Figure 2.

Different cellular processes may lead to dynamic changes in myelination during adulthood. Axons (green) in the CNS can be wrapped by myelin segments (blue), which are formed by oligodendrocytes (OLs) during development. Converging evidence indicates that myelination can be dynamically remodelled by activity-dependent and experience-dependant mechanism, even during adulthood. (a) Oligodendrocyte precursor cells (OPCs, orange) are an abundant proliferating cell population in the adult CNS and have the potential to differentiate into new myelinating oligodendrocytes [36]. Such de novo myelination (orange segment) can occur at previously unmyelinated segments, or (b) can replace retracting or damaged myelin segments of pre-existing OLs. (c,d) Additionally, pre-existing oligodendrocytes may also adjust structural parameters of their myelin sheath to modify nerve conduction velocity. Such myelin remodelling can be achieved by (c) altering the thickness of myelin segments though the addition or removal of membrane layers [38^•], or (d) varying the length of myelin segments [44^•]. Yet, it remains unclear if and to what extent myelin remodelling in the adult CNS can be mediated by pre-existing OLs. (e) Additionally, adjustment of node of Ranvier length has been suggested as another potential mechanism for tuning the arrival time of information in the CNS [46^•]. While all of these mechanisms have the potential to change the information flow within neural networks, their relative contribution to adaptive myelin plasticity, as well as their mechanistic complexity, remain poorly understood.