Development and Aging

The white matter of the brain undergoes a continuous process of remodeling throughout the life span. It has

Depolarizing current (between nodes) yS. Myelin

Depolarizing current (between nodes) yS. Myelin

Figure 3 Illustration of saltatory conduction in a myelinated nerve. The action potential is propagated by sodium influx at the nodes of Ranvier, and the impulse ''jumps'' rapidly throughout the entire length of the axon (adapted from B. Pansky, D. J. Allen, and C. G. Budd, Review of Neuroscience, 2nd ed., p. 357. Macmillan, New York, 1988).

Figure 3 Illustration of saltatory conduction in a myelinated nerve. The action potential is propagated by sodium influx at the nodes of Ranvier, and the impulse ''jumps'' rapidly throughout the entire length of the axon (adapted from B. Pansky, D. J. Allen, and C. G. Budd, Review of Neuroscience, 2nd ed., p. 357. Macmillan, New York, 1988).

been estimated that the ratio of cerebral gray to white matter at age 50 is approximately 1:1, whereas in infancy and old age there is a relative preponderance of gray matter. In comparison to gray matter, white matter requires a longer time to develop and then is preferentially lost in senescence.

During development, white matter requires several decades to complete its formation. Myelination of the brain begins in utero and continues until adulthood. As a general rule, myelination occurs earlier in areas devoted to elemental motor and sensory functions and later in areas concerned with higher cerebral functions. Thus, the cerebral commissures and the association areas, for example, are not fully myelinated until the third decade of life. Because there is a strong likelihood that the degree of myelination parallels functional maturity, the fact that frontal lobe myelination is not complete until young adulthood suggests that the integrity of white matter contributes importantly to the maturation of personality and comportment.

In contrast, aging in the brain involves not only a loss of neurons and their dendritic trees but also a selective loss of white matter. Recent findings from neuroimaging and autopsy studies have led to a revision of the traditional view that neuronal loss is the major characteristic of brain aging. It now appears that whereas neuronal loss is not as marked as previously thought, gradual loss of white matter in aging is a consistent observation. The implications of this white matter change are not entirely clear, but as in development, there may be a clinical correlation. For example, the cognitive slowing often seen in the elderly may be explained in part by the loss of white matter.

of diffusion weighting, a means by which specific white matter tracts can be identified. Recently, functional imaging techniques have attracted much interest, including positron emission tomography and functional MRI, both designed to evaluate the metabolic activity of brain regions. Although these instruments are most suited to imaging gray matter areas with high metabolic activity, they are likely to assist in the study of white matter function by identifying cortical and subcortical regions participating in neural networks that also include white matter tracts.

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