 In the peripheral nervous system (PNS) axons can be either myelinated or unmyelinated.Myelination refers to the insulation of an axon with concentric surrounding layers of lipid membrane (myelin) produced by Schwann cells.These layers are generally uniform and continuous, but due to imperfect nature of the process by which Schwann cells wrap the nerve axon, this wrapping process can sometimes leave behind small pockets of residual cytoplasm displaced to the periphery during the formation of the myelin sheath.