How does myelination of axons in the brain regulate the timing of postsynaptic activations?

The myelination of axons or the presence of the myelin sheath regulates the conduction velocity of the nerve impulses by acting as an insulator to the nerve fibers. The spaces or gaps between each myelin sheath are called the nodes of Ranvier and the impulse go from one node to another. This phenomenon is called saltatory conduction. 

Nice to know: The myelin sheath in the central nervous system is provided by cells called oligodendrocytes. On the other hand, the myelin sheath in the peripheral nervous system is provided by cells called the Schwann cells.