Brain Anatomy

What Is the Brain?

The brain is the command centre of the human body. Sitting inside the protective casing of the skull, it receives information from every sense, processes it, stores some as memory and sends instructions back out to muscles, glands and organs — all in fractions of a second.

Despite weighing only about 1.4 kilograms — roughly the same as a small pineapple — the brain is extraordinarily complex. It contains an estimated 86 billion neurons (nerve cells), each forming thousands of connections with its neighbours. The total number of synapses (connection points) in a single human brain is estimated in the hundreds of trillions.

The brain never truly rests. Even during deep sleep it is actively consolidating memories, regulating breathing and heartbeat, and running maintenance processes. It accounts for only about 2% of body weight but consumes roughly 20% of the body's total energy supply.

The Three Major Divisions

Anatomists divide the brain into three broad regions: the cerebrum, the cerebellum and the brainstem. Each has distinct responsibilities, though they communicate constantly with each other and with the spinal cord.

The Cerebrum

The cerebrum is by far the largest part of the brain, making up about 85% of its total weight. It is the dome-shaped structure you picture when you imagine a brain. Its outer surface — the cerebral cortex — is heavily folded into ridges (gyri) and grooves (sulci), a design that dramatically increases surface area without requiring a much larger skull.

The cerebrum is responsible for conscious thought, voluntary movement, sensory perception, language, memory, emotion and higher reasoning. These functions are not randomly distributed — they are organised into specific regions with specialised roles.

The Cerebellum

The cerebellum (Latin for "little brain") sits beneath the back of the cerebrum. Although it makes up only about 10% of the brain's volume, it contains more than half of all the brain's neurons. It is particularly densely packed with a cell type called Purkinje cells.

The cerebellum's primary job is coordination and fine-tuning of movement. It does not initiate movements — that is the cerebral cortex's role — but it receives information about what movement was intended and compares it with what the muscles and joints are actually doing, then makes tiny corrections to keep actions smooth, precise and balanced.

The Brainstem

The brainstem connects the cerebrum and cerebellum to the spinal cord. It consists of three parts stacked from top to bottom: the midbrain, the pons and the medulla oblongata. The brainstem controls the body's most fundamental automatic functions — breathing, heart rate, blood pressure, swallowing and the sleep-wake cycle.

Ten of the twelve cranial nerves — the nerves that connect the brain directly to the head, face and many internal organs — emerge from the brainstem. Damage to the brainstem, even in a small area, can be immediately life-threatening because of the vital functions it governs.

The Cerebral Cortex and Its Lobes

The cerebral cortex is divided into two hemispheres — left and right — connected by a thick bundle of nerve fibres called the corpus callosum. Each hemisphere is further divided into four lobes, named after the skull bones closest to them.

Each lobe contains both primary sensory or motor areas and association areas. Primary areas deal with raw incoming or outgoing signals. Association areas integrate information from multiple sources to produce complex perceptions, decisions and actions.

An important note about brain lateralisation: the left hemisphere controls movement and receives sensory information from the right side of the body, and vice versa. This crossover happens in the brainstem. For most right-handed people, and a majority of left-handed people, the left hemisphere is dominant for language.

Important Deep Brain Structures

Beneath the cerebral cortex lie several critical structures embedded within the white matter of the brain.

The Thalamus

The thalamus is a paired structure sitting near the centre of the brain, often described as the brain's relay station. Almost all sensory information travelling toward the cerebral cortex passes through the thalamus first, where it is sorted and directed to the appropriate cortical area. The thalamus also plays a role in regulating consciousness and alertness.

The Hypothalamus

Sitting just below the thalamus, the hypothalamus is small but extraordinarily powerful. It links the nervous system to the hormonal (endocrine) system through its control of the pituitary gland — often called the master gland. The hypothalamus regulates body temperature, hunger, thirst, sleep-wake cycles, and the hormonal responses to stress. It is the bridge between brain activity and the body's chemical environment.

The Limbic System

The limbic system is a set of interconnected structures involved in emotion, motivation and memory. Key components include the amygdala (emotion processing, particularly fear and threat responses) and the hippocampus (essential for converting short-term experiences into long-term memories). The limbic system works closely with the prefrontal cortex of the frontal lobe to balance emotional reactions with rational thought.

The Basal Ganglia

The basal ganglia are clusters of neurons deep in the cerebrum that help regulate the initiation and smoothness of movement. They work in a circuit with the cerebral cortex and cerebellum. When the basal ganglia are disrupted — as happens in Parkinson's disease — movement becomes slow and tremors can appear. The basal ganglia also contribute to habit formation and reward-based learning.

How the Brain Is Protected

Given how vital the brain is, the body goes to considerable lengths to protect it. There are multiple layers of defence.

The outermost protection is the skull — a rigid casing of fused bones. Beneath the skull are three layers of protective membranes called the meninges: the tough outer dura mater, the web-like arachnoid mater and the delicate pia mater that clings to the brain surface. Between the arachnoid and pia layers flows cerebrospinal fluid (CSF), a clear liquid that cushions the brain against impacts, removes metabolic waste and provides a stable chemical environment.

The brain also benefits from the blood-brain barrier — a specialised lining of the blood vessels within the brain that selectively controls which substances can cross from the bloodstream into brain tissue. This barrier keeps out many potentially harmful substances, including bacteria and many drugs, while allowing oxygen, glucose and other essentials through freely.

Blood Supply and Oxygen Dependency

The brain is exceptionally dependent on a steady blood supply. It receives blood through four main arteries: the two internal carotid arteries and the two vertebral arteries. These feed into a ring of connected vessels at the base of the brain called the Circle of Willis, which provides some protection against blockages in individual arteries by allowing blood to reroute.

If blood flow to a region of the brain is interrupted — as occurs in a stroke — neurons in that area begin to die within minutes, because the brain has almost no energy reserves of its own. Even a brief interruption of a few seconds can cause loss of consciousness. This oxygen sensitivity is why maintaining a healthy cardiovascular system is directly connected to long-term brain health.

To understand how the brain communicates with the rest of the body, read our guide to the nervous system. For a closer look at automatic protective responses, see the guide to human reflexes.

The Brain During Sleep

Sleep is far from passive for the brain. During a typical night of sleep, the brain cycles through several stages: lighter NREM (non-rapid eye movement) sleep, progressively deeper NREM sleep and REM (rapid eye movement) sleep, in which most vivid dreaming occurs.

During deep sleep, the brain consolidates memories — moving information from temporary short-term storage in the hippocampus to more permanent storage in the cortex. It also clears metabolic waste products, including proteins associated with neurodegenerative conditions, through a recently discovered system called the glymphatic system.

During REM sleep, the brain is nearly as active as when you are awake. Neural circuits involved in memory and emotion are reactivated and processed. This is believed to play a role in emotional regulation and creative problem-solving. Disrupting REM sleep consistently impairs both memory and mood.

Brain Plasticity: The Ability to Change

For a long time, scientists believed the adult brain was fixed — that the neurons you were born with were essentially all you would ever have. This view has been overturned by decades of research revealing that the brain retains a remarkable capacity for change throughout life, a property called neuroplasticity.

Neuroplasticity refers to the brain's ability to reorganise itself by forming new synaptic connections and, in some regions, even new neurons. Learning a new skill, recovering from a brain injury, or adapting to the loss of a sense all involve neuroplasticity at work.

The most dramatic neuroplasticity occurs in childhood — the brain's "critical periods" — when huge numbers of new connections form and unnecessary ones are pruned away. But meaningful plasticity continues throughout adulthood. This is the biological basis for lifelong learning and rehabilitation after injury.

Explore the brain's role in the body's overall communication network with the body systems explorer, or test your knowledge with the anatomy quiz. The nervous system guide covers how the brain and spinal cord work as a team.

The Cranial Nerves

Unlike most nerves in the body, which emerge from the spinal cord, the 12 cranial nerves emerge directly from the brain or brainstem. They are numbered I through XII and are responsible for an enormous range of functions in the head and some internal organs.

• Cranial nerve I (Olfactory) — carries smell from the nose to the brain.
• Cranial nerve II (Optic) — carries visual information from the eyes.
• Cranial nerves III, IV and VI — control eye movements.
• Cranial nerve V (Trigeminal) — provides sensation to most of the face and controls chewing muscles.
• Cranial nerve VII (Facial) — controls facial expressions and carries taste from the front of the tongue.
• Cranial nerve VIII (Vestibulocochlear) — carries hearing and balance information from the inner ear.
• Cranial nerve X (Vagus) — the longest cranial nerve, extending into the chest and abdomen to influence heart rate, digestion and breathing.

The cranial nerves illustrate how intimately the brain is connected to the senses, the face and the body's internal state. Damage to a single cranial nerve produces very specific, recognisable patterns of symptoms — a feature that helps clinicians locate the source of a neurological problem.

Brain Development Across the Lifespan

The brain undergoes its most dramatic changes in the earliest years of life, but development continues long into adulthood. A newborn's brain weighs about 350 g — roughly a quarter of its adult size — and grows rapidly in the first few years, reaching about 80% of adult volume by age five.

During childhood and adolescence, the brain produces far more neural connections than it will ultimately keep. A process called synaptic pruning then eliminates unused connections, refining neural circuits based on experience. This is why early childhood experiences and environments have a particularly profound and lasting influence on brain organisation.

The prefrontal cortex — responsible for planning, impulse control and complex decision-making — is one of the last regions to fully mature, typically completing development in the early to mid-twenties. This biological timeline has practical implications for risk-taking behaviour and emotional regulation in adolescents and young adults.

In later life, the brain undergoes gradual structural changes: some regions lose volume, processing speed tends to slow and certain types of memory become less reliable. However, accumulated knowledge, vocabulary and many skills remain robust or even improve with age. Individual variation is large; brain health in older age is influenced by genetics, education, physical activity, social engagement and cardiovascular health throughout life.

What the Brain Needs to Function

The brain's enormous energy demand requires a continuous and stable supply of glucose and oxygen. Unlike most body tissues, the brain cannot store meaningful reserves of either; a few minutes without oxygen causes irreversible neuron death, beginning in the most sensitive areas.

Glucose is the brain's preferred and primary fuel under normal circumstances. During prolonged fasting or very low carbohydrate intake, the liver produces alternative fuel molecules called ketone bodies, which the brain can use to a considerable degree — a metabolic flexibility that likely evolved as a survival advantage during food scarcity.

Several micronutrients are critical for brain function. Iron is needed for oxygen transport to the brain and for synthesis of several neurotransmitters. Iodine deficiency during early brain development causes irreversible cognitive impairment. B vitamins, particularly B12 and folate, are needed for myelin synthesis and DNA replication in new neurons. Omega-3 fatty acids — especially DHA — are structural components of neuron membranes and are highly concentrated in the brain.

Test your knowledge of the brain's structure and functions using the anatomy quiz, or explore related content in the human sleep guide, which explains what the brain does during the hours you are not consciously aware of it.