Effects of aging on myelinated nerve fibers in monkey primary visual cortex

In monkeys, myelin sheaths of the axons in the vertical bundles of nerve fi bers passing through the deeper layers of primary visual carter show age-re lated alterations in their structure. These alterations have been examined by comparing the myelin sheaths in young monkeys, 5-10 years old, with thos e in old monkeys, between 25 and 33 years of age. The age-related alteratio ns are of four basic types. In some sheaths, there is local splitting of th e major dense line to accommodate dense cytoplasm derived from the oligoden drocytes. Other sheaths balloon out, and in these locations, the intraperio d line in that part of the sheath opens up to surround a fluid-filled space . Other alterations are the formation of redundant myelin so that a sheath is too large for the enclosed axon and the formation of double sheaths in w hich one layer of compact myelin is surrounded by another one. These altera tions in myelin increase in frequency with the ages of the monkeys, and the re is a significant correlation between the breakdown of the myelin and the impairments in cognition exhibited by individual monkeys. This correlation also holds even when the old monkeys, 25 to 33 years of age, are considere d as a group. It is suggested that the correlation between the breakdown of myelin in the old monkeys and their impairments in cognition has not to do specifically with visual function but to the role of myelin in axonal cond uction throughout the brain. The breakdown of myelin could impair cognition by leading to a change in the conduction rates along axons, resulting in a loss of synchrony in cortical neuronal circuits. J. Comp. Neurol. 419:364- 376, 2000. (C) 2000 Wiley-Liss, Inc.