Distribution of mitochondria along small-diameter myelinated central nervous system axons.

Small-diameter myelinated CNS axons are preferentially affected in multiple sclerosis (MS) and in the hereditary spastic paraplegias (HSP), in which the distal axon degenerates. Mitochondrial dysfunction has been implicated in the pathogenesis of these and other disorders involving axonal degeneration. The aim of this study was to determine whether the frequency of axonal mitochondria changes along the length of small-diameter fibers and whether there is a preferential localization to the region of the node of Ranvier. We find that mitochondrial numbers do not change along the length of a myelinated small-diameter fiber, and, in contrast to the peripheral nervous system, there is no tendency for mitochondrial numbers to increase at the node.