The brain is the principal information-processing organ of the central nervous system, which also includes the spinal cord. It receives information about the body and its surroundings, coordinates movement and internal functions, and supports perception, thought, emotion, and memory. In humans, these activities depend on interacting networks of neurons and other cells rather than on a single controlling center. The scientific study of the brain and the rest of the nervous system is called neuroscience. (nichd.nih.gov)
Major anatomical divisions
The human brain lies within the skull and is continuous with the spinal cord. Its major anatomical divisions include the cerebrum, diencephalon, brainstem, and cerebellum. These divisions contain specialized structures connected by extensive pathways, allowing sensory information and motor commands to pass between different levels of the nervous system. (training.seer.cancer.gov)
The cerebrum is the largest portion. It consists of left and right hemispheres connected by the corpus callosum, a substantial bundle of nerve fibers. Its outer layer, the cerebral cortex, participates in conscious sensation, thought, and voluntary activity. Each hemisphere includes frontal, parietal, temporal, and occipital lobes, as well as the insula, which lies within a deep lateral fold. (training.seer.cancer.gov)
The lobes have different functional specializations. Frontal regions contribute to complex thinking and problem-solving; parietal regions process sensory information; temporal regions participate in hearing and understanding sounds; and occipital regions process visual information. These descriptions identify broad associations, not independent compartments: brain functions depend on communication among regions. Language, for example, involves more than one cortical region. (nimh.nih.gov)
The diencephalon includes the thalamus and hypothalamus. The hypothalamus helps maintain homeostasis by regulating internal bodily activities. The brainstem consists of the midbrain, pons, and medulla oblongata; it connects higher brain regions with the spinal cord and contains systems involved in breathing and heart-rate control. The cerebellum, behind the brainstem, contributes to balance and the coordination of movement. (training.seer.cancer.gov)
Cells and communication
Neurons are specialized cells that receive and transmit signals. A typical neuron has a cell body, branching dendrites that receive inputs, and an axon that carries signals toward other cells. Sensory neurons convey information associated with stimuli such as light, sound, and pressure, while motor neurons transmit commands that produce muscular movement. Other neurons connect and process information within nervous-system circuits. (nichd.nih.gov)
Communication commonly involves both electrical and chemical signaling. An electrical signal travels along an axon and triggers the release of a neurotransmitter at a synapse, the specialized junction between communicating cells. The chemical messenger influences the receiving cell, allowing information to continue through a circuit. Large networks of such connections enable different brain regions to coordinate their activity. (nichd.nih.gov)
The brain also contains glial cells, which perform essential functions beyond neuronal signaling. They support neurons, regulate their chemical surroundings, remove cellular debris, and produce myelin, the insulating material around many axons. Myelin helps electrical impulses travel efficiently, and communication between neurons and glia contributes to its formation. (nichd.nih.gov)
Protection and internal environment
The skull provides physical protection, while three membranes called the meninges surround the brain and spinal cord. From outside inward, these are the dura mater, arachnoid mater, and pia mater. The pia closely follows the brain’s surface; the space beneath the arachnoid contains blood vessels and cerebrospinal fluid. This fluid also occupies interconnected cavities within the brain called ventricles and circulates around the central nervous system. (training.seer.cancer.gov)
The blood–brain barrier regulates movement of substances between circulating blood and the environment surrounding brain cells. It is formed principally by specialized properties of the lining of cerebral blood vessels. Disruption of this barrier can permit blood components to enter brain tissue and contribute to swelling and inflammatory responses. The barrier is therefore distinct from the mechanical protection supplied by bone and membranes. (ncbi.nlm.nih.gov)
Development and plasticity
Brain development involves the production, migration, and differentiation of neurons, followed by the establishment of connections. Developing neurons respond to signals from other cells that help determine their location, shape, chemical characteristics, and circuit connections. Genes influence these processes, while environmental conditions and experience also affect how the brain develops and functions. (ninds.nih.gov)
Development continues through adolescence and into early adulthood. Different systems mature at different rates; prefrontal regions involved in planning and prioritizing are among those with prolonged development. Neuroplasticity refers to the nervous system’s capacity to adapt and reorganize. It includes changes in circuit activity and connections associated with learning and can also contribute to adaptation after injury. Plasticity does not imply that all damaged tissue can be replaced or that recovery is complete. (nimh.nih.gov)
Disorders and investigation
Brain disorders include stroke, epilepsy, traumatic injury, developmental conditions, infections, tumors, and degenerative diseases such as Alzheimer’s and Parkinson’s diseases. Their effects may involve movement, sensation, cognition, behavior, or combinations of these functions. Neurological examinations assess such capacities as strength, coordination, reflexes, memory, speech, and sensory responses. (ninds.nih.gov)
Investigators and clinicians use complementary methods to examine brain structure and activity. Magnetic resonance imaging produces detailed tissue images using magnetic fields and radio waves. Functional MRI measures blood-related changes associated with activity in particular regions, rather than recording neuronal signals directly. Electroencephalography records electrical activity through electrodes placed on the scalp. Laboratory analysis, genetic testing, and cerebrospinal-fluid examination can provide additional information about infection, inherited conditions, inflammation, and other neurological processes. (ninds.nih.gov)