White matter is so named because of its glistening white appearance on the cut surface of the brain. This feature is attributable to myelin, a fatty substance that wraps around the axons of cerebral neurons as they course through the brain. Myelin is a complex mixture of lipids (70%) and protein (30%) manufactured by oligodendrocytes, cells of glial origin that are analogous to the Schwann cells of the peripheral nervous system (Fig. 1). At the microscopic level, white matter consists of collections of axons coated with myelin; these axons travel together in various fiber systems called tracts to form structural connections between many different gray matter regions.
At the macroscopic level, white matter makes up about 50% of the adult cerebrum. Three kinds of white matter fiber systems are recognized. First are the projection fibers, which travel corticofugally (from the cortex) or corticopetally (to the cortex) in relation to more caudal destinations. An example of a corticofu-
gal tract is the corticospinal tract, and the optic radiation from the thalamus to the occipital lobe is a corticopetal tract. Second, there are several tracts of commissural fibers that connect the two hemispheres. The most prominent of these is the corpus callosum, a massive structure carrying approximately 300 million axons. Finally, various association fibers serve to connect cortical regions within each hemisphere. There are numerous short association fibers (arcuate or U fibers) that link adjacent cortical gyri, and there are five long association tracts—the superior occipitofrontal fasciculus, the inferior occipitofrontal fasciculus, the arcuate fasciculus, the cingulum, and the uncinate fasciculus—that connect more remote cortical areas (Fig. 2).
Two other anatomic observations are relevant. First is the close relationship of white matter with the frontal lobes, the largest of the human cortical regions: The cerebral white matter is most abundant under the frontal lobes, where all the major association tracts have one of their termini. These features imply that the frontal lobes exert a unique influence in part through the actions of the white matter. Second, there is a higher proportion of white matter compared to gray matter in the right hemisphere than in the left hemisphere. This asymmetry suggests a functional specialization of the hemispheres whereby the white matter in the right cerebrum plays a distinctive role.
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