Myelinated axons
Most axons in the CNS and PNS are insulated by a myelin sheath produced by glial cells. The myelin sheath is produced by Schwann cells in the PNS and by oligodendrocytes in the CNS. The sheath consists of segmental units, called internodes, formed by the concentric wrappings of the plasma membranes of these cells. The myelin sheath increases conduction velocity along the axon. 1000x, 1000x, 5000x
Myelinated axons >
These cross sections of peripheral nerves show axons surrounded by myelin sheaths produced by Schwann cells. A Schwann cell produces a single internodal segment of myelin, formed by concentric wrappings of its plasma membrane around the axon. This membrane wrapping is uniquely rich in lipid, which provides the myelin with its insulating ability.
Myelin sheath
These cross sections of peripheral nerves show axons surrounded by myelin sheaths produced by Schwann cells. A Schwann cell produces a single internodal segment of myelin by concentric wrapping of its plasma membrane around the axon. This membrane wrapping is uniquely rich in lipid, which provides the myelin with its insulating ability.
Schwann cells
These cross sections of peripheral nerves show axons surrounded by myelin sheaths produced by Schwann cells. A Schwann cell produces a single internodal segment of myelin by concentric wrapping of its plasma membrane around the axon. This membrane wrapping is uniquely rich in lipid, which provides the myelin with its insulating ability.
Unmyelinated axons >
In general, axons with diameters of less than one micron are not myelinated. In this case, multiple axons indent the surface of a single Schwann cell and are surrounded by a single wrapping of the Schwann cell plasma membrane.
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