learning a new skill, a language or a city's streets measurably alters its structure. After injury, undamaged regions can take over lost functions, especially in the young.

What we still don't know

We know a great deal about how neurons signal and which regions do what. We do not know how patterns of electrical activity become the experience of seeing red, feeling pain or being you. This is the problem of consciousness, and it remains unsolved. Nor do we fully understand sleep, why we dream, how memories are stored at the level of molecules, or what goes wrong in conditions from depression to Alzheimer's disease. The organ that studies the brain is the brain, and it has not yet understood itself.

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Images

A cut through the middle of the brain, showing the deep structures beneath the cortex.
A cut through the middle of the brain, showing the deep structures beneath the cortex. Patrick J. Lynch, medical illustrator, CC BY 2.5 via Wikimedia Commons
A neuron. Branching dendrites receive signals; the long axon sends them on to the next cell.
A neuron. Branching dendrites receive signals; the long axon sends them on to the next cell. BruceBlaus, CC BY 3.0 via Wikimedia Commons
A single neuron from the cortex, stained to show its branches. The brain contains 86 billion of these.
A single neuron from the cortex, stained to show its branches. The brain contains 86 billion of these. The original uploader was Cahass at English Wikipedia., CC BY-SA 3.0 via Wikimedia Commons

Test yourself

4 questions · answers explained at the end

  1. 1.About how many neurons does the human brain have?

  2. 2.What share of the body's energy does the brain use?

  3. 3.Which part of the brain coordinates balance and movement?

  4. 4.Which side of the brain controls the right side of the body?

Answer every question to check.
Topics:brainneuronnervous systemmemorycerebrumcerebellumsynapseconsciousnessneuroscience