The adult human brain measures approximately 16.7 centimeters in length, 14 centimeters in width, and 9.3 centimeters in height. In composition, it is roughly 78 percent water, while lipids account for 60 percent of its dry weight, supplemented by proteins, carbohydrates, and inorganic salts. Neural tissue is visibly divided into gray matter and white matter. Gray matter, accounting for about 40 percent of the brain, consists largely of neuronal cell bodies (somas) and dendrites, whereas white matter, composing the remaining 60 percent, consists primarily of myelinated axon tracts that transmit signals between regions.⁹¹⁰

Surrounding the brain within the skull are three connective tissue membranes collectively termed the cranial meninges. The outermost layer is the thick, fibrous dura mater, which consists of a periosteal layer fused to the interior of the cranium and an inner meningeal layer.¹¹¹² Inward folds of the meningeal dura divide the cranial cavity into compartments, including the falx cerebri between the cerebral hemispheres and the tentorium cerebelli between the occipital lobes and the cerebellum. Beneath the dura lies the web-like, avascular arachnoid mater, followed by the delicate pia mater, which adheres directly to the contours of the brain's surface.²

The functional tissue of the brain is composed of two broad classes of cells: neurons and glial cells. Quantitative anatomical studies estimate that the adult human brain contains approximately 86 billion neurons and a roughly equal number of glial cells, though the glia-to-neuron ratio varies substantially across different brain structures. A single cubic millimeter of human cerebral cortex contains roughly 57,000 cells and 150 million synapses.¹⁸

Neurons are electrically excitable cells specialized for rapid communication. While diverse in morphology, nearly all neurons share three structural components: a cell body (soma) housing the nucleus and protein-synthesizing organelles; branched extensions called dendrites that receive incoming signals; and a single projection called an axon that conducts electrical impulses away from the soma toward target cells.¹⁹ In the cerebral cortex, approximately 70 to 80 percent of neurons are excitatory projection neurons, while 20 to 30 percent are inhibitory interneurons that modulate local circuit activity.

Glial cells (neuroglia) maintain homeostasis, provide structural and metabolic support, and actively modulate synaptic signaling.²⁰ They are categorized into four major types within the central nervous system: Astrocytes are star-shaped cells connected by gap junctions that regulate extracellular ion concentrations, clear excess neurotransmitters from synaptic clefts, influence local cerebral blood flow, and engage in chemical signaling known as gliotransmission. Oligodendrocytes wrap axons in concentric layers of myelin, a lipid-rich insulating sheath that increases the speed of electrical propagation up to 120 meters per second. Microglia serve as the resident macrophages and primary immune defense of the central nervous system, scavenging cellular debris and eliminating redundant synaptic connections. Ependymal cells line the internal ventricles of the brain and assist in the production and circulation of cerebrospinal fluid.

Communication between neurons occurs primarily at junctions called synapses. When an electrical action potential travels down an axon and reaches the presynaptic axon terminal, it triggers the release of chemical messengers—neurotransmitters—across the microscopic synaptic cleft to bind with receptors on the postsynaptic dendrite or soma. In addition to chemical synapses, some neural populations are coupled by electrical synapses (gap junctions) that permit direct, bidirectional ion flow, alongside retrograde signaling mechanisms in which postsynaptic cells transmit chemical feedback to presynaptic terminals.

Despite its small relative mass, the human brain has an intense and uninterrupted demand for glucose and oxygen, generating between 10 and 23 watts of power during both wakefulness and sleep. Arterial blood reaches the brain via two paired vascular systems: the anterior circulation, supplied by the left and right internal carotid arteries, and the posterior circulation, supplied by the left and right vertebral arteries.²¹²² The internal carotid arteries deliver blood to the frontal, parietal, and lateral temporal lobes as well as anterior deep structures, whereas the vertebral arteries unite to form the basilar artery, supplying the brainstem, cerebellum, occipital lobes, and thalamus.

At the base of the brain within the subarachnoid space, these two vascular systems join to form an anastomotic pentagonal ring known as the circle of Willis. Composed of the anterior, middle, and posterior cerebral arteries linked by anterior and posterior communicating arteries, this ring provides collateral blood flow in the event of an arterial occlusion and helps dissipate hemodynamic pulse pressure waves.²³ Venous blood drains from the brain tissue into dural venous sinuses situated between the layers of the dura mater, which subsequently empty into the internal jugular veins.

To protect delicate neural circuits from blood-borne pathogens, toxins, and fluctuations in plasma composition, cerebral capillaries are lined by tightly joined endothelial cells supported by astrocyte end-feet, forming the blood-brain barrier. This selective barrier strictly regulates the transport of molecules between the bloodstream and the brain's extracellular fluid, preventing the passage of most large or lipid-insoluble compounds.

Mechanical cushioning and waste clearance are further supported by cerebrospinal fluid (CSF), a clear, colorless liquid filtered from blood plasma by specialized capillary networks called choroid plexuses located within the brain's four internal cavities: the paired lateral ventricles, the third ventricle, and the fourth ventricle.²⁴²⁵ The choroid plexuses produce roughly 400 to 600 milliliters of CSF daily—at a rate of about 21 milliliters per hour—though the ventricular system and surrounding subarachnoid space hold only 140 to 150 milliliters at any single time, resulting in a complete turnover of fluid four to five times per day. CSF circulates from the ventricles into the subarachnoid space enveloping the brain and spinal cord, acting as a physiological sink that collects metabolic waste products before being reabsorbed into the dural venous sinuses.

In the human embryo, the central nervous system originates from the ectoderm through a process called dorsal induction between the 18th and 26th days of gestation, when the neural plate folds inward to form the neural tube.²⁶ Once the neural tube closes around the fourth to fifth week of gestation, its anterior portion expands into three primary vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain).²⁷ These further differentiate into five secondary subdivisions—the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon—while the interior cavity of the tube develops into the ventricular system.

Subsequent development proceeds through overlapping cellular stages: neurogenesis, cell migration, differentiation, synaptogenesis, synaptic pruning, and myelination.²⁸ Almost all of the brain's 86 billion neurons are generated prenatally, migrating along chemical gradients from proliferative zones to their permanent cortical and subcortical destinations.²⁹ At birth, the human brain weighs roughly 350 to 400 grams, or about 25 percent of its adult weight. It expands rapidly during infancy, reaching 75 percent of adult weight by age two and 95 percent by age six. Because total neuron numbers remain largely static after birth, this fivefold postnatal volume expansion is driven by the growth of dendritic arbors, intense synaptogenesis ("synaptic blooming"), and the progressive myelination of axons by oligodendrocytes. From childhood through adolescence and into early adulthood, microglia and activity-dependent mechanisms eliminate weaker or redundant connections through synaptic pruning, refining neural circuitry in response to environmental experience and learning.³⁰

From an evolutionary perspective, the human brain shares a conserved vertebrate Bauplan with other mammals, contradicting older "triune brain" models proposed in the twentieth century that depicted evolution as adding separate rational cortical layers atop primitive reptilian and limbic cores.³¹³² Instead, human evolution is characterized by extreme encephalization—a brain-to-body mass ratio far exceeding that of any other primate—driven by coordinated modifications across cortical, subcortical, and cerebellar networks.³³ Most notable is the disproportionate expansion of the neocortex, particularly distributed association cortices, paired with prolonged developmental maturation that extends human neural plasticity and learning across decades.

The brain processes sensory information and directs motor output through contralaterally organized pathways.³⁴ In general, the left cerebral hemisphere receives somatosensory, visual, and auditory input from—and exerts motor control over—the right side of the body and right visual field, while the right hemisphere governs the left side. Motor commands originate in the primary motor cortex of the frontal lobe and descend via white matter tracts such as the corona radiata and internal capsule through the brainstem to the spinal cord, modulated continuously by the basal ganglia and cerebellum. Sensory signals from peripheral receptors ascend through the spinal cord or cranial nerves, synapsing in the thalamus (with the exception of olfaction) before reaching primary sensory cortices in the parietal, occipital, and temporal lobes.

While the two cerebral hemispheres appear largely symmetrical in gross structure, many higher cognitive functions exhibit hemispheric lateralization, working in tandem across the corpus callosum. In the majority of individuals, language production and grammatical processing are localized predominantly to the left hemisphere, centered on Broca's area in the inferior frontal lobe and Wernicke's area in the posterior temporal lobe. Conversely, the right hemisphere frequently shows greater specialization for visuospatial orientation, prosody, and holistic attention.³⁵

Higher-order cognition—including working memory, abstract reasoning, decision-making, and impulse control—relies heavily on the prefrontal cortex and its extensive reciprocal connections with parietal, temporal, and subcortical networks. Long-term declarative memory formation depends on synaptic plasticity within the hippocampus, where dendritic spines strengthen or weaken their connections before memories are gradually consolidated across the cerebral cortex. Meanwhile, brainstem nuclei, the hypothalamus, and the pineal gland regulate sleep-wake cycles, during which the brain remains electrically active to consolidate memories and clear metabolic byproducts.³⁶

Disruptions to the brain's vascular supply, cellular integrity, or electrical signaling produce a wide spectrum of neurological and psychiatric disorders.³⁷³⁸ Cerebrovascular disease, most notably stroke, occurs when blood flow to a region of the brain is interrupted either by an arterial blockage (ischemic stroke) or by the rupture of a blood vessel (hemorrhagic stroke), such as an intracranial aneurysm along the circle of Willis.³⁹ Deprived of oxygen and glucose, affected neurons undergo rapid necrosis, resulting in focal deficits in speech, movement, or cognition depending on the territory damaged.⁴⁰

Neurodegenerative diseases involve the progressive dysfunction and death of specific neuronal populations over years or decades, frequently accompanied by abnormal protein aggregation and microglial neuroinflammation. Alzheimer's disease, the leading cause of dementia, is characterized by intracellular neurofibrillary tangles and extracellular amyloid plaques that degrade memory and cognitive networks. Parkinson's disease results from the degeneration of dopaminergic neurons in the substantia nigra of the midbrain, causing tremor, rigidity, and bradykinesia. Other chronic degenerative conditions include amyotrophic lateral sclerosis (ALS), which targets motor neurons; Huntington's disease, a hereditary striatal disorder; and multiple sclerosis, an autoimmune condition in which the immune system attacks oligodendrocytes and strips myelin sheaths from central nervous system axons.

Mechanical forces to the head can cause traumatic brain injury (TBI), ranging from transient concussions to severe contusions, diffuse axonal injury, and epidural or subdural hematomas between the meningeal layers.⁴¹ Repeated head trauma is associated with chronic traumatic encephalopathy (CTE) and increases long-term risks for epilepsy, stroke, and dementia.⁴² Structural and electrical pathologies also include primary brain tumors—such as astrocytomas arising from glial cells or meningiomas arising from the meninges—as well as epilepsy, a disorder marked by recurrent seizures driven by abnormal, synchronized electrical discharges across cortical circuits.⁴³⁴⁴

Understanding of the human brain evolved slowly from antiquity.⁴⁵ In 335 BC, the Greek philosopher Aristotle held a cardiocentric view, proposing that the heart was the seat of intelligence while the brain functioned merely as a radiator to cool the blood. Around 170 AD, the Roman physician Galen argued that thought, personality, and movement were governed by humors flowing through the brain's fluid-filled ventricles. This ventricular doctrine persisted until the sixteenth century, when the Belgian anatomist Andreas Vesalius published detailed dissections of the human nervous system and attributed brain function to the solid neural tissue rather than the cavities.

In 1791, Luigi Galvani demonstrated that electrical stimulation of nerves caused muscle contraction, establishing the bioelectrical nature of neural signaling. During the nineteenth century, clinical case studies established the principle of functional localization: following the 1848 accident in which an iron rod pierced the left frontal lobe of railroad worker Phineas Gage and altered his personality, physicians Paul Broca and Carl Wernicke in the 1860s and 1870s linked specific left-hemisphere lesions to distinct speech deficits. At the turn of the twentieth century, Camillo Golgi developed a silver-staining technique that allowed Santiago Ramón y Cajal to visualize individual nerve cells under the microscope, establishing the "neuron doctrine"—that the nervous system is made of discrete cells rather than a continuous syncytium—for which both shared the 1906 Nobel Prize.

In the twenty-first century, neuroscience has shifted toward large-scale connectomics, single-cell transcriptomics, and computational modeling.⁴⁶⁴⁷ In May 2024, researchers at Harvard University and Google Research published a nanoscale three-dimensional reconstruction of a single cubic millimeter of human temporal cortex, using electron microscopy and artificial intelligence to map 57,000 cells, 230 millimeters of blood vessels, and 150 million synapses across 1.4 petabytes of data.⁴⁸⁴⁹ Concurrently, investigators at Massachusetts General Hospital deployed the high-resolution Connectome 2.0 magnetic resonance imaging scanner to trace microscopic white matter tracts in living humans, while the National Institutes of Health's BRAIN Initiative Cell Atlas Network (BICAN) integrated data from over 30 million cells by 2026 to map the genetic and cellular trajectories of the developing and adult human brain.⁵⁰