Grey and White Matter
What is the brain tissue composed of?
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Our brain tissue is no laughing matter…instead, it’s gray and white matter 😬. Bundles of neurons, or nervous system cells, make up the tissue in our brains. If you’ve ever seen a picture of a real, preserved brain or a neuroimage snapshot, you might remember that the tissue at the outer edge of the brain is darker than the tissue along the inner part of the brain. This darker tissue is referred to as “gray matter,” whereas the lighter tissue is called “white matter”.

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Why does our brain tissue have different shades? Gray matter contains neuron bodies, dendrites, and axons without a substance called “myelin”. White matter, on the other hand, gets its lighter appearance because neurons contributing to this tissue all have myelinated axons. Myelin is a layer of fat and proteins that allows for rapid communication through "saltatory conduction"


Is this difference important? Yes! Evolutionary biologists theorize that our brains are segregated into gray and white matter to optimize communication. Gray matter is excellent for communication among closely located brain regions, and white matter is key in communication among distantly located regions.


You can think about this difference as road systems: local roads (i.e., gray matter) are most helpful when you want to transport goods within your neighboring regions, but highways (i.e., white matter) are needed when you want to transport goods to a distant region. Having myelin on axons in white matter is analogous to having a higher speed limit on a highway than on a local road, so information can travel more quickly over long distances.


[1] US National Library of Medicine, Medical Encyclopedia, “Gray and white matter of the brain”
[2] Wen, Q., & Chklovskii, D. B. (2005). Segregation of the brain into gray and white matter: a design minimizing conduction delays. PLoS computational biology, 1(7), e78.
[3] US National Library of Medicine, Medical Encyclopedia, “Myelin”

© 2018 by Janelle E. Letzen, PhD