Two types of matter comprise our central nervous system: gray matter, situated in cortical and subcortical brain regions, containing most of our neuronal cell bodies as well as dendrites, unmyelinated axons, and synapses that facilitate processing of sensory, motor, and other relevant information; and white matter, consisting of long-range myelinated axon tracts that coordinate signals between these regions. During adolescence, synaptic pruning (refining of neuronal connections) decreases gray matter while increased axon myelination (improving speed and quality of signal relay) increases white matter. These structural changes are thought to enhance cognitive efficiency as teenagers brave the transition to adulthood, but it is unclear exactly when and how substance use may derail this progression.
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