Generation of new oligodendrocytes: optogenetic stimulation or motor learning increases differentiation of oligodendrocyte precursor cells (OPCs) [15,48]. 2) Myelin sheath thickness: life experience or optogenetic stimulation increases myelin thickness relative to axonal diameter (g ratio) [44,48]. 3) Myelin sheath length: axon conduction times are regulated by differential axon diameter and myelin sheath length [50, (Ford,Grothe 15), 81]. 4) Myelin sheath number: electrical activity of axons or life experience modulates the number of myelin sheaths [47,49]. 5) Selection of axons to myelinate: activity-evoked vesicle release increases the stability and growth of nascent myelin sheaths [54].