A field guide to your own brain

Somewhere behind your eyes, about three pounds of soft, folded tissue is turning these black marks into meaning. It is doing this without any effort on your part, while also keeping your heart beating, your balance steady and a background hum of half-formed thoughts ticking over. The human brain is, as far as we know, the most complex object in the universe — and most of us carry one around without ever being introduced to it. Here is a short field guide.

The three-pound universe

The brain is not a single thing so much as a committee of specialised regions that have learned to work together. The largest is the cerebrum, the great wrinkled dome that fills most of the skull. Its folds are not decorative: they are a packing trick. Flattened out, the outer layer — the cerebral cortex — would cover an area far larger than the skull could hold, so evolution crumpled it up to fit more thinking surface into the same space.

Beneath and behind the cerebrum sit the older, quieter parts. The cerebellum fine-tunes movement and balance. The brainstem, where the brain meets the spinal cord, runs the essentials you never think about: breathing, heart rate, blood pressure. You could lose a surprising amount of the cerebrum and survive; damage the brainstem and the lights go out.

A forest of neurons

The working unit of all this is the neuron, a cell built for sending signals. The adult brain contains an estimated 86 billion of them — a number so large it stops meaning anything. What matters is not the count but the connections. Each neuron can link to thousands of others, and it is in those connections, not the cells themselves, that thought and memory seem to live.

Neurons talk to each other across tiny gaps called synapses. When a signal reaches the end of one neuron, it releases chemical messengers — neurotransmitters — that drift across the gap and nudge the next cell into firing. To speed things along, the long fibres of neurons are wrapped in myelin, a fatty insulation that works exactly like the plastic coating on an electrical wire. Signals on a myelinated fibre travel more than a hundred times faster than on a bare one.

The divided brain

The cerebrum comes in two halves, and they are wired to the body in a way that surprises almost everyone: each hemisphere controls the opposite side. Your left hemisphere moves your right hand; your right hemisphere moves your left. The two halves are kept in constant conversation by the corpus callosum, a thick cable of some 200 million nerve fibres running between them.

The popular idea of a "logical left brain" and a "creative right brain" is mostly a myth — almost everything you do draws on both sides at once. But there are real specialisations. In most people, the machinery of language leans heavily on the left, which is why a stroke on that side can rob someone of words while leaving them otherwise sharp.

Where memories are made

If you remember anything from this article tomorrow, you will have a small, seahorse-shaped structure to thank. The hippocampus is essential for turning fleeting experiences into lasting memories. Its importance became clear through the famous case of a patient known as H.M., who had both hippocampi removed to treat epilepsy. His seizures eased — but he could no longer form new long-term memories, greeting the same doctors as strangers every day for the rest of his life.

Memories are not filed away whole, like documents. They are scattered across the cortex and reassembled each time you recall them, which is why remembering something actually changes it, and why the simple act of testing yourself strengthens a memory more than reading it again.

An energy-hungry organ

All this activity has a cost. Though the brain is only about 2% of your body weight, it burns roughly 20% of your energy, running day and night whether you are solving a problem or fast asleep. It never truly switches off. Even in deep sleep it is busy — clearing out waste, consolidating the day's memories and rehearsing what it has learned.

How to be kind to it

The brain is not fixed hardware. It rewires itself throughout life in response to what you do — a property called neuroplasticity. Learning a new skill physically changes the connections inside your head. A few things reliably help it along:

• Sleep. It is during sleep that the brain files away memories and flushes out metabolic waste. Cutting it short sabotages both.
• Movement. Physical exercise increases blood flow and encourages the growth of new connections, and is one of the best-evidenced protectors of the ageing brain.
• Effortful learning. The brain strengthens what it has to work for. Retrieving a fact from memory — the small strain of a quiz — does more than passively re-reading it ever could.

None of this requires a special gift. The organ doing your reading right now is already extraordinary, already curious, already rebuilding itself in response to whatever you feed it. The least we can do is get to know it a little.