The brain is the central organ of the nervous system and the principal site of integration and processing of information in humans. It consists of billions of neurons together with large populations of glial cells, blood vessels, connective tissues and specialised supporting structures. The brain receives sensory information from the body and environment, integrates it with internal physiological states and stored information, and produces motor, autonomic, emotional and cognitive responses.
It supports perception, movement, language, attention, learning, memory, reasoning, decision making, consciousness and many other functions. It also regulates essential processes such as breathing, cardiovascular control, temperature regulation and feeding through interconnected neural networks. The brain is not a single homogeneous organ. It contains numerous regions with specialised but overlapping functions connected through extraordinarily complex networks.
Anatomically, the brain is commonly described in terms of the cerebrum, diencephalon, brainstem and cerebellum, with additional subdivisions used by neuroscientists. The cerebrum consists of two cerebral hemispheres connected by major fibre pathways, including the corpus callosum. Each hemisphere contains a cerebral cortex consisting of grey matter overlying extensive white matter pathways. Deep structures include the basal ganglia and limbic related regions. The diencephalon includes structures such as the thalamus and hypothalamus. The brainstem consists of the midbrain, pons and medulla, while the cerebellum lies posteriorly and contributes strongly to coordination, motor learning and other functions. These anatomical divisions are useful but do not imply that functions are isolated within individual structures.
The brain processes information through networks of interconnected neurons. Sensory signals enter through specialised receptors and are transformed into neural representations. The brain combines these signals with context, memory, expectations and internal states to construct perceptions and guide behaviour. Motor systems transform goals and sensory information into coordinated patterns of muscle activity. Feedback from the body continuously updates these processes. This means that brain function is fundamentally dynamic and network based.
A single region can participate in multiple functions, while a single function can depend on many regions. Modern neuroscience therefore combines anatomical localisation with network analysis, electrophysiology, neuroimaging, computational modelling and molecular biology rather than treating each mental ability as the product of one isolated anatomical spot.
The brain is protected by several specialised structures and physiological mechanisms. The skull provides mechanical protection, while the meninges surround the brain and spinal cord. Cerebrospinal fluid cushions neural tissue and participates in chemical and waste transport functions. The blood brain barrier restricts the movement of many substances from blood into neural tissue and helps maintain a controlled extracellular environment. Brain blood flow is tightly regulated because neurons require continuous oxygen and glucose. Although the brain represents only a fraction of total body mass, it has substantial metabolic demands. Interruption of blood supply can rapidly cause neuronal injury, which explains the urgency of conditions such as ischaemic stroke.
Brain development begins early in embryonic life and continues through childhood, adolescence and adulthood. Neural progenitor cells generate different populations of neurons and glia, which migrate and differentiate into specialised structures. Axons grow towards target regions, synapses form and neural circuits are refined through activity and experience. Myelination continues for many years after birth in several brain regions. Plasticity allows neural circuits to alter their connectivity and function in response to learning, experience, injury and environmental conditions. Plasticity is not unlimited, and different brain regions and developmental stages have different capacities for change. The mature brain therefore remains biologically dynamic rather than being a completely fixed structure.
Brain function can be studied using many complementary approaches. Neuroanatomy examines structures and connections, electrophysiology measures electrical activity, neurochemistry examines signalling molecules and receptors, and functional imaging can reveal changes associated with neural activity. Techniques such as magnetic resonance imaging can show anatomical structure, while functional magnetic resonance imaging can measure changes related to blood oxygenation. Electroencephalography measures electrical activity at the scalp with high temporal resolution.
Animal studies, cellular experiments and computational models provide additional information that cannot always be obtained directly from humans. No single method provides a complete picture of brain function, so modern neuroscience combines evidence across scales ranging from molecules and synapses to whole brain networks and behaviour.
Brain disorders can result from vascular injury, trauma, infection, tumours, inflammation, abnormal electrical activity, neurodegeneration, developmental differences and many other causes. Stroke can damage brain tissue by interrupting blood flow or causing bleeding. Traumatic brain injury can disrupt neural circuits and supporting structures. Epilepsy involves abnormal patterns of neuronal activity. Neurodegenerative disorders can progressively affect particular populations of neurons and connected networks. Brain tumours can interfere with function through direct tissue effects, altered pressure or disruption of neural pathways. Because different brain regions contribute to different functions, injury can produce characteristic combinations of sensory, motor, language, memory or behavioural changes, although individual outcomes vary greatly.
The brain should not be understood as a biological computer with a simple central processor. It has no single location where all information is collected and no single region that independently produces every aspect of consciousness or behaviour. Its functions emerge from interactions among distributed circuits, chemical signalling systems, bodily feedback and environmental experience. The brain also does not work separately from the spinal cord and peripheral nervous system. Perception and behaviour depend on continuous communication between brain and body. Current neuroscience increasingly emphasises distributed networks, plasticity, prediction, feedback and interactions across multiple levels of organisation. Despite enormous progress, major questions remain about consciousness, subjective experience, memory organisation and how large scale neural activity gives rise to complex cognition.
