Your brain
What each part does, how one neuron fires, and how neurons are made
Meet your brain
This is your brain, seen through the skull. It weighs about as much as a bag of sugar, 1.4 kilograms, and it is where you think, feel, remember and decide. It is made of two halves that talk to each other through a thick bridge in the middle. The wrinkly outside is where most of the thinking happens. The brain is like a team of experts: each region has its own main job, but they all work together. Watch the parts light up.
The brain weighs about 1.4 kg and contains roughly 86 billion neurons and a similar number of other cells, including the glial (support) cells. The cerebrum, the big wrinkled part, is split into two hemispheres joined by the corpus callosum, a thick bundle of nerve fibres. Its surface layer, the cerebral cortex ("grey matter", a few millimetres thick), does most of the information processing; the folds pack more of it into the skull. Each hemisphere has four lobes: frontal (planning, reasoning, short-term working memory, movement), parietal (touch and other body senses, sense of space, reading, arithmetic), occipital (vision) and temporal (hearing, memory, language). Under and behind the cerebrum sit the cerebellum (coordination and learned movements) and the brain stem (breathing, heart rate, blood pressure). Many pathways for limb movement and body sensation cross over, so each hemisphere mainly serves the opposite side of the body. The colours and highlights only help tell structures apart; they are not the brain’s real colours. The surface looks patchy because the gyri (ridges) and sulci (grooves) of the cortex are separate meshes in the model.
Seeing and hearing
Your eyes turn light into nerve signals, and your ears turn vibrations into signals. Making sense of them is the brain’s job. Seeing happens mostly at the very back of your brain, and hearing on the sides, just above your ears. Several brain areas then work together to turn those signals into the pictures and sounds you know.
The primary visual cortex lies in the occipital lobe, along the calcarine sulcus at the back of the brain (the cuneus above it and the lingual gyrus below). The retina has already done a first round of processing before the signal leaves the eye; damage to the visual cortex can wipe out part or all of the visual field even though the eyes are fine. The primary auditory cortex is in the transverse temporal gyri (Heschl’s gyri), tucked into the top of each temporal lobe. Each of these areas is a map: neighbouring points of the retina, or neighbouring pitches, are handled by neighbouring patches of cortex. Sight and sound signals are then passed on to many surrounding areas that recognise faces, words, tunes and places.
Moving and feeling
Two strips run over the top of your brain, like a headband. The front strip sends orders to your muscles: wiggle a finger, kick a ball. The strip just behind it receives touch: hot, cold, tickly, sharp. Each strip is a map of your body, and the parts that need fine control or feel the most detail, like your hands and lips, get the biggest space. The left side of the brain mostly runs the right side of the body, and the other way round.
The primary motor cortex is the precentral gyrus, the strip at the back of the frontal lobe, which plans, controls and executes voluntary movement. Directly behind it, across the central sulcus, the postcentral gyrus is the primary somatosensory cortex, which receives touch, temperature, pain and body-position information from the whole body. Both strips are organised as a distorted map of the body (the "homunculus"): the hands, lips and tongue get far more cortex than the trunk, in proportion to how finely they move or feel, not their size. The highlights show the outer part of each strip; the leg and foot areas continue over the top onto the inner surface between the hemispheres. The main pathways for limb movement and body sensation cross, so the right hemisphere mostly moves and feels the left side of the body, which is why a stroke affecting these pathways can weaken the opposite side.
Talking and remembering
A patch low on the left side of the front helps you put words together to speak. Deep inside each side, a curled shape the size of your little finger, called the hippocampus, works like a librarian: it takes what happens to you today and files it away as a memory you can find later.
Producing speech depends on Broca’s area, in the inferior frontal gyrus, and in most people mainly on the left hemisphere. Damage to this left frontal language network makes speech slow and effortful, and understanding complex sentences can suffer too. The hippocampus, in the medial temporal lobe, is needed to form new long-term memories of facts and events: it indexes an experience and sends it out to the cortex for storage, and helps retrieve it later. Damage to both hippocampi severely impairs new memories of this kind, while some older memories and the ability to learn skills can remain. It is also one of the few regions where some new neurons may be made in adults.
Balance and staying alive
At the back, under the big brain, sits a smaller wrinkly ball: the cerebellum, or "little brain". It keeps you balanced and makes your movements smooth, so you can ride a bike or catch a ball without thinking. Below it, where the brain joins the spinal cord, is the brain stem. It keeps you breathing and speeds your heart up or slows it down, even while you sleep. (The heart has its own built-in pacemaker; the brain stem adjusts it.)
The cerebellum coordinates movement and stores learned movements such as playing an instrument or hitting a tennis ball; although it is small, it holds about 69 billion of the brain’s 86 billion neurons, most of them tiny granule cells. The brain stem (midbrain, pons and medulla oblongata) connects the brain to the spinal cord and controls vital functions: it drives breathing and regulates heart rate and blood pressure (the heartbeat itself is generated by the heart’s own pacemaker cells), and handles swallowing and many reflexes. The midbrain also takes part in eye movements. Damage to the brain stem is life-threatening for exactly this reason.
Hidden deep inside
Now the outside is faded away so you can see the parts hidden deep in the middle. The thalamus is a switchboard: almost everything your senses pick up passes through it. The hypothalamus, the size of a pea, wakes you up, makes you hungry or thirsty and keeps you warm. The amygdala fires up when you are scared. The basal ganglia help you start a movement smoothly.
The thalamus relays nearly all sensory input (except smell) to the cortex and passes signals between cortical areas. The hypothalamus, about the size of a pearl, controls hunger, thirst, body temperature, sleep and waking, and drives the pituitary gland, so it links the nervous system to hormones; it also shapes emotional states. The amygdala, next to the hippocampus, tags experiences with emotion, especially fear. The basal ganglia (caudate nucleus, putamen and globus pallidus) surround the thalamus and initiate and smooth movements; in Parkinson’s disease the loss of dopamine-producing neurons that feed them causes tremor, rigidity and a shuffling walk.
One neuron
Zoom in until the brain is just cells. This is a neuron, a nerve cell. It has a body, lots of little branches called dendrites that catch messages, and one long tail called the axon that sends a message on. Watch the yellow spark: it comes in along the dendrites, the body flashes, and the spark races down the axon. The pale beads are insulation that makes it go faster.
A neuron has a cell body (with the nucleus), dendrites that receive input and one axon that carries output. When enough input arrives, the cell body’s trigger zone fires an action potential: a brief electrical pulse (about a tenth of a volt, a thousandth of a second) that regenerates itself along the axon rather than fading. Many axons are wrapped in myelin, a fatty insulation made by glial cells, with bare gaps (nodes) between the sleeves; the pulse jumps from node to node. Speed depends on diameter and myelin: unmyelinated axons conduct at roughly 0.5 to 10 metres per second, while large myelinated axons can reach about 150. Axons range from less than a hair’s width, between neighbouring cortical cells, to a metre, from the brain down the spinal cord. This is a cartoon: real dendrite trees are far bushier, and the cell body is drawn much larger relative to the axon than it is.
Jumping the gap
At this kind of meeting point, the two neurons do not quite touch. At the end of the axon there is a tiny gap before the next neuron. The meeting point is called a synapse. The spark cannot jump the gap, so the sending end lets out tiny pink messenger molecules that drift across it. When they land, they can encourage the next neuron to fire, or tell it to stay quiet. Here they are the "go" kind: the next neuron fires, and the message carries on.
At a chemical synapse the arriving action potential makes the axon terminal release neurotransmitter molecules from small sacs (vesicles) into the synaptic cleft, a gap of about 20 to 40 nanometres. The molecules diffuse across and bind to receptors on the receiving cell, which can excite it (glutamate, acetylcholine), inhibit it (GABA) or modulate it (dopamine, serotonin). A single message rarely decides anything: the receiving neuron adds up excitation and inhibition from many synapses, and fires only if the total is enough. (Electrical synapses, where cells are directly coupled, also exist but are less common.) Synapses are where learning happens, because their strength can change with use. Most medicines that act on the brain, and most drugs, work by changing what happens at synapses.
Billions, talking
Now imagine 86 billion of them. Many neurons have thousands of meeting points with other neurons, and a message hops from one to the next. Tap any neuron to make it fire and watch the signal spread through its neighbours, then fade out. (Real brains never fall silent: something is always firing somewhere.) This is a slowed-down cartoon of a tiny patch. Thinking is many such patches all over the brain working together.
The human brain has about 86 billion neurons, 16 billion of them in the cerebral cortex, and many neurons carry thousands of synapses, so the total runs to hundreds of trillions of connections. Signals spread along these connections in patterns shaped by which synapses excite and which inhibit, and a thought or a movement involves coordinated activity across many circuits at once. This model is a display, not a simulation: each neuron talks to its three nearest neighbours (about a third also reach one neuron further away), a wave stops after four hops, and a neuron that has just fired waits two seconds before it can fire again. Real refractory periods last milliseconds and do not by themselves make activity stop, and real wiring is not nearest-neighbour: axons run to precise targets, sometimes across the whole brain.
How a neuron is born
Where do neurons come from? Before you were born, deep inside your growing brain, special stem cells kept splitting in two. Some of the new cells stayed stem cells; others became baby neurons (this picture shows the part of the brain that becomes the cortex, the wrinkly outside). A new neuron then climbed up a long rope-like cell, like climbing a ladder, until it reached the place where it would live and work.
Most cerebral-cortex neurons form before birth, while many cerebellar neurons (the tiny granule cells) are made during infancy. Neural stem cells near the fluid-filled centre of the developing brain divide: a division can make more stem cells, or progenitor cells that go on to become neurons or glia. A newborn cortical neuron then migrates outward, most often by gliding along the long fibres of radial glia that stretch from the inner layers to the surface, and also by following chemical cues on nearby cells. Not all make it: some developing neurons die, and some survive but settle in the wrong place, which can cause disorders such as some childhood epilepsies. In the adult brain, new neurons appear in only a few places, such as the hippocampus, and how much this matters is still being worked out.
Growing branches
Once the young neuron is in its place, it starts to grow. Branches sprout up and out to catch messages, and one long tail reaches out to send them. The bright tip of each branch is feeling its way, like a plant shoot looking for light.
This is differentiation. Signals from surrounding cells tell the settled neuron what shape to take, which neurotransmitter to make and which other cells to connect to. It sprouts dendrites and extends a single axon, each led by a growth cone, a mobile tip that follows chemical guidance cues, attractive and repulsive, to its target. Some axons must travel a long way: from the motor cortex down the spinal cord, or from one hemisphere to the other through the corpus callosum. The shape a neuron ends up with, bushy or sparse, short-axoned or long, is what makes the hundreds of different types of neuron.
Making friends
Wherever a branch meets another neuron, a tiny meeting point can form: a synapse. Each pink dot here is a new one. A young brain makes them in enormous numbers, far more than it will keep.
Where an axon terminal meets a dendrite, a synapse can form, with the sending machinery on one side and receptors on the other. Synapse formation runs fastest in late pregnancy and the first years of life, when the cortex gains synapses faster than at any other time; a toddler’s brain has more synapses than an adult’s. Which connections form depends on the neuron’s type and on activity: neurons that fire together tend to wire together.
Use it or lose it
The brain is a bit like a town that keeps the roads people drive on and lets the others grow over. As you learn, some connections get stronger, some get weaker, and some are removed altogether. Connections that carry lots of messages usually grow thicker and stronger; ones that stay quiet often fade away. This tidying, called pruning, goes on all through childhood and your teens.
Synaptic pruning removes many of the connections made in early life, while others are strengthened, so circuits become faster and more efficient. Activity patterns help decide which synapses strengthen, weaken or survive: it is not a simple "used = kept" rule, since active synapses can also be weakened. Pruning is heaviest in childhood and adolescence, and different regions finish at different times: sensory areas early, the prefrontal cortex (behind the forehead, used for planning and judgement) last. Myelination, the insulation that speeds signals, also continues into early adulthood, and the brain is generally considered mature in the mid-to-late twenties. Learning, exercise and sleep all shape this process. An unused pathway can be lost: an eye that is covered for long periods in early childhood may never gain normal vision.