Zenuwweefsel: Myeline

Myelin is a protective, multilayered covering around axons, composed of lipids and proteins, that provides electrical insulation.

In the PNS, individual Schwann cells repeatedly coil around short segments of an axon. The inner layers of this wrapping form the myelin sheath, while the outer portion, the neurolemma, contains the Schwann cell's nucleus and cytoplasm.

Two adjacent myelinated regions, or internodes, are separated by exposed areas of the axon, known as nodes of Ranvier. Because myelin is insulating, a nerve impulse jumps along an axon from one node to the next, speeding up the propagation of nerve impulses. 

In the CNS, oligodendrocytes send out flat, cellular processes that wrap around axons. Several oligodendrocytes contribute to the myelination of one axon, and one oligodendrocyte myelinates parts of many axons.

Myelinated axons in the CNS also have nodes of Ranvier and internodes but no neurolemma.

Unmyelinated PNS and CNS axons are thin and lack myelin sheaths. In the PNS, a single Schwann cell sometimes encloses many unmyelinated axons without coiling around them.