Special cells called Schwann cells grow and wrap around some axons many times laying down two phospholipid bilayers each time.The membrane contains a fatty substance called myelin. There may be up to 20 insulating layers around the axon called the myelin sheathMyelinated neurones can conduct impulses at up to 100 m/s compared to unmyelinated neurones at 1m/s. There are 2-3 μm gaps between the Schwann cells called the Nodes of Ranvier.