White Matter

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The white matter of the brain constitutes nearly one-half of its volume and serves the important general function of linking cortical and subcortical gray matter regions with each other. White matter consists of collections of axons ensheathed with myelin that are most often called tracts, but that may also be termed fasciculi, bundles, lemnisci, funiculi, and peduncles. Traveling extensively throughout the brain to link widely dispersed gray matter areas, these groups of fibers integrate cortical and subcortical regions into functionally unified neural networks. These networks subserve the many unique functions of the brain, from basic sensory and motor activities to complex cognition and emotion. By virtue of the dramatic increase in axonal conduction velocity that is afforded by myelin, rapid and efficient transfer of information across white matter tracts occurs, which enables the highest functions in the cerebral hemispheres. The many neurologic and neurobehavioral deficits sustained by individuals with the white matter disease multiple sclerosis (MS) testify to the importance of white matter in brain function.

White matter tracts in the brain can be considered under three general categories: projection fibers, commissural fibers, and association fibers. Projection fibers are those that ascend to the cortex from lower structures (corticopetally) or descend from the cortex to lower regions (corticofugally). Major corticopetal tracts are the thalamic radiations, relaying somato-sensory information from the thalamus to the parietal cortex, and the optic radiations, projecting from the

Figure 14 Cutaway illustration of the hippocampus, which lies deep in the medial temporal lobe. Abbreviations: H, hippocampus; A, amygdala; F, fornix; MB, mammillary body; DM, dorsal medial nucleus of the thalamus; BFB, basal forebrain; CG, cingulate gyrus. Reprinted with permission from Filley, C. M. (2001). Neurobehavioral Anatomy, 2nd ed., p. 59. University Press of Colorado, Boulder, CO.

Figure 14 Cutaway illustration of the hippocampus, which lies deep in the medial temporal lobe. Abbreviations: H, hippocampus; A, amygdala; F, fornix; MB, mammillary body; DM, dorsal medial nucleus of the thalamus; BFB, basal forebrain; CG, cingulate gyrus. Reprinted with permission from Filley, C. M. (2001). Neurobehavioral Anatomy, 2nd ed., p. 59. University Press of Colorado, Boulder, CO.

lateral geniculate body of the thalamus to the occipital cortex. The major corticofugal tract is the corticosp-inal tract, which projects from the motor cortex to lower motor neurons in the spinal cord. A similar role is played by the corticobulbar tract, which also originates in the motor cortex but terminates on lower motor neurons in the brain stem. Both of these tracts first travel through the large internal capsule; cortico-bulbar fibers then cross to join various brain stem motor nuclei, whereas corticospinal fibers continue rostrally to the spinal cord, with most first decussating in the lower medulla. Projection fibers are therefore involved solely with elemental sensory and motor functions.

More important for behavior are the commissural and association fiber systems (Fig. 15). Commissural fibers are those that travel between the hemispheres in the cerebral commissures. The largest of these by far is the corpus callosum, a massive white matter tract that connects the four lobes of the brain on each side with their counterparts on the other. Much smaller com-missural fiber systems include the anterior commissure and the hippocampal commissure. The association fibers join gray matter regions within each hemisphere. Among these, anatomists have distinguished two types: short and long association fibers. Short association fibers, also called arcuate or U fibers, connect adjacent cortical gyri and are found throughout the cerebrum. Long association fibers primarily link the lobes of the brain: these are the superior occipitofrontal fasciculus, the inferior occipitofrontal fasciculus, the arcuate fasciculus, the uncinate fasciculus, and the cingulum. All of these tracts have one termination in the frontal lobe, whereas their other terminus is variably in more posterior regions.

Two other white matter tracts deserve mention. The fornix is a prominent curved structure in the limbic system that connects the hippocampus and the mam-millary bodies. The medial forebrain bundle connects the hypothalamus with both caudal and rostral brain regions and participates in hypothalamic control of the autonomic nervous system.

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