Brain Anatomy and Function

Brain Anatomy and Function

Linas Juozenas
Intelligence Unleashed · Biological foundations

Inside the living architecture of the mind

Every memory, plan, feeling and act of understanding depends on living tissue working across many scales. The brain is not a cabinet of isolated “centers.” It is an energy-demanding, self-adjusting system in which cells form circuits, circuits join networks, and networks cooperate with the body and environment.

AnatomyNeurons & gliaCommunicationNetworksPlasticityBrain protection
  1. CellsNeurons, glia, vessels and molecular machinery
  2. CircuitsLocal patterns of excitation, inhibition and feedback
  3. NetworksLong-range systems coordinating information and state
  4. Mind in actionLearning, reasoning, emotion, choice and behaviour
The essential correction

No region thinks alone

Localization is real. Damage to a small pathway can selectively disturb speech, vision, movement or memory. Yet complex abilities arise from communication among specialized regions. Reasoning, language, emotion and intelligence recruit distributed systems whose contribution changes with the task.

A brain map is therefore a map of prominent participation, not a collection of independent boxes. Even a primary sensory region works through recurrent exchange with attention, memory, action and bodily-state systems.

The human meaning

A mind is a living achievement

A brain embodies inheritance, development, education, practice, relationships, memory and years of adaptation. Knowledge does not float separately from biology: it is preserved through changes in living cells and networks.

Exceptional intelligence and deeply cultivated expertise can be unusually consequential. A rare insight may guide other people or solve a problem that available tools cannot solve by themselves. Respect means giving intelligent people safety, intellectual freedom, challenge, health, uninterrupted time and collaborators—while every person’s dignity remains unconditional.

01 · One system, many scales

From molecules to a thinking person

Ion channels explain how a spike begins; circuits explain selection; networks explain coordination; psychology describes what the whole person learns, feels and does. No scale replaces another.

Structure constrains function; activity reshapes structure

Existing circuitry influences what is easy or possible, while development, learning, injury and recovery can alter synapses, myelin, dendrites and patterns of network use.

02 · Orientation and support

A demanding organ, protected and supplied

The nervous system must keep delicate electrical tissue alive while continuously exchanging information with the rest of the body.

Central nervous system

The brain and spinal cord integrate sensation, action and internal regulation. The spinal cord is not merely a cable: it contains circuits for reflexes, rhythmic movement and early processing.

Peripheral nervous system

Sensory, motor and autonomic nerves link the central nervous system with skin, muscles and organs. Brain and body continually influence one another through neural, hormonal, immune and metabolic signals.

Protection and circulation

Skull, vertebrae and three meningeal layers provide mechanical protection. Cerebrospinal fluid cushions and transports; blood vessels deliver oxygen and glucose; specialized endothelial tight junctions form the core of the blood–brain barrier, supported by pericytes and astrocytic endfeet.

≈86 billionAn influential estimate of neurons in the adult human brain—not an exact count for every person. The same study estimated a similar number of non-neuronal cells.1
≈2% / ≈20%Roughly two percent of body mass, yet about one-fifth of resting energy use. Much of that energy maintains signalling and readiness.2
Thousands of typesModern molecular atlases reveal extraordinary cellular diversity rather than one generic neuron and one generic glial cell.3

Grey matter, white matter and fluid spaces

Grey matter contains many neuronal cell bodies, dendrites, synapses, local axons and glia; it forms cortex and deep nuclei. White matter contains long-range axons, many wrapped in lipid-rich myelin, plus supporting cells and vessels. Neither colour means more or less important: cognition requires local processing and communication between processors.

The ventricular system contains interconnected, CSF-filled spaces. Most CSF is secreted by specialized choroid-plexus epithelium. It circulates through ventricles and spaces around the brain and spinal cord, participating in buoyancy, chemical stability and waste transport. Fluid clearance involves several interacting routes, including perivascular and meningeal lymphatic pathways; important details remain under study.29

03 · Specialization within cooperation

Major structures and what they contribute

The safest question is not “What does this part do?” but “What does it contribute, through which connections, under which conditions?”

The folded cerebral cortex

The cortex is a layered sheet folded into gyri and sulci. Most neocortex has six layers, but their thickness, cell types and connections vary markedly by region. Primary sensory and motor maps sit alongside association cortex, where information is integrated across longer timescales and wider networks.

Cortical regions: prominent roles without single-function mythology
Region Prominent contributions Important correction
Frontal Voluntary movement, planning, working memory, inhibition, rule use, language production and evaluation. “Executive function” is distributed; the frontal lobe depends on parietal, subcortical, sensory and cerebellar partners.
Parietal Touch and body maps, multisensory integration, spatial attention, quantity and transformations in space. There is no isolated mathematics center; numerical thought recruits task-dependent networks.
Temporal Hearing, language comprehension, semantic knowledge, object and face processing, and access to memory systems. Memory is not one faculty or one location; different systems support episodes, skills, facts and working memory.
Occipital Early and intermediate analysis of visual contrast, orientation, colour, depth and motion. Seeing continues through pathways extending into temporal, parietal, frontal and subcortical systems.
Insula Interoception, taste, pain integration, salience and coordination between bodily state and action. It participates in emotion but does not independently manufacture a feeling.

Deep structures and connecting systems

Hippocampal formation

Medial-temporal circuits help form relational and episodic memories and represent places and contexts. H.M.’s profound new-learning impairment after bilateral medial-temporal surgery transformed memory science.4 Place-responsive cells support spatial coding, but the hippocampus is not a permanent warehouse for every memory.5

Amygdala

A family of nuclei that learns about biological relevance, threat, reward and uncertainty and helps adjust attention, memory and bodily responses. Amygdala damage can disturb recognition of fear signals, but the structure is not simply a universal “fear button.”8

Thalamus

Organized nuclei exchange information with cortex, basal ganglia and cerebellum. Beyond sensory routing, thalamic hubs help regulate attention, arousal and communication between cortical systems.7

Hypothalamus

A compact set of nuclei coordinating temperature, appetite, thirst, circadian timing, reproduction, stress responses and endocrine control through the pituitary. It turns internal measurements into neural, hormonal and behavioural action.

Basal ganglia

Interconnected nuclei participate in selecting and learning actions, habits, motivation and reinforcement.9 Some midbrain dopamine signals carry prediction-error information used to update behaviour; dopamine is not merely a “pleasure chemical.”32 Addiction reflects adaptations across reward, habit, stress, memory and control circuits—not just too much dopamine.33

Cerebellum

Dense, regular circuitry predicts and corrects timing in movement and learning. Cerebellar lesions can also affect language, planning, spatial thought and emotional regulation, demonstrating loops far beyond motor control.6

Brainstem

Midbrain, pons and medulla contain pathways and nuclei essential to breathing, cardiovascular control, arousal, sleep, eye movement, facial sensation, swallowing and posture. These functions emerge through circuits extending into hypothalamus, spinal cord and forebrain.

Commissures and pathways

The corpus callosum is the largest bridge between hemispheres; other commissures and projection tracts connect cortex with deep structures and the spinal cord. White matter carries coordinated traffic, but direction, timing and inhibition determine what that traffic means.

Laterality without the “left-brain/right-brain” myth

Some operations are lateralized: language is left-dominant in many people, while aspects of spatial attention often show right-hemisphere advantages. Yet both hemispheres participate in nearly every complex activity and continuously exchange information. People are not divided into logical left-brained and creative right-brained types.10

04 · The cellular society

Neurons signal; glia make signalling possible

A neuron is not a wire, and glia are not packing material. Both exist in diverse forms whose properties depend on region, development and role.

  1. DendritesReceive and locally transform many inputs
  2. SomaIntegrates metabolism, genes and incoming activity
  3. AxonCarries regenerative electrical signals
  4. TerminalReleases a transmitter or other signal
  5. SynapseChanges the next cell’s state and probability of responding

Neurons: selective, dynamic integrators

Dendrites contain active channels and can perform local computations; the cell body integrates electrical and chemical state; the axon initial segment often determines whether an action potential begins. Some axons travel long distances, others remain local. Neurons may release fast transmitters, slower modulators, peptides or combinations of signals.

In neocortex, excitatory projection neurons are numerous and inhibitory interneurons are highly diverse. Inhibition does not merely stop thought: it sharpens timing, stabilizes networks, separates competing representations and prevents uncontrolled excitation.

Glia: support, defence and regulation

Astrocytes regulate ions, take up transmitters, help match blood flow to activity and support the blood–brain barrier. Oligodendrocytes form CNS myelin; Schwann cells myelinate peripheral axons. Microglia survey tissue, respond to injury and participate in synaptic remodelling. Ependymal cells line ventricles, while specialized choroid-plexus epithelium secretes most CSF.

Whole-brain neuron and non-neuronal counts are roughly comparable in the influential 86-billion estimate, but ratios vary dramatically by region. The cerebellum contains most brain neurons; the cerebral cortex contains a larger proportion of non-neuronal cells.1

Single-cell sequencing now distinguishes thousands of transcriptomic cell populations and states across the adult human brain. These molecular categories do not map one-to-one onto mental functions, but they reveal how much biology hides beneath broad labels such as “neuron.”3

05 · Communication

Electric within cells, often chemical between them

Neural information is carried not by electricity alone but by patterns of voltage, transmitter release, receptor response, timing and changing connection strength.

From membrane potential to action potential

Cell membranes separate unequal concentrations of ions. Selective channels and pumps help create a voltage difference, commonly near −70 mV in a resting neuron but varying by cell and moment. Inputs produce graded voltage changes. If the axon’s trigger region reaches threshold, voltage-gated channels generate an all-or-none action potential that regenerates along the axon. Hodgkin and Huxley’s equations first captured the underlying sodium and potassium conductances in squid giant axon; modern neurons add many more channel types and geometries.11

Myelin permits rapid, energy-efficient saltatory conduction between nodes of Ranvier. Not every axon is myelinated, and myelin is not inert insulation: oligodendrocyte behaviour can respond to activity, making adaptive myelination another possible mechanism of learning and network refinement.30

A chemical synapse in five steps

  1. An action potential reaches a presynaptic terminal.
  2. Voltage-gated calcium channels open and calcium enters.
  3. Vesicles fuse with the membrane and release transmitter.
  4. Transmitter binds ionotropic or metabotropic receptors.
  5. Reuptake, diffusion or enzymes clear the signal.
  6. The receiving cell integrates this input with thousands of others.
Different signals solve different coordination problems
Signal family Broad role Typical timescale Correction
Fast excitation Raises the probability of downstream activation; glutamate is the major fast excitatory transmitter in the CNS. Milliseconds Excitation is not inherently “good”; excessive activation can be destructive.
Fast inhibition Shapes timing and competition; GABA is the major fast inhibitory transmitter in the adult brain. Milliseconds Inhibition enables precision and stable information processing.
Neuromodulation Dopamine, serotonin, noradrenaline and acetylcholine alter gain, learning, state and plasticity across circuits. Seconds to longer No transmitter corresponds to one emotion or personality.
Body and hormones Autonomic, endocrine, immune and metabolic signals coordinate brain state with the organism. Fast to hours The brain cannot be understood as detached from the body.

Plastic synapses are not simply “stronger is better”

Long-term potentiation can increase later synaptic responses.12 Long-term depression, stabilization, inhibition, structural change and forgetting also matter. A healthy network must remain selective—not maximize every connection.

06 · Coordination in time and space

Circuits, rhythms and large-scale networks

The connectome describes possible routes; moment-to-moment activity selects which routes cooperate now.

Local circuitry

Feedforward, feedback, recurrent and lateral connections allow a circuit to amplify evidence, suppress competitors, maintain information and compare prediction with input. The same named motif can compute differently depending on cell type and state.

Structural connectivity

Axons constrain who can communicate directly. Diffusion MRI estimates bundles non-invasively, while tracing and microscopy provide finer evidence in animals or tissue. A tract is a route, not proof that it is active during a particular thought.

Functional coordination

Regions whose activity fluctuates together form functional networks. Correlation can reveal organization, but it does not by itself establish direct wiring, direction or cause. Human Connectome Project data transformed this large-scale mapping.16

Networks repeatedly observed in human imaging

Default-modeInternally oriented thought, autobiographical and conceptual processing
Frontoparietal controlFlexible goal maintenance, rule use and coordination
Attention systemsOrienting toward goals or behaviourally relevant events
Salience-relatedIntegrating bodily and contextual importance with control
Sensory & motorDistinct maps for vision, hearing, touch and action

Network boundaries vary by method, resolution and individual. Maps based on 1,000 people revealed both topographic sensory systems and distributed association networks, but a seven- or seventeen-network atlas is a useful model—not the final natural number of networks.1314

Brain rhythms are timing patterns, not personality labels

Conventional frequency bands—the borders and meanings vary
Band Approximate range Often studied in relation to What cannot be inferred alone
Delta ~0.5–4 Hz Deep sleep and slow cortical dynamics Health, intelligence or consciousness from “more delta” alone
Theta ~4–8 Hz Memory, navigation and cognitive control in particular circuits One universal memory state
Alpha ~8–13 Hz Sensory excitability, attention and eyes-closed rest Relaxation or creativity in every region
Beta ~13–30 Hz Sensorimotor maintenance, prediction and active cognition A single mental operation
Gamma ~30 Hz and above Local coordination and task-related processing Binding, insight or superior intelligence by itself

EEG and MEG record mixtures generated by many cells. Frequency only becomes informative with location, timing, phase, task, amplitude, recording quality and comparison conditions. Consumer devices that translate one band directly into a thought or trait exceed what the signal alone can justify.

07 · A developing architecture

Plasticity, learning and intelligence

The adult brain retains substantial capacity for change. Plasticity is real, meaningful and worth celebrating—but it is specific, constrained and dependent on conditions.

General intelligence is distributed—and consequential

There is no single intelligence center. Structural imaging links general cognitive ability with many small differences across total brain volume, cortical regions, white-matter microstructure and connectivity. In UK Biobank, total brain volume correlated modestly with general intelligence, while regional measures added information but did not provide a deterministic anatomy of a person.19

Functional studies likewise favour coordination across the brain. Resting connectivity can predict part of the variation in general intelligence under cross-validation, and global connectivity profiles often outperform isolated-region theories.18 A 2026 Human Connectome Project study further supported a flexible, brain-wide network architecture rather than one privileged region or network.17

Biology matters

Intelligence differences reflect interacting genetic and environmental influences. Brain development depends on inherited variation, prenatal and childhood conditions, health, education, exposure and chance. Heritability describes variation in a particular population and environment; it does not make an individual’s future fixed.

Education matters

Across more than 600,000 participants, a meta-analysis estimated that an additional year of education was associated with roughly 1–5 points of benefit on intelligence tests, depending on design and assumptions.22 This is evidence for meaningful development, not a guaranteed annual dose or unlimited increase.

Transfer matters

Improvement on a practised task is not automatically broader intelligence growth. Strong evidence asks whether gains transfer to unfamiliar problems, endure, and improve learning or life outside the training program.

Adult neurogenesis: progress without overstatement

Studies published in 2025 and 2026 identified proliferating progenitors, neuroblasts and immature granule neurons in the adult human dentate gyrus, strengthening evidence that some neuron production continues.2331 Its rate, variation between people and contribution to memory remain unresolved. Most adult learning is explained through changes in existing cells and connections, not widespread birth of new neurons.

08 · Attention and originality

Original thought needs inner space and other minds

The social environment does not sit outside brain function. It selects what receives attention, which questions are permitted and whether an unfinished idea has time to become coherent.

Space to wander

Chosen solitude can reduce interruption, immediate imitation and the pressure to follow someone else’s priorities. Undirected time may leave room for remote associations, private questions and incomplete possibilities to coexist long enough for something original to form.

This is not a claim that isolation automatically creates genius, nor that the default-mode network proves creativity. It is a practical recognition that constant instruction can accidentally occupy the very attention from which originality grows.

Return, test and build

Other people supply knowledge, criticism, encouragement, tools, resources and complementary abilities. Community helps distinguish a valuable idea from an attractive mistake, turn insight into working form, protect its creator and carry it farther than one person could alone.

Voluntary solitude differs from unwanted isolation. Healthy intellectual life includes freedom to withdraw for thought and freedom to return when connection is wanted—to people who support, promote and celebrate the mind rather than demand that it disappear.

Protect the unfinished thought from constant direction; then welcome it back into a community strong enough to test it, grow it and celebrate what it becomes.
Independence and belonging are partners, not opposites
09 · Evidence and its limits

How neuroscience studies the living brain

No method reads the whole system directly. Confidence grows when anatomy, timing, behaviour, perturbation and molecular evidence converge.

Every method answers a different question
Method Shows Strength Central limitation
Structural / diffusion MRI Anatomy and water-diffusion orientation In-vivo tissue and pathway estimates Does not measure thought; tractography is probabilistic
fMRI Blood-oxygen-level-dependent changes Whole-brain spatial patterns Indirect and slow relative to neural signalling15
EEG / MEG Electrical potentials / magnetic fields Millisecond timing Exact source localization is difficult
PET Radioactive molecular tracers Selected metabolism or receptors Radiation and modest temporal resolution
Lesions / stimulation Effects of damage or perturbation Causal clues about necessity and contribution Boundaries, reorganization and protocol complicate inference
Cells and tissue Spikes, structure, genes and molecules Exceptional local detail Human samples are limited and often clinical or post-mortem

A colourful brain image is a reconstruction, not a photograph of thought

Imaging maps depend on preprocessing, statistical thresholds, comparison groups and models. Activation does not prove that a region alone produced the experience; reduced activation can mean impairment, efficiency, strategy change or timing missed by the method. Replication, adequate samples, transparent analysis and out-of-sample prediction matter.

10 · Preserve the possibility

Brain health is part of intellectual development

Protecting cognition is not only about avoiding disease. It preserves memory already earned, learning still possible and the future work a person may contribute.

Sleep and rhythm

Sleep supports memory consolidation, attention, metabolic regulation and emotional control. Stable timing, adequate duration and treatment of persistent sleep disorders matter more than chasing a single “perfect” sleep stage.27

Movement and circulation

Physical activity supports vascular health and is associated with cognitive benefits. In one randomized study of older adults, aerobic exercise increased hippocampal volume and improved spatial memory, though no one program guarantees the same response for every person.26

Learning and access

Challenge, education, hearing and vision care, useful tools and cognitively rich activity help maintain participation and build reserve. Cognitive reserve may let people sustain function despite some age- or disease-related brain changes.28

Safety and recovery

Seat belts, helmets where appropriate, fall prevention and safe work practices reduce avoidable head injury. Rehabilitation uses repetition, task relevance, compensation and remaining networks; plasticity supports recovery but does not make injury harmless.

Alcohol and other drugs

Alcohol is ethanol: a psychoactive, dependence-producing drug. Legal status and cultural familiarity do not make it harmless. In 36,678 middle-aged and older UK Biobank participants, greater reported intake was associated with lower global and regional grey-matter measures and differences in white-matter microstructure; the observational design cannot prove what alcohol caused in an individual.24

Care without stigma

Neurological and psychiatric conditions arise from interacting biology, development, body systems and environments—not weak character. Focal injuries such as stroke can cause specific syndromes, while many conditions involve distributed circuits rather than one chemical imbalance. Accurate assessment and qualified care protect agency and help people use remaining strengths.

Protect informed choice, not a cultural double standard

Different substances have different acute and long-term risks depending on dose, route, purity, age, health, interactions and context. A legal drug should not be advertised as harmless while illegal drugs alone are treated as “real drugs.” Nor does one comparative ranking make another substance safe. Honest education names alcohol’s risks plainly, avoids glamorizing intoxication and rejects coercion or social pressure to impair a mind.

Longitudinal research has linked higher alcohol consumption with adverse brain outcomes, including hippocampal and white-matter differences, while uncertainty remains at low levels and between individuals.25 The protective message is simple: less exposure generally means less exposure-related risk, and people deserve truthful information without moral condemnation.

Myths worth retiring

“We use only 10% of the brain.”Brain systems are active across daily life, though not every neuron fires simultaneously—and that would not be desirable.
“People are left-brained or right-brained.”Specialization exists, but complex thought depends on both hemispheres and their connections.
“More activation means a better brain.”Efficient performance can require less activity, while excess synchronized activity can be pathological.
“The adult brain cannot change.”It remains plastic, though age, health, prior structure and the kind of practice constrain change.
“A scan can read a person exactly.”Group patterns and probabilistic predictions are not transparent access to an individual’s private thought.
“One app, frequency or pill upgrades everything.”Broad claims require broad, durable transfer—not improvement on one trained task or biomarker.
11 · The whole picture

The brain is specialized, connected, embodied and changeable

Cells enable circuits; circuits coordinate networks; networks operate through a living body; experience reshapes the system. This architecture makes learning and intelligence possible without reducing a person to one region, transmitter or scan.

Intelligence matters because it expands the capacity to understand complexity, learn quickly, foresee consequences and create what did not exist. Developing it deserves celebration. Protecting exceptional minds deserves seriousness. When we preserve health, freedom, education, quiet thought and chosen collaboration, we protect accumulated knowledge and future discoveries—not only for one person, but for everyone those discoveries may help.

Evidence library

Sources and further reading

Foundational neuroscience, major studies, reviews and authoritative resources supporting this guide.

  1. Azevedo et al. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain (2009).
  2. Raichle & Gusnard. Appraising the brain’s energy budget (2002).
  3. Siletti et al. Transcriptomic diversity of cell types across the adult human brain (2023).
  4. Scoville & Milner. Loss of recent memory after bilateral hippocampal lesions (1957).
  5. O’Keefe & Dostrovsky. The hippocampus as a spatial map: preliminary evidence from unit activity in the freely-moving rat (1971).
  6. Schmahmann & Sherman. The cerebellar cognitive affective syndrome (1998).
  7. Hwang, Bertolero, Liu & D’Esposito. The human thalamus is an integrative hub for functional brain networks (2017).
  8. Adolphs et al. Impaired recognition of emotion in facial expressions following bilateral damage to the human amygdala (1994).
  9. Alexander, DeLong & Strick. Parallel organization of functionally segregated circuits linking basal ganglia and cortex (1986).
  10. Gazzaniga. Cerebral specialization and interhemispheric communication (2000).
  11. Hodgkin & Huxley. A quantitative description of membrane current and its application to conduction and excitation in nerve (1952).
  12. Bliss & Lømo. Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit (1973).
  13. Yeo et al. The organization of the human cerebral cortex estimated by intrinsic functional connectivity (2011).
  14. Power et al. Functional network organization of the human brain (2011).
  15. Logothetis et al. Neurophysiological investigation of the basis of the fMRI signal (2001).
  16. Van Essen et al. The WU-Minn Human Connectome Project: an overview (2013).
  17. Wilcox, Hemmatian, Varshney & Barbey. The network architecture of general intelligence in the human connectome (2026).
  18. Anderson & Barbey. Investigating cognitive neuroscience theories of human intelligence: a connectome-based predictive modeling approach (2023).
  19. Cox et al. Structural brain imaging correlates of general intelligence in UK Biobank (2019).
  20. Maguire et al. Navigation-related structural change in the hippocampi of taxi drivers (2000).
  21. Draganski et al. Neuroplasticity: changes in grey matter induced by training (2004).
  22. Ritchie & Tucker-Drob. How much does education improve intelligence? A meta-analysis (2018).
  23. Dumitru et al. Identification of proliferating neural progenitors in the adult human hippocampus (2025).
  24. Daviet et al. Associations between alcohol consumption and gray and white matter volumes in the UK Biobank (2022).
  25. Topiwala et al. Moderate alcohol consumption as risk factor for adverse brain outcomes and cognitive decline: longitudinal cohort study (2017).
  26. Erickson et al. Exercise training increases size of hippocampus and improves memory (2011).
  27. Rasch & Born. About sleep’s role in memory (2013).
  28. Stern. Cognitive reserve in ageing and Alzheimer’s disease (2012).
  29. Louveau et al. Structural and functional features of central nervous system lymphatic vessels (2015).
  30. Fields. A new mechanism of nervous system plasticity: activity-dependent myelination (2015).
  31. Disouky et al. Human hippocampal neurogenesis in adulthood, ageing and Alzheimer’s disease (2026).
  32. Schultz, Dayan & Montague. A neural substrate of prediction and reward (1997).
  33. Koob & Volkow. Neurobiology of addiction: a neurocircuitry analysis (2016).

Educational note: This article explains general neuroscience and does not diagnose symptoms, interpret an individual scan or replace assessment by a qualified health professional. Brain measures are probabilistic, and clinical meaning depends on history, examination, methods and context.

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