Gray matter
Also known as: grey matter, GM
The darker, neuron-rich tissue that forms the outer cortex and the deep nuclei of the brain — where most signal processing happens.
Gray matter is the neuron-dense tissue that gives the brain its characteristic folded outer layer and its deep subcortical structures. It appears grey-pink in fresh tissue because of its concentration of cell bodies, dendrites, and unmyelinated axons.
It's where most of the brain's computation happens — neurons receive inputs, integrate them, and send signals onward through their long axons (which form the white matter).
Where it is
Gray matter is found in two main places:
- The cortex — a 1 to 4.5 mm thick sheet covering the outside of the brain, folded into gyri and sulci. This is what people usually picture when they imagine "the brain."
- Subcortical nuclei — clusters of gray matter buried inside the brain, including the thalamus, hippocampus, basal ganglia, and amygdala.
Why it's measured
Total and regional gray matter volume changes meaningfully with age, disease, and life experience:
- It declines gradually across adulthood and more steeply in conditions like Alzheimer's
- Specific regions shrink in specific conditions — early gray matter loss in the hippocampus is a hallmark of Alzheimer's; basal ganglia changes feature in Parkinson's
- Some lifestyle factors (exercise, education, social engagement) are associated with relatively preserved gray matter at older ages
In neuroimaging analytics, total gray matter volume, regional volumes, and cortical thickness are standard outputs of a volumetric pipeline.
Related terms
White matter
The bundles of myelinated axons that connect different regions of the brain — the brain's wiring, as opposed to its computing cells.
Cerebral cortex
The folded outer layer of the brain — only a few millimetres thick, but where most conscious thought, perception, and action happens.
Cortical thickness
The thickness of the brain's outer grey-matter ribbon (the cortex), measured millimetre-by-millimetre across its surface from a structural MRI.
Hippocampus
A small, seahorse-shaped structure in the temporal lobe of each hemisphere — central to memory formation and spatial navigation.