Myelination is the wrapping of an insulating layer of myelin around the axons of neuronal cells, much like wrapping a bare wire in insulating rubber, which not only blocks out noise but greatly increases the speed at which information can be transmitted. These processes that speed up the transmission of information during the development of the nervous system are myelination, and the myelination that takes place in the brain is even more far-reaching. Myelination proceeds from the head to the feet, from the middle to the sides, which explains very well why children first learn to raise their heads and rollover, which require control of only the upper body, and then learn to control their legs to learn walk. The process of myelination is essentially complete by the around 12th month, which explains why most children learn during this time.
Myelination (myelin is the substance that coats the axons in the nerves allowing increased motor control) follows two paths. One path starts at the head at birth and moves slowly down to the feet, making gross motor movements possible. The other path starts at the chest and slowly moves outward toward the fingers. These two processes mean that our baby is usually developing aspects of gross motor.
Action potential propagation in myelinated neurons is faster than in unmyelinated neurons because of saltatory conduction.
Source: Frullano L, Zhu J, Miller RH, Wang Y. Synthesis and characterization of a novel Gd-based contrast agent for magnetic resonance imaging of myelination, J Med Chem 2013 Jan