The Human Brain

What the brain is

The brain is the central organ of the human nervous system. Together with the spinal cord it forms the central nervous system, the part of the body that gathers information, makes sense of it, and issues the commands that produce action. Physically it is unimpressive to look at: a soft, pinkish-grey mass with the consistency of firm jelly, weighing between about 1.3 and 1.4 kilograms in a typical adult. Its power lies not in bulk but in organisation.

That organisation is built from cells. The brain contains an estimated 86 billion neurons, the electrically active cells that carry and process information, and a comparable number of glial cells, which support, insulate, feed, and defend the neurons. Each neuron may connect to hundreds or thousands of others, so the total number of connections runs into the trillions. It is this dense web of connections, rather than the count of cells alone, that gives the brain its capacity for thought, memory, and learning.

Grey matter and white matter

Cut into the brain and two kinds of tissue appear. Grey matter is where the neuron cell bodies gather, and it is where the actual processing of information happens. White matter lies beneath it and is made of the neurons' long fibres, their axons, many wrapped in a pale, fatty myelin sheath that gives the tissue its colour. If grey matter is where the computation takes place, white matter is the wiring that connects one region to another.

In the cerebrum this arrangement is turned inside out compared with the spinal cord: the grey matter forms a thin sheet on the outside, the cerebral cortex, wrapped around a bulk of white matter. Deep within that white matter sit further islands of grey matter, the deep nuclei, meaning clusters of cell bodies buried inside the white matter rather than laid out on the surface. They include the basal ganglia and the relay stations of the thalamus.

It is worth stopping on that inversion, because it is not an accident and it explains the brain's most famous feature. Putting the computing cells on the outside as a sheet has two consequences. The first is that a sheet can be folded, and folding is the only way to fit a large surface into a small container. The human cortex, spread flat, would cover something close to the area of a large dinner napkin. It fits inside a skull that has to pass through a birth canal only because it is crumpled, and the ridges and grooves you see on any picture of a brain are that crumpling. About two thirds of the cortical surface is hidden down inside the folds. An unfolded brain of the same cortical area would need a head no human could be born with.

The second consequence is about wiring. If the cells that do the computing sit on the surface, the cables connecting them can run underneath, through the interior, taking short routes between any two points on the sheet. That is exactly what the white matter is: the wiring layer, placed where the wires are shortest. Get the arrangement the other way round and the same connections would have to take longer paths, cost more energy, and take longer to traverse. The folded sheet over a core of wiring is not a quirk of anatomy. It is a solution to the problem of packing a large, densely interconnected surface into a small, cheap, fast package.

The major divisions

Anatomists describe the brain in several ways. A developmental scheme divides it into forebrain, midbrain, and hindbrain, reflecting how it grows from the embryo. A simpler working scheme, and the one used through this library, splits the adult brain into three parts that are easy to see and to reason about: the cerebrum, the cerebellum, and the brainstem.

The developmental forebrain corresponds broadly to the cerebrum plus deep structures such as the thalamus and hypothalamus; the hindbrain gives rise to the cerebellum and to parts of the brainstem. Both schemes describe the same organ, only from different angles, and it is worth being comfortable with each.

Hemispheres and the corpus callosum

The cerebrum is divided down the middle into two halves, the left and right cerebral hemispheres, separated by a deep groove called the longitudinal fissure. Broadly, each hemisphere controls and receives sensation from the opposite side of the body: the left hemisphere governs the right hand, the right hemisphere the left. This crossing over is a consistent feature of the nervous system.

The two hemispheres are not isolated. They are joined by a thick bridge of white matter, the corpus callosum, made of some 200 million fibres that carry a constant traffic of signals between the sides. Because of this bridge the hemispheres work as a single, coordinated organ. It is worth stressing that the popular idea of people being ruled by a logical left brain or a creative right brain is a myth: while some functions do lean to one side, such as language in most people, the two hemispheres are in continuous communication and personality is not divided between them.

A map of the main parts

Beyond the three broad divisions, a handful of named structures do most of the work that a general reader will meet. The table below is a quick orientation; each is treated more fully on its own page or on the pages linked further down.

These parts do not act alone. Almost every capacity we might name, seeing a face, recalling a name, reaching for a cup, draws on many regions at once, working together through the brain's dense connections. The map is a guide to where processing is concentrated, not a set of sealed compartments.

How the brain is protected

The brain is delicate and cannot repair itself easily, so it is shielded by several layers working together. Understanding this protection explains a good deal about how the brain stays stable and why certain injuries and infections are so serious.

1. The skull

   A rigid case of bone surrounds the brain and absorbs most physical impacts. It is the outermost and most obvious line of defence, though its hardness also means the brain has no room to swell without harm.

2. The meninges

   Three membranes lie between skull and brain: the tough outer dura mater, the delicate middle arachnoid mater, and the thin inner pia mater that clings to the brain's surface. Together they cushion the brain and hold it in place.

3. Cerebrospinal fluid

   A clear fluid fills the space around the brain and its internal chambers, the ventricles. The brain floats in it, and the effect is larger than it sounds: a brain weighing around 1.4 kg has an effective weight of only about 50 g once suspended, because it is very nearly as dense as the fluid holding it up. That matters, because brain tissue has roughly the consistency of firm jelly. Resting unsupported on the base of the skull, it would be deformed and torn by its own weight. Floating, it is not. The fluid also buffers it against jolts and carries away waste.

4. The blood-brain barrier

   The walls of the brain's blood vessels are unusually tight, forming a chemical gatekeeper that lets in fuel and oxygen but keeps out many toxins and pathogens. This blood-brain barrier protects the brain's finely tuned chemistry.

These defences are remarkably effective, but they have costs. The rigid skull means swelling has nowhere to go, so a bleed or bruise inside it can raise pressure dangerously. The blood-brain barrier, meanwhile, blocks many drugs as effectively as it blocks toxins, which is one reason treating brain disease is difficult.

The brain's energy use

For its modest size the brain is extraordinarily hungry. It makes up only about 2 per cent of an adult's body weight, yet it consumes roughly 20 per cent of the body's energy, even at rest. Almost all of that energy goes on the constant work of pumping ions across neuron membranes to keep the cells ready to fire and to send signals.

The reason is worth stating plainly, because it turns a striking number into an understandable one. Signalling itself is nearly free: a neuron fires by letting ions rush down the gradients it has already built, which is downhill and costs nothing at the moment it happens. The expense is putting them back. Every impulse and every synaptic potential runs the gradients down a little, and molecular pumps have to haul each ion back up against its gradient afterwards, burning fuel to do it. The brain's enormous energy bill is therefore not the cost of thinking. It is the cost of undoing thinking, of resetting the machinery so that it is ready to fire again. That is also why the brain cannot coast: it stores essentially no fuel, so a few minutes without blood flow is catastrophic.

The brain relies almost entirely on a steady flow of glucose and oxygen carried by the blood, and it stores almost no fuel of its own. That dependence explains why a loss of blood supply, as in a stroke, damages tissue within minutes, and why the brain guards its supply so jealously through the dense network of vessels feeding it. Fuel and protection, in other words, are two sides of the same problem: keeping a delicate, power-hungry organ running without interruption.

The brain reference library

This overview is the front door to a larger reference library. The pages are grouped into seven sections, each with its own hub. Anatomy works at the level of whole structures, cells and signalling at the level of individual neurons and their messages, and the remaining sections take on what the brain actually does: how it remembers, how it thinks, how its state shifts between sleep, stress, and reward, and how it is built and studied. Start with whichever section matches your question.

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