whitematter

White matter refers to regions of the central nervous system dominated by myelinated axons and supporting glial cells, in contrast to gray matter, which contains neuron cell bodies and unmyelinated fibers. In the brain, white matter forms the communication highways that connect different gray-matter areas, enabling rapid transmission of neural signals. The myelin sheath surrounding many axons is produced by oligodendrocytes in the central nervous system and increases conduction velocity through saltatory conduction at the nodes of Ranvier.

Anatomically, white matter contains fiber tracts organized into fasciculi, bundles, and commissures. Major brain tracts include the corpus callosum, internal capsule, corona radiata, optic radiations, and long association fibers such as the arcuate fasciculus. In the spinal cord, white matter comprises ascending sensory and descending motor pathways that run in posterior, lateral, and anterior columns.

Developmentally, myelination begins in late gestation and continues through childhood and adolescence, with regional variation in timing. Oligodendrocytes extend myelin sheaths along axons, while astrocytes and microglia provide metabolic support and immune surveillance.

Clinical relevance and imaging: white matter integrity can be assessed with magnetic resonance imaging (MRI). On MRI, white matter appears relatively hyperintense on certain sequences, and diffusion tensor imaging measures such as fractional anisotropy reflect fiber organization and integrity. White matter is affected in various disorders, including demyelinating diseases like multiple sclerosis, leukodystrophies, and age-related vascular white matter changes, which can impact processing speed, motor function, and cognitive performance. Aging commonly features white matter hyperintensities, particularly in periventricular regions.