Numbers we measured
Every number here comes from a public brain map. Most we measured ourselves; a few come straight from the paper, and those are marked. Each one lists how it was measured and what it does not tell you.
12 numbers so far · the datasets are under Maps · our approach is described in Methodology.
37%of the cells in layer 1 are astrocytes — the most common cell in the cortex's top layer
Map: H01 · human cortex · our measurement · 2026-10-30
How: Cells by the dataset's layer labels (somas.csv): layer 1 — 1,688 astrocytes of 4,595 cells; 1,688 of the sample's 5,474 astrocytes (30.8%) sit in layer 1. Consistent with the paper's Table S4 (astrocyte volume fraction 21.6% in layer 1 vs 14–15% in layers 2–6).
Caveat: One sample; layer labels are the dataset's own. The map does not tell astrocyte subtypes apart (e.g. primate-specific interlaminar astrocytes, Oberheim et al. 2009).
18% vs 70%of the cells are neurons in layer 1, the cortex's top layer, versus layer 2 just below
Map: H01 · human cortex · our measurement · 2026-10-29
How: Cells by the dataset's layer labels (somas.csv): layer 1 — 827 neurons of 4,595 cells (586 interneurons, 72 pyramidal); layer 2 — 4,656 of 6,651.
Caveat: Layer labels are the dataset's own; one sample. Layer 1 is mostly wiring: pyramidal-cell branch tips meeting long-range inputs (Schuman et al. 2021).
20,139 vs 16,087oligodendrocytes outnumber neurons in this human cortex sample
Map: H01 · human cortex · ours + paper · 2026-10-28
How: Cell types in H01 somas.csv; matches the paper's Table S5 exactly.
Caveat: Driven by the white matter in the sample: in the grey-matter layers alone neurons outnumber oligodendrocytes (15,005 vs 9,350).
77%of the cells in the sample's white matter are oligodendrocytes
Map: H01 · human cortex · our measurement · 2026-10-28
How: 10,110 of 13,063 white-matter cells in somas.csv (half of all oligodendrocytes in the sample sit there).
Caveat: Layer labels are the dataset's own.
98%of neurons sit within 50 µm of a blood vessel
Map: H01 · human cortex · our measurement · 2026-10-27
How: 10,441 neuron cell-body centres (somas.csv, away from the sample edges) to the nearest vertex of the finest blood-vessel meshes (1,477 segments). Median 22.9 µm; max 141 µm.
Caveat: Distance to mesh vertices (spacing ~1.6 µm); the vessel segmentation may miss some capillaries, so true distances may be shorter. One ~1 mm³ sample of human temporal cortex.
245 µm²of a glial cell's surface pressed against a neuron's cell body
Map: H01 · human cortex · our measurement · 2026-10-27
How: Finest meshes of the gallery's 'parasitic_glial' cells: glial surface within 0.25 µm of the neuron (8% of the glial surface; ~5% of the neuron's cell-body surface).
Caveat: One pair of cells; the glial cell is labelled microglia/OPC.
390predicted dopamine neurons in a complete fruit-fly connectome
Map: Male CNS · fruit fly · our measurement · 2026-10-27
How: Neurotransmitter predictions of named cell types; all 390 are named neurons, together making 803,309 output synapses.
Caveat: Transmitter identity is predicted from EM, not measured chemically.
≈7 µmacross: a flat, whorled axon coiled on a neuron's cell body
Map: H01 · human cortex · our measurement · 2026-10-26
How: Finest public meshes of the gallery's 'axon_whorl' (97 segments); disc ≈6 × 7 µm, ~1.6 µm thick; checked against the raw electron-microscopy images.
Caveat: Automated segmentation; which pieces form the axon was chosen by the H01 team's gallery.
22,196 of 164,606fruit-fly neurons (about 1 in 7) are wired into the nerve cord
Map: Male CNS · fruit fly · our measurement · 2026-10-26
How: Superclass tags in the Janelia Male CNS segment properties (nerve-cord classes + ascending neurons). Independent check: MANC ~23,500 incl. 1,328 descending → ~22,200.
Caveat: Only neurons with a superclass tag counted.
8named histamine neurons outside the fly's visual system
Map: Male CNS · fruit fly · our measurement · 2026-10-20
How: Connectome transmitter predictions, outside the optic lobes.
Caveat: Predicted, not measured chemically.
almost allinputs within 20 µm of one pyramidal neuron's cell body are inhibitory
Map: H01 · human cortex · our measurement · 2026-10-19
How: All input synapses of cell 6644185383 in both public synapse releases; within 20 µm of the soma centre: 97.7% (20210601) and 96.5% (20210729, the release that matches the paper).
Caveat: One cell. Labels partly depend on whether a synapse sits on a spine.
~150 millionsynapses in just over one cubic millimetre of human cortex
Map: H01 · human cortex · from the paper · 2026-10-18
How: Paper's automated count: 149,871,669 synapses in the volume.
Caveat: Automated detection; the paper reports missed and false detections.