The myelinated nerve fibres are enveloped with

Myelinated nerve fibers are enveloped by Schwann cells. They are a type of glial cell in the peripheral nervous system (PNS). They produce the myelin sheath, which wraps around and insulates axons of neurons in the PNS. This myelin sheath is composed of multiple layers of lipid-rich membrane, which greatly increases the speed of nerve impulse conduction along the axon. Also note that there are non-myelinating Schwann cells as well.