ABSTRACT
In longitudinal sectioned nerve fibers, the paranodal loops of myelin normally terminate in a regular sequence, and all of them are in contact with the underlying membrane of the axon. The most common age-related morphological alteration in myelin sheaths is the accumulation of pockets of dense cytoplasm between splits of the lamellae at the major dense line. In the monkey, the process of myelin formation appears to continue throughout life in the central nervous system and results in the formation of thicker myelin sheaths and in the formation of redundant myelin. For a nerve fiber to completely degenerate, it is necessary for the ensheathed axon to degenerate, and in turn this causes the myelin sheath surrounding the axon to also degenerate. The loss of some nerve fibers from white matter tracts with age would lead to some disconnection between groups of neurons in the brain, which could adversely affect cognition.
