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Robotic Brain Universe

How the brain is organised

A map of the whole brain: its divisions, cells and energy budget, how it develops from a tube, how its network is wired, and how it changes over a lifetime.

Introductory · about 8 min · updated 2026-10-02 · awaiting clinical review

See it in 3D:

Illustrative simulation excitatory inhibitory

Forebrain, midbrain and hindbrain and the five vesicles; grey matter, white matter and ventricles; how this atlas's layers fit together; 86 billion neurons and the glia myth; the energy cost of signalling and sparse codes; the economy of brain networks and small worlds; neural tube to adult brain; lifespan brain charts; scaling, spike cost and network efficiency.

Contents
  1. A tour of the whole brain
  2. What the parts are
  3. Why the brain is organised this way
  4. How the brain is built
  5. When the brain grows and changes
  6. When organisation goes wrong
  7. Numbers that describe the brain
  8. Technology: counting, charting and mapping
  9. Milestones
  10. Frontiers
  11. Check yourself

A tour of the whole brain

The adult human brain contains about 86 billion neurons and a similar number of other cells. Its cerebral cortex makes up 82% of its mass yet holds only 19% of its neurons.[1]

It is also expensive: the brain's energy demands account for at least 20% of the body's energy consumption, and much of that energy goes on signalling between neurons.[2,3]

This reading is the map for the rest of the atlas: how the brain is divided, how it develops from a simple tube, how its parts are wired into an economical network, and how it changes across a lifetime. Many of the regions it mentions have their own deeper readings.[4,5,6]

What the parts are

Three divisions. Early in development the front of the neural tube swells into three vesicles: the prosencephalon (future forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain). The forebrain divides into the telencephalon and diencephalon, and the hindbrain into the metencephalon and myelencephalon, establishing the basic organisation of the central nervous system.[4]

Grey and white matter. Regions containing neuron cell bodies look grey, giving grey matter: the neocortex, a 2–5 mm thick layer on the brain's surface, and the subcortical nuclei that relay information to and from it. Axons wrapped in fatty, white myelin form the white matter pathways that connect regions. At the centre lie the interconnected ventricles, filled with cerebrospinal fluid.[4]

The atlas's layers. In this atlas the frontal, parietal, temporal, occipital, cingulate and insular lobes are built from cortical parcellations; deep structures such as the thalamus and hippocampus come from an automated segmentation that assigns one of 37 labels to each voxel; white matter appears as population-averaged tracts; and the brainstem, cerebellum and ventricles complete the picture.[7,8,9,10]

Key numbers

Neurons in the adult male human brain
86.1 ± 8.1 billion[1]
Non-neuronal cells
84.6 ± 9.8 billion[1]
Share of all neurons that are in the cerebral cortex
19%[1]
Thickness of the neocortex
2–5 mm[4]
Cortical areas per hemisphere in a multimodal parcellation
180[11]

Why the brain is organised this way

Economy. Brain networks are costly in material and energy, and many features of their organisation can be explained by a drive to minimise wiring cost. Yet they also have high efficiency, robustness, modularity and a 'rich club' of well-connected hubs, which cost extra. Bullmore and Sporns propose that brain organisation is an economic trade-off between minimising cost and gaining valuable network topology.[5]

Energy-efficient codes. An energy budget for grey matter estimates that action potentials and the postsynaptic effects of glutamate use most of the signalling energy (47% and 34%), which favours sparse, distributed codes in which no more than about 15% of neurons are active at once.[3]

Small worlds. Networks that are highly clustered locally yet have short paths between any two points, called small-world networks, exchange information efficiently both globally and locally; neural networks share this organising principle.[12,13]

How the brain is built

The neural tube. The first well-defined neural structure, the neural tube, forms in the third week of gestation; its last openings close around embryonic days 25 and 27. Cells at its front end will form the brain, and those further back the hindbrain and spinal cord.[4]

Neurons and their journeys. Neuron production begins on embryonic day 42 and is largely complete by mid-gestation. Newborn neurons migrate away from where they were produced; those destined for the neocortex form its six layers in an orderly way, then grow the dendrites and axons that build networks. By the end of pregnancy the major fibre pathways, including the thalamocortical pathway, are in place.[4]

Folding. The longitudinal fissure separating the hemispheres begins to form as early as gestational week 8 and is complete by week 22; the central sulcus appears between weeks 20 and 24, and secondary sulci between weeks 30 and 35.[4]

From neural tube to adult brain (simplified)Neural tube formsthird week of gestationThree primary vesiclesforebrain, midbrain, hindbrainFive secondary vesiclestelencephalon to myelencephalonNeurons produced and migratefrom embryonic day 42;six-layered neocortexFissures and sulci formlongitudinal fissure from week8; secondary sulci weeks 30–35Growth after birthgrey matter volume peaks near 6yearsWhite matter peaksnear 29 years, then declines
From neural tube to adult brain (simplified). Milestones from embryology and from lifespan charts of more than 100,000 MRI scans; individual timing varies.[4,6]
Text version of the diagram
  1. Neural tube forms: third week of gestation. Leads to Three primary vesicles.
  2. Three primary vesicles: forebrain, midbrain, hindbrain. Leads to Five secondary vesicles.
  3. Five secondary vesicles: telencephalon to myelencephalon. Leads to Neurons produced and migrate.
  4. Neurons produced and migrate: from embryonic day 42; six-layered neocortex. Leads to Fissures and sulci form.
  5. Fissures and sulci form: longitudinal fissure from week 8; secondary sulci weeks 30–35. Leads to Growth after birth.
  6. Growth after birth: grey matter volume peaks near 6 years. Leads to White matter peaks.
  7. White matter peaks: near 29 years, then declines.

When the brain grows and changes

Brain charts. Pooling 123,984 MRI scans from 101,457 people aged from 115 days after conception to 100 years, Bethlehem and colleagues built growth charts for the brain. Grey matter volume rose from mid-gestation to a peak at 5.9 years, then fell almost linearly; white matter volume peaked at 28.7 years and declined faster after 50; subcortical grey matter peaked at 14.4 years.[6]

Surface and thickness. Total cortical surface area peaked at about 11 years and total cerebrum volume at about 12.5 years, but cortical thickness peaked remarkably early, at 1.7 years.[6]

When organisation goes wrong

Costly elements are vulnerable. An economical view of brain networks highlights that their more costly elements, such as well-connected hubs, are especially vulnerable to pathological attack and abnormal development.[5]

Measuring the atypical. Centile scores from the brain charts provide a standardised measure of atypical brain structure and revealed patterns of variation across neurological and psychiatric disorders.[6]

Numbers that describe the brain

Scaling, energy and network efficiency can each be captured in a simple equation.[3,13,15]

Linear scaling of primate brains[1,15]
Mbrain∝NneuronsM_{\text{brain}} \propto N_{\text{neurons}}

Across primates, brain mass grows in proportion to the number of neurons, so the human brain is a linearly scaled-up primate brain; other orders, such as rodents, follow different cellular scaling rules, so brain size alone is not a reliable guide to neuron number.

Symbols in Linear scaling of primate brains
SymbolMeaningUnit
MbrainM_{\text{brain}}brain massg
NneuronsN_{\text{neurons}}number of neurons—
The cost of a spike[3]
ΔCMRO2≈145 mL O2100 g⋅h×Δν\Delta \mathrm{CMR}_{\mathrm{O_2}} \approx 145\ \frac{\mathrm{mL\ O_2}}{100\ \mathrm{g \cdot h}} \times \Delta\nu

In the grey-matter energy budget, raising the firing rate of every cortical neuron by one spike per second increases oxygen consumption by about 145 mL per 100 g of grey matter per hour, which is why the brain favours sparse codes.

Symbols in The cost of a spike
SymbolMeaningUnit
ΔCMRO2\Delta \mathrm{CMR}_{\mathrm{O_2}}change in the rate of oxygen consumptionmL/(100 g·h)
Δν\Delta\nuchange in mean firing rate per neuronspikes/s
Network efficiency[13]
Eglob=1N(N−1)∑i≠j1dijE_{\text{glob}} = \frac{1}{N(N-1)} \sum_{i \ne j} \frac{1}{d_{ij}}

The average of the inverse shortest path lengths between all pairs of nodes. It is high when information can travel between any two regions in few steps; small-world networks are efficient both globally and locally.

Symbols in Network efficiency
SymbolMeaningUnit
NNnumber of nodes (regions)—
dijd_{ij}shortest path length between nodes i and j—

Technology: counting, charting and mapping

Counting cells. A newer counting method, the isotropic fractionator, has corrected half a century of overestimates of glial numbers.[1,14]

Open brain charts. The lifespan brain charts are an open, interactive resource against which any new MRI scan can be benchmarked, and their centile scores proved stable across repeated scans and more heritable than raw measurements.[6]

Maps of the cortex. Combining structure, function and connectivity from MRI, a multimodal parcellation divided each hemisphere into 180 areas; in this atlas, older parcellations with fewer regions are shown, each described on the cortex reading.[8,11,16]

The embryonic brain vesicles[4]
Primary vesicleFuture divisionSecondary vesicles
ProsencephalonForebrainTelencephalon and diencephalon
MesencephalonMidbrainDoes not divide further
RhombencephalonHindbrainMetencephalon and myelencephalon

Milestones

Counting, wiring and charting the brain

  1. 1998Small-world networks are described.[12]
  2. 2001An energy budget for signalling in grey matter; network efficiency is defined.[3,13]
  3. 2009The human brain is counted: about 86 billion neurons.[1,15]
  4. 2010A review of the basics of brain development.[4]
  5. 2012The economy of brain network organisation.[5]
  6. 2015Brain energy metabolism reviewed from the cell upwards.[2]
  7. 2016180 cortical areas per hemisphere; 150 years of cell counting reviewed.[11,14]
  8. 2022Brain charts for the human lifespan from over 100,000 people.[6]

Frontiers

Growth charts for every brain. Just as height and weight have growth charts, brain charts now let researchers place any person's brain measurements relative to normative trajectories, although their authors acknowledge that existing MRI data under-represent the diversity of the world's population.[6]

Neurons, not brain size. The finding that brain size does not reliably predict neuron number argues for a view of cognitive ability centred on absolute numbers of neurons rather than on body size or encephalisation.[15]

Check yourself

Check yourself

  1. About how many neurons are in the adult human brain?
    Show answer

    About 86 billion.

  2. What share of the brain's neurons are in the cerebral cortex?
    Show answer

    About 19%, even though the cortex is 82% of brain mass.

  3. Name the three primary brain vesicles and what they become.
    Show answer

    Prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain).

  4. Why is white matter white?
    Show answer

    Because its axons are wrapped in fatty, white myelin.

  5. When does grey matter volume peak, according to the brain charts?
    Show answer

    At about 5.9 years.

  6. What share of the body's energy does the brain use?
    Show answer

    At least 20%.

  7. What trade-off shapes brain networks, according to Bullmore and Sporns?
    Show answer

    Minimising wiring cost against gaining valuable network properties such as efficiency and hubs.

  8. What is the glia-to-neuron ratio in the human brain?
    Show answer

    Less than 1:1, not 10:1.

Glossary[1,4,6,12,14]

Neural tube
The first well-defined neural structure of the embryo, from which brain and spinal cord develop.
Forebrain
The front division of the brain, developing from the prosencephalon (telencephalon and diencephalon).
Midbrain
The middle division, from the mesencephalon.
Hindbrain
The rear division, developing from the rhombencephalon (metencephalon and myelencephalon).
Grey matter
Brain tissue rich in neuron cell bodies.
White matter
Brain tissue made of myelinated axons.
Neocortex
The six-layered cortex on the brain's surface.
Glia
Non-neuronal cells of the brain.
Small-world network
A network that is highly clustered yet has short paths between nodes.
Centile score
Where a measurement lies relative to a reference population of the same age.

References

  1. Azevedo FAC, Carvalho LRB, Grinberg LT, Farfel JM, Ferretti REL, Leite REP, Jacob Filho W, Lent R, Herculano-Houzel S. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. Journal of Comparative Neurology 2009;513(5):532-541. doi:10.1002/cne.21974
  2. Magistretti PJ, Allaman I. A cellular perspective on brain energy metabolism and functional imaging. Neuron 2015;86(4):883-901. doi:10.1016/j.neuron.2015.03.035
  3. Attwell D, Laughlin SB. An energy budget for signaling in the grey matter of the brain. Journal of Cerebral Blood Flow and Metabolism 2001;21(10):1133-1145. doi:10.1097/00004647-200110000-00001
  4. Stiles J, Jernigan TL. The basics of brain development. Neuropsychology Review 2010;20(4):327-348. doi:10.1007/s11065-010-9148-4
  5. Bullmore E, Sporns O. The economy of brain network organization. Nature Reviews Neuroscience 2012;13(5):336-349. doi:10.1038/nrn3214
  6. Bethlehem RAI, Seidlitz J, White SR, Vogel JW, Anderson KM, Adamson C, et al.. Brain charts for the human lifespan. Nature 2022;604(7906):525-533. doi:10.1038/s41586-022-04554-y
  7. FreeSurfer developers. CorticalParcellation (FreeSurfer wiki), section "Lobe mapping". FreeSurfer wiki. https://surfer.nmr.mgh.harvard.edu/fswiki/CorticalParcellation
  8. Desikan RS, Ségonne F, Fischl B, Quinn BT, Dickerson BC, Blacker D, et al.. An automated labeling system for subdividing the human cerebral cortex on MRI scans into gyral based regions of interest. NeuroImage 2006;31(3):968-980. doi:10.1016/j.neuroimage.2006.01.021
  9. Fischl B, Salat DH, Busa E, Albert M, Dieterich M, Haselgrove C, et al.. Whole Brain Segmentation. Neuron 2002;33(3):341-355. doi:10.1016/S0896-6273(02)00569-X
  10. Yeh F-C. Population-Probability Atlas and Tract-to-Region Connectome (HCP-1065), data release. brain.labsolver.org 2022. https://brain.labsolver.org/hcp_trk_atlas.html
  11. Glasser MF, Coalson TS, Robinson EC, Hacker CD, Harwell J, Yacoub E, et al.. A multi-modal parcellation of human cerebral cortex. Nature 2016;536(7615):171-178. doi:10.1038/nature18933
  12. Watts DJ, Strogatz SH. Collective dynamics of 'small-world' networks. Nature 1998;393(6684):440-442. doi:10.1038/30918
  13. Latora V, Marchiori M. Efficient behavior of small-world networks. Physical Review Letters 2001;87(19):198701. doi:10.1103/PhysRevLett.87.198701
  14. von Bartheld CS, Bahney J, Herculano-Houzel S. The search for true numbers of neurons and glial cells in the human brain: a review of 150 years of cell counting. Journal of Comparative Neurology 2016;524(18):3865-3895. doi:10.1002/cne.24040
  15. Herculano-Houzel S. The human brain in numbers: a linearly scaled-up primate brain. Frontiers in Human Neuroscience 2009;3:31. doi:10.3389/neuro.09.031.2009
  16. Destrieux C, Fischl B, Dale A, Halgren E. Automatic parcellation of human cortical gyri and sulci using standard anatomical nomenclature. NeuroImage 2010;53(1):1-15. doi:10.1016/j.neuroimage.2010.06.010

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Template anatomy for education. Not patient-specific. Not for clinical decision-making.