What primarily makes up white matter in the CNS?

• A. Bundles of axons
• B. Neuron cell bodies
• C. Blood vessels
• D. Dendrites

Answer: (A)

White matter is defined by its bundles of axons. These long neural projections carry information between different brain regions and between the brain and spinal cord, forming the communication highways of the CNS. In the brain, these axons are organized into tracts that connect cortical areas (association fibers), link the two hemispheres (commissural fibers like the corpus callosum), or run between brain and spinal cord (projection fibers). The white color comes mainly from the myelin sheath surrounding many of these axons, which is lipid-rich and produced by oligodendrocytes in the CNS. In contrast, gray matter houses most neuron cell bodies, dendrites, and synapses, so it’s more involved in processing. Blood vessels are present throughout but don’t constitute the primary component of white matter.