White matter is one of the two components of the central nervous system and consists mostly of glial cells and myelinated axons that transmit signals from one region of the cerebrum to another and between the cerebrum and lower brain centers. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to its usual preservation in formaldehyde. The other main component of the brain is grey matter (actually pinkish tan due to blood capillaries), which is composed of neurons. A third colored component found in the brain that appears darker due to higher levels of melanin in dopaminergic neurons than its nearby areas is the substantia nigra. Note that white matter can sometimes appear darker than grey matter on a microscope slide because of the type of stain used. White matter, long thought to be passive tissue, actively affects how the brain learns and dysfunctions. Whilst grey matter is primarily associated with processing and cognition, white matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.