3D brain atlas
A three-dimensional model of the human brain with 34 regions. For each region the atlas describes its function, how it works and the effects of disease. Three layers show the lobes, the functional areas of the cortex and the deep structures.
The model is simplified. Shapes, sizes and boundaries are approximate, functional areas are shown at their typical locations, and individual brains vary. It is not intended for medical use.
Systems of the brain
Ten systems of the brain are described in turn. For each system, the model shows its location, how it works and the changes caused by a condition that affects it.
The cerebral cortex
The cortex is the folded outer layer of the brain, a few millimetres thick. Its ridges (gyri) and grooves (sulci) allow a large surface to fit inside the skull. Each hemisphere is divided into four lobes, whose boundaries are marked by deep grooves such as the central and lateral sulci.
Functions of the lobes
Each lobe has particular functions: planning and movement in the frontal lobe, touch and spatial awareness in the parietal lobe, hearing and memory in the temporal lobe, and vision in the occipital lobe. Most tasks draw on several lobes at once. Recognising a friend’s face and voice, for example, depends on connections between the temporal and occipital lobes and many other areas.
Epilepsy
In epilepsy, groups of neurons fire in sudden, abnormal and excessive bursts. A focal seizure begins in one area, often the temporal lobe, and causes symptoms related to the function of that area. The activity sometimes spreads to both hemispheres.
Related conditions: focal seizures, generalised seizures, temporal lobe epilepsy.
The motor pathway
The primary motor cortex, a strip of cortex in front of the central sulcus, contains a map of the body. Its signals travel down the corticospinal tract, through the brainstem, to the spinal cord. Most of these fibres cross to the other side in the lower brainstem, so the left side of the brain moves the right side of the body.
The basal ganglia
The basal ganglia are deep nuclei (the caudate, putamen and globus pallidus) that help to select movements and keep them smooth. Dopamine released by neurons of the substantia nigra keeps this circuit in balance.
Parkinson’s disease
In Parkinson’s disease, the dopamine-producing neurons of the substantia nigra are gradually lost. Tremor, stiffness and slow movement usually appear only after a large proportion of these cells has gone. Medicines that replace or mimic dopamine can relieve the symptoms.
Related conditions: Parkinson’s disease, Huntington’s disease, dystonia.
The cerebellum
The cerebellum lies beneath the back of the brain. It occupies about a tenth of the brain’s volume but contains roughly four out of every five of its neurons.
How the cerebellum corrects movement
The cerebellum receives a copy of each movement plan from the cortex, relayed through the pons, and feedback from the body about the movement actually made. It sends corrections back through the thalamus, which makes movements accurate and well timed. It also contributes to learning motor skills such as riding a bicycle.
Ataxia
Damage to the cerebellum from stroke, multiple sclerosis, alcohol or inherited conditions causes ataxia: unsteady walking, clumsy hand movements, slurred speech and jerky eye movements. Strength is usually normal; coordination is impaired.
Related conditions: ataxia, cerebellar stroke, multiple sclerosis.
The sensory cortices
The somatosensory cortex, behind the central sulcus, contains a map of touch across the body. The auditory cortex lies on the upper surface of the temporal lobe, and the visual cortex at the back of the brain.
The visual pathway
Signals leave each eye along the optic nerve. At the optic chiasm about half of the fibres cross, so each side of the brain receives the opposite half of the visual field. The signals are relayed in the thalamus before they reach the visual cortex. Every sense except smell passes through the thalamus first.
Migraine aura
Roughly a quarter to a third of people with migraine experience an aura, often flickering lights or zigzag lines. The aura is thought to be caused by cortical spreading depression, a slow wave of activity that moves across the visual cortex and is followed by a period of reduced activity.
Related conditions: migraine, occipital stroke, visual field loss.
Language areas
In most people, language depends mainly on the left hemisphere. Broca’s area, in the frontal lobe, contributes to producing speech; Wernicke’s area, in the temporal lobe, contributes to understanding it.
The arcuate fasciculus
The arcuate fasciculus is a curved bundle of fibres that links Wernicke’s and Broca’s areas. Hearing a word, understanding it and repeating it depend on this pathway together with the auditory cortex and many other regions.
Aphasia after stroke
A stroke that damages Broca’s area can cause slow, effortful speech while comprehension remains relatively good. Damage to Wernicke’s area can cause fluent speech that makes little sense, with poor comprehension. Speech and language therapy helps many people to recover.
Related conditions: stroke, aphasia, primary progressive aphasia.
The limbic system
The limbic system is a ring of structures on the inner side of each hemisphere: the hippocampus, the amygdala, the fornix and the cingulate gyrus. Together they support memory and emotion.
The Papez circuit
The hippocampus binds the details of an experience into a new memory. Its signals travel through the fornix to the hypothalamus and thalamus, then to the cingulate cortex and back, a loop known as the Papez circuit. The amygdala marks which experiences are emotionally important, which is why emotional events are often remembered vividly.
Alzheimer’s disease
In Alzheimer’s disease, abnormal amyloid and tau proteins accumulate and neurons are lost. Damage often begins in and around the hippocampus, which is why recent memories are lost first. As brain tissue shrinks, the fluid-filled ventricles enlarge.
Related conditions: Alzheimer’s disease, other dementias, amnesia.
The hypothalamus and pituitary gland
The hypothalamus, no larger than an almond, lies at the base of the brain, and the pituitary gland hangs below it. Further back, the small pineal gland produces melatonin.
Hormonal control
The hypothalamus monitors body temperature, hunger, thirst and stress, and sends chemical signals to the pituitary gland. The pituitary then releases hormones into the blood that control growth, metabolism, stress responses, water balance and reproduction.
Pituitary tumours
Most pituitary tumours are benign, but they can disturb hormone levels. Because the gland lies just below the optic chiasm, a growing tumour can press upwards on it and cause loss of the outer half of the visual field in both eyes.
Related conditions: pituitary adenoma, hormone disorders, bitemporal hemianopia.
White matter and the corpus callosum
Beneath the cortex lies white matter: bundles of nerve fibres wrapped in fatty myelin. The largest bundle is the corpus callosum, an arch of hundreds of millions of fibres that connects the left and right hemispheres.
Myelin
Myelin insulates nerve fibres so that electrical signals travel along them quickly. Through the corpus callosum, each hemisphere shares sensory information and movement plans with the other.
Multiple sclerosis
In multiple sclerosis, the immune system attacks myelin and leaves areas of damage called lesions, often near the ventricles and in the corpus callosum. Signals slow down or fail, which causes symptoms such as numbness, weakness, visual problems and fatigue.
Related conditions: multiple sclerosis, other demyelinating diseases.
The ventricles
The brain contains four connected, fluid-filled spaces called ventricles: two large lateral ventricles, a narrow third ventricle and the fourth ventricle near the brainstem.
Circulation of cerebrospinal fluid
The choroid plexus produces cerebrospinal fluid, which flows from the lateral ventricles to the third ventricle, through a narrow channel (the cerebral aqueduct) to the fourth, and then around the brain and spinal cord before it is absorbed into the blood. The fluid cushions the brain and helps to clear waste.
Hydrocephalus
When the fluid cannot drain, for example because the aqueduct is blocked, it accumulates and the ventricles swell and press on the brain. The symptoms depend on age and cause. A surgically placed shunt, or a small opening made inside the ventricles, can relieve the pressure.
Related conditions: hydrocephalus, normal pressure hydrocephalus.
The brainstem
The brainstem has three parts: the midbrain, the pons and the medulla. It connects the brain to the spinal cord and contains the centres that control breathing and heart rate.
Pathways through the brainstem
All signals between the brain and the body pass through the brainstem: motor commands travel down and sensations travel up. It also contains the nuclei of most cranial nerves, which control the face, eyes, swallowing and hearing.
Locked-in syndrome
A stroke in the pons can interrupt the pathways for movement while leaving awareness intact. People with locked-in syndrome are awake and can think, see and hear, but cannot move or speak; many communicate with blinks or vertical eye movements.
Related conditions: brainstem stroke, locked-in syndrome, motor neurone disease.
How brain systems work together
Even a simple act, such as reaching for a cup, draws on the cortex, basal ganglia, cerebellum, brainstem and spinal cord at once. For this reason, damage in one place can have effects elsewhere, and recovery after injury is often possible as other areas adapt.
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