Group
White matter pathways
Also called Substantia alba, Cerebral white matter, Brain wiring
The brain's wiring: bundles of fat-wrapped cables that let distant regions talk fast.
If the gray outer layer of the brain is where the thinking gets done, white matter is the wiring that connects it all. It is made of long nerve fibers, called axons, wrapped in a fatty coating called myelin. That fat is what makes it look pale. It fills much of the inside of your brain and lets signals travel many times faster than they could along bare fibers.
Where it is
Under the wrinkled gray outer layer (the cortex), filling the deep inside of each half of the brain. It also forms the big bridge between the two halves and the cables running down into the brainstem and spinal cord.
What it does
Fast connections
Myelin lets signals jump from gap to gap along each fiber, so they travel much faster.
Association fibers
These link areas within the same half of the brain, like the cable between the areas for understanding and producing speech.
Commissural fibers
These cross between the left and right halves. The corpus callosum is by far the biggest.
Projection fibers
These run up and down between the cortex and deeper parts, the brainstem, and the spinal cord, carrying senses in and movement commands out.
Good timing
By setting how fast signals travel, white matter is thought to help messages from far-apart areas arrive in sync.
Think of it as the brain's internet cables: gray matter is the computers, and white matter is the fiber network that lets them share information almost instantly.
Inside the white matter pathways
- Corpus callosumThe giant bridge of about 200 to 300 million fibers that lets your left and right brain talk.
- Corticospinal tractThe main highway for movement, running from your brain all the way down the spinal cord.
- Arcuate fasciculusThe curved cable linking the brain's areas for speaking and understanding words.
- Optic radiationA fan of fibers carrying vision from the thalamus to the back of the brain.
- Acoustic radiationThe last stretch of the hearing pathway, from the thalamus to the hearing cortex.
- CingulumA belt of fibers looping around the brain's middle, linking emotion, focus, and memory.
- Uncinate fasciculusA hook-shaped cable linking memory and emotion to decision-making.
- Inferior fronto-occipital fasciculusOne of the brain's longest cables, running from the back of your head to your forehead.
- Optic chiasmThe X-shaped crossing where half of each eye's fibers swap sides.
Works with
- AxonWhite matter is made mostly of axons, the long output cables of nerve cells.
- Myelin sheathThis fatty wrapping around each axon gives white matter its color and its speed.
- OligodendrocyteThese helper cells wrap myelin around axons, each one insulating many fibers at once.
- Node of RanvierSignals leap between these tiny bare gaps in the myelin, which is what makes them so fast.
- Corpus callosumThe largest white matter structure, a bridge of about 200 to 300 million fibers between the two halves.
- White matter bundleZoom in to see what a white matter bundle looks like up close.
When it goes wrong
Multiple sclerosis
The immune system attacks myelin, leaving scarred patches in the white matter. Signals slow down or get blocked, which can cause numbness, weakness, or vision problems that come and go.
Head injuries
A hard jolt can stretch and tear long fibers throughout the white matter, even when nothing shows on the outside. This is one reason a head injury can affect thinking.
Blood vessel health
Damage to tiny blood vessels, often from high blood pressure, can leave small bright spots in the white matter on brain scans. Keeping blood pressure healthy helps protect the brain.
For learning only, not medical advice. If you have a health concern, talk to a doctor.
Surprising facts
- White matter keeps maturing well into your twenties. The front of the brain, which handles planning and self-control, is among the last parts to finish.
By the numbers
Where the name comes from
Named for its pale color, which comes from myelin, a fatty coating. In Latin it is substantia alba, "white substance".
Quick quiz
3 questions. Your answers are saved and come back in your daily review.
Which of these is a job of the white matter pathways?
Show the answer
Fast connections. Myelin lets signals jump from gap to gap along each fiber, so they travel much faster.
Fast connections. Myelin lets signals jump from gap to gap along each fiber, so they travel much faster.
The white matter pathways is part of which larger structure?
Show the answer
Inside the brain. It belongs to the deep parts of the brain.
Inside the brain. It belongs to the deep parts of the brain.
Which part fits this description? "Like the brain's internet cables: gray matter is the computers, and ___ is the fiber network that lets them share information almost instantly."
Show the answer
White matter pathways. It is the white matter pathways. The brain's wiring: bundles of fat-wrapped cables that let distant regions talk fast.
White matter pathways. It is the white matter pathways. The brain's wiring: bundles of fat-wrapped cables that let distant regions talk fast.
These parts now come back in your daily review, just before you would forget them.
Practice these parts in 3DSources
- Brain Basics: Know Your Brain, National Institute of Neurological Disorders and Stroke (NINDS)
- White matter tracts, Radiopaedia
- 2.4 Mechanisms of Neural Signaling, OpenStax, Introduction to Behavioral Neuroscience
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Explore the white matter pathways in 3D
Turn it around, zoom inside and see how it connects to the rest of the brain, then test yourself with quick quizzes.
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