Grey Matter vs White Matter: The Brain's Processors and Wiring
What's the difference between grey matter and white matter?
Side view
Midline view
The Short Answer
Grey matter is made mostly of neuron cell bodies and their connections (synapses) - it's where information is processed. White matter is made of long nerve fibres (axons) wrapped in fatty myelin, which makes it look pale - it's the wiring that carries signals between grey matter areas, fast. Each makes up roughly half of the brain.
At a Glance
| Aspect | Grey matter | White matter |
|---|---|---|
| Made of | Neuron cell bodies, dendrites, synapses, and support cells | Bundles of axons insulated with myelin |
| Job | Processing: computing, storing, and responding | Communication: carrying signals between regions quickly |
| Where it's found | The outer cortex, and deep clusters such as the thalamus and basal ganglia | Beneath the cortex, and large tracts such as the corpus callosum |
| Why that colour | Lots of cell bodies and blood vessels; pinkish-grey in a living brain | Fatty myelin looks pale |
| Change with age | Volume peaks in childhood or early adolescence, then slowly thins | Keeps increasing through adolescence into the 20s and beyond |
| Examples of conditions | Alzheimer's disease, Huntington's disease | Multiple sclerosis, diffuse axonal injury after head trauma |
Processors and Cables
If you slice through a preserved brain, two kinds of tissue stand out. A thin outer layer and some deep clusters look darker - the grey matter. Between them is paler tissue - the white matter. The difference comes from what each contains.
Grey matter is packed with neuron cell bodies, their branching dendrites, and the synapses where neurons connect. This is where the brain's computation happens. Most of it forms the cerebral cortex, a sheet about 2–4 mm thick folded into the brain's wrinkles, but there are also deep grey-matter hubs such as the thalamus and basal ganglia.
White matter is made of axons - the long output fibres of neurons - wrapped in myelin, a fatty insulating layer that makes signals travel many times faster. The largest single white-matter structure is the corpus callosum, which links the two hemispheres. White matter makes up roughly half of the human brain - a larger share than in most other animals.
How They Change Through Life
In 1999, Jay Giedd and colleagues scanned children and teenagers repeatedly over several years. Grey matter volume in the frontal and parietal lobes rose through childhood, peaked around the start of adolescence, and then declined - partly reflecting the pruning of unused connections as the brain becomes more efficient. White matter, by contrast, kept increasing. A 2011 study by Catherine Lebel and Christian Beaulieu found that white-matter wiring continues to mature into the 20s and beyond, especially in tracts linking the frontal lobes.
White matter can also change with learning. In a 2009 study, Jan Scholz and colleagues taught adults to juggle for six weeks and found changes in white matter in a region involved in tracking moving objects - one of the first signs that white matter responds to experience, not just development.
When Things Go Wrong
Because they do different jobs, damage to each causes different problems. In multiple sclerosis, the immune system attacks myelin, so signals along white-matter tracts slow or fail. In traumatic brain injury, twisting forces can stretch and tear long axons throughout the white matter. Diseases such as Alzheimer's and Huntington's, on the other hand, mainly destroy neurons in grey matter.
MRI scans show grey and white matter clearly. A technique called diffusion imaging can even trace the paths of white-matter tracts, by tracking how water moves along the fibres.
Common Myths
"More grey matter means more intelligence."
Brain size and grey matter volume are only loosely linked to intelligence. How efficiently regions are connected - through white matter - matters too, and thinning grey matter in adolescence is a normal part of healthy development.
"White matter is just packing material."
White matter is active, adaptable wiring. Its speed and timing shape how well brain networks work together, and it changes with learning.
Read More
Key Terms
This article is for general education and isn't a substitute for professional medical advice. See our Terms of Use.
Sources & Further Reading
- White matter in learning, cognition and psychiatric disorders - Fields (2008), Trends in Neurosciences
- Brain development during childhood and adolescence: a longitudinal MRI study - Giedd et al. (1999), Nature Neuroscience
- Longitudinal development of human brain wiring continues from childhood into adulthood - Lebel & Beaulieu (2011), Journal of Neuroscience
- Training induces changes in white-matter architecture - Scholz et al. (2009), Nature Neuroscience
- Brain Basics: Know Your Brain - National Institute of Neurological Disorders and Stroke