Axoglial interactions in myelin plasticity: Evaluating the relationship between neuronal activity and oligodendrocyte dynamics

Myelin is a critical component of the vertebrate nervous system, both increasing the conduction velocity of myelinated axons and allowing for metabolic coupling between the myelinating cells and axons. An increasing number of studies demonstrate that myelination is not simply a developmentally hardwired program, but rather that new myelinating oligodendrocytes can be generated throughout life. The generation of these oligodendrocytes and the formation of myelin are influenced both during development and adulthood by experience and levels of neuronal activity. This led to the concept of adaptive myelination, where ongoing activity-dependent changes to myelin represent a form of neural plasticity, refining neuronal functioning, and circuitry. Although human neuroimaging experiments support the concept of dynamic changes within specific white matter tracts relevant to individual tasks, animal studies have only just begun to probe the extent to which neuronal activity may alter myelination at the level of individual circuits and axons. Uncovering the role of adaptive myelination requires a detailed understanding of the localized interactions that occur between active axons and myelinating cells. In this review, we focus on recent animal studies that have begun to investigate the interactions between active axons and myelinating cells and review the evidence for—and against—the ability of neuronal activity to alter myelination at an axon-specific level.