THE WIRING
Bundles of axons
Axons carry signals away from nerve cells. NINDS explains that some are tiny, while others run from the brain all the way down the spinal cord.
When they matureBRAIN SCIENCE EXPLAINED
The brain's
hidden wiring.
Grey matter thinks; white matter connects.
150,000 km of insulated fibres
150–180k km Myelinated nerve fibres in an adult brain
- Bundles of axons
- Wrapped in myelin
- Mapped by water
- The connections, explained
Deep inside the brain, bundles of nerve fibres called tracts link one region to another. Their fatty insulation gives white matter its colour and lets signals travel far faster.
THE SHORT ANSWER
Tracts are cables
of nerve fibres.
A tract is a bundle of axons running together.
Myelin insulation is what makes white matter white.
THE BRIDGE
The corpus callosum
The two halves of the brain talk through a thick tract of nerve fibres called the corpus callosum, estimated to hold about 250 million fibres.
See the numbers
THE MAP
Following the water
Diffusion MRI tracks how water moves. In white matter it moves preferentially along fibre bundles, which lets scientists trace the tracts.
How mapping worksWHEN TRACTS MATURE
Which pathways
get myelin first?
Pick a brain pathway to see when it gains its myelin insulation. The bar shows how late in life that process carries on.
From the Lancaster Glossary of Child Development, summarising the classic studies of Yakovlev and Lecours (1967). General information, not medical advice.
HOW TRACTS ARE MAPPED
Tracing wires
without surgery.
Tractography uses an MRI scanner and careful analysis. The signal comes from the movement of water inside the brain itself.
Measure water movement
Diffusion MRI records how water molecules move in each small block of brain tissue.
Spot the direction
In open fluid water wanders at random; in white matter it moves more easily along the fibre bundles.
Work out the tracts
Combining those directions across the brain reveals the size and orientation of the fibre tracts, and how regions may be connected.
Compare many brains
The Human Connectome Project collected high-quality imaging from over 1,100 healthy young adults to chart these pathways.
TWO NUMBERS WORTH KNOWING
A lot of cable
in a small space.
One estimate of total fibre length,
one of the size of the brain's biggest bridge.
150–180k km
Myelinated nerve fibres in an adult brain
Harvard's BioNumbers database records estimates of 150,000 to 180,000 km of myelinated fibres in the white matter at about age 20, from Danish brain studies.
TOTAL LENGTH · BIONUMBERS
≈ 250 million
Fibres in the corpus callosum
The corpus callosum joins the left and right hemispheres. Its cross-section in adults is about 6.2 cm², and its fibres have been estimated at 250 million.
THE BRIDGE · LANCASTER
The Lancaster glossary notes that myelin is 70 to 80% lipid, and that myelinated fibres can conduct signals about a hundred times faster than unmyelinated ones.
GREY AND WHITE
Why white matter
looks white.
NINDS explains that the grey matter of the cortex is grey because its nerves lack the insulation that makes most other parts of the brain look white. That insulation is myelin, a fatty sheath made by support cells wrapped around axons.
Myelin has small regular gaps called nodes of Ranvier. The signal jumps from node to node, which the Lancaster glossary says reaches speeds about a hundred times those of uninsulated fibres.
Questions
Questions people ask
What is a white matter tract?
A bundle of nerve fibres, or axons, running together between brain regions. White matter is made up of these fibres and their myelin sheaths, with support cells and blood vessels.
What does myelin do?
NINDS describes myelin as a fatty insulation around axons that helps nerve signals travel faster and farther. Gaps in it let the signal jump along the fibre.
What is the corpus callosum?
The thick tract at the base of the deep fissure between the two halves of the cerebrum. It lets the hemispheres communicate and is estimated to contain about 250 million fibres.
How can a scan show tracts?
Diffusion MRI measures the direction water moves. In white matter it moves preferentially along fibre bundles, so the pattern of movement reveals the likely route of each tract.
When is the brain's wiring finished?
Myelination is mainly a process after birth. It continues at least through the first decade, frontal areas mature in the second, and some cortical connections keep myelinating throughout life.
Where the facts come from
Updated
- NINDS (US National Institutes of Health): Brain Basics, Know Your Brain
- BioNumbers (Harvard Medical School): Length of myelinated nerve fibers in brain
- Lancaster Glossary of Child Development: Myelination
- Lancaster Glossary of Child Development: Myelin, and Corpus callosum
- Lancaster Glossary of Child Development: Diffusion tensor magnetic resonance imaging
- Human Connectome Project: HCP Young Adult study overview