Brain Anatomy

How to read a brain

Before any structure has a name, three ideas make the whole organ legible. The first is the distinction between grey and white matter. Grey matter is tissue crowded with neuron cell bodies and the synapses between them; it is where information is processed. White matter is bundled axons wrapped in pale myelin; it is the cabling that carries information from one processing site to another. Every region you meet is one, the other, or a mixture, and asking which it is tells you immediately whether you are looking at a computer or a cable.

The second idea is that grey matter comes in two arrangements. It can be spread out as a sheet, which is what the cerebral cortex is, or it can be gathered into a lump, which anatomists call a nucleus. The thalamus, the basal ganglia, and the hypothalamus are all collections of nuclei. Sheets tend to do layered, general-purpose processing; nuclei tend to be specialist relay and control stations.

The third idea is directional vocabulary. Rostral means toward the front, caudal toward the tail, dorsal toward the back or top, ventral toward the belly or underside. Medial means toward the midline and lateral away from it. Superior and inferior mean above and below, and anterior and posterior mean in front of and behind. These terms recur constantly: the medial temporal lobe, the ventral striatum, the dorsolateral prefrontal cortex. Once the vocabulary is fluent, the name of a structure often tells you where to find it.

One more principle is worth carrying through everything below. Most brain structures come in pairs, one in each hemisphere, and most pathways cross the midline at some point. Sensation and movement for the right side of the body are handled by the left hemisphere, and vice versa. Damage on one side therefore shows up on the other side of the body, a rule so reliable that clinicians use it to localise injury before any scan is taken.

The three great divisions

The adult brain looks complicated, but it grows from a simple tube. Early in development that tube swells into three vesicles: the forebrain, the midbrain, and the hindbrain. Every structure in the mature brain descends from one of the three, and grouping them this way is still the cleanest way to hold the whole organ in mind.

Something important follows from this developmental picture. The parts of the brain that appeared earliest in evolution and that mature earliest in the individual are the ones that regulate the body: breathing, heart rate, temperature, arousal. The parts that expanded most in humans, the cerebral cortex above all, are the ones that arrived last. Anatomy is layered history, and the layers still show.

The cerebrum and its lobes

The cerebrum dominates the human brain, accounting for the great bulk of its volume and almost all of what you can see from the outside. It is divided by the deep longitudinal fissure into two hemispheres, joined underneath by a thick bridge of white matter called the corpus callosum, and each hemisphere is further divided by prominent grooves into four lobes.

Draped over the cerebrum is the cerebral cortex, a folded sheet of grey matter only 2 to 4 millimetres thick. The folding is a packing solution: ridges called gyri and grooves called sulci let a large sheet fit into a compact skull, and roughly two-thirds of the cortical surface is hidden inside the folds. It is worth being precise about the difference between two words that are often used loosely. The cerebrum is the whole mass, cortex plus the white matter and deep nuclei beneath it. The cerebral cortex is only the outer sheet.

Two smaller regions are usually mentioned alongside the four. The insula lies hidden deep within the lateral sulcus and is involved in taste, visceral sensation, and the awareness of internal bodily states. The limbic lobe is a ring of cortex on the medial surface, including the cingulate gyrus, which forms part of the emotional and memory circuitry described below. The four lobes page treats each in detail.

The deep structures

Beneath the cortex, embedded in white matter, sit the deep grey-matter nuclei. These are not primitive leftovers. They are indispensable, and a great deal of what feels like effortless behaviour, moving without thinking about it, remembering where you left your keys, feeling hungry, is their work.

The diencephalon

At the centre of the brain, straddling the third ventricle, lies the diencephalon. Its two headline structures are the thalamus and the hypothalamus. The thalamus is the great relay: with the single exception of smell, essentially all sensory information passes through a thalamic nucleus before it reaches the cortex, and the thalamus does far more than pass signals on, gating and shaping them and forming reciprocal loops with the cortex that shape attention and consciousness. The hypothalamus, just below it and weighing only about four grams, runs homeostasis: temperature, hunger, thirst, circadian timing, and, through its control of the pituitary gland, much of the endocrine system.

The basal ganglia

The basal ganglia are a group of interconnected nuclei, the caudate nucleus and putamen (together the striatum), the globus pallidus, the subthalamic nucleus, and the substantia nigra. They form loops with the cortex and the thalamus that select which movements to release and which to suppress, and the same circuitry underlies habit formation and reward-based learning. When the dopamine neurons of the substantia nigra die, the result is Parkinson's disease; when the striatum degenerates, the result is Huntington's disease.

The limbic system

The limbic system is less a single organ than a loosely defined network on the medial and inner surfaces of the hemispheres. Its best-known members are the hippocampus, essential for forming new conscious memories, and the amygdala, central to fear, threat detection, and the emotional colouring of experience. The cingulate gyrus, the fornix, the mammillary bodies, and parts of the hypothalamus and thalamus are usually included as well.

Brainstem and cerebellum

The brainstem is the stalk that carries every signal travelling between the brain and the body. It has three segments, the midbrain at the top, then the pons, then the medulla oblongata, which becomes the spinal cord. Ten of the twelve cranial nerves emerge from it, and packed into a few cubic centimetres are the centres that control breathing, heart rate, blood pressure, swallowing, and the reflexes of the eye. Running through its core is the reticular formation, a diffuse network that drives arousal and, with the thalamus, sets the level of consciousness.

Because so much is packed into so little space, the brainstem tolerates injury poorly. A small lesion here can be fatal where a much larger one in the cortex would not be, and this is exactly why the anatomical layout matters clinically.

Behind the brainstem, tucked under the occipital lobes, sits the cerebellum, the "little brain". Its tightly folded cortex holds more neurons than the rest of the brain combined, a striking fact that reflects the enormous computational load of coordinating movement. It does not initiate action; it refines it, comparing intended movement with actual movement and correcting the difference, which is why cerebellar damage produces clumsy, poorly timed, badly aimed movement rather than paralysis. It also contributes to motor learning and, increasingly recognised, to timing and prediction in cognitive tasks.

The protective and support layer

Nervous tissue is soft, fragile, and metabolically greedy. An entire apparatus exists to protect and supply it, and it is as much a part of brain anatomy as any nucleus.

The meninges

Three membranes wrap the brain. The tough outer dura mater lines the skull and folds inward to form partitions such as the falx cerebri between the hemispheres. The delicate arachnoid mater lies beneath it, and beneath that the subarachnoid space, filled with cerebrospinal fluid, in which the major arteries run. The innermost pia mater is a thin film that clings to every gyrus and dips into every sulcus. Bleeding into the space above the dura, below the dura, or within the subarachnoid space produces the three classic types of intracranial haemorrhage, each with a distinct pattern on imaging precisely because the layers are distinct.

Ventricles and cerebrospinal fluid

Inside the brain is a connected system of four fluid-filled cavities: two lateral ventricles, the third ventricle in the midline of the diencephalon, and the fourth between the brainstem and the cerebellum. The choroid plexus in their walls produces cerebrospinal fluid, which circulates through the ventricles and out over the surface of the brain. Buoyancy is the headline function: a 1.4 kg brain suspended in CSF has an effective weight of only around 50 grams, which is why it does not crush itself.

Blood supply

Four arteries feed the brain: the two internal carotids in front and the two vertebral arteries behind, which merge into the basilar artery. They join in a ring at the base of the brain, the circle of Willis, which allows blood to reach an area even if one supply route is blocked. From the ring arise the anterior, middle, and posterior cerebral arteries, each irrigating a defined territory. Because the territories are defined, the pattern of symptoms after a stroke identifies the artery involved with remarkable precision, a fact that turns anatomy into a diagnostic tool.

Finally, the vessels themselves are specialised. Their walls form the blood-brain barrier, a tightly sealed lining that admits oxygen, glucose, and selected molecules while excluding most toxins, pathogens, and drugs. It is one of the reasons treating brain disease is so hard.

Anatomy by the numbers

Figures are approximate and vary between individuals and between sources; they are given here to convey scale rather than to be quoted as exact constants.

Every page in this section

Each structure introduced above has its own full reference page. Start anywhere, but the cerebrum and the cortex are the natural entry points if the anatomy is new to you.

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