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 sheath.Myelinated neurones can conduct impulses at up to 100m/s compared to unmyelinated neurones at 1m/s. There are 2-3 μm gaps between the Schwann cells called the Nodes of Ranvier.Schwann cells carry out phagocytosis to remove cell debris and are also involved in the regeneration of nerve cells.