Concentric wrapping of plasma membrane with nodes of Ranvier between
Expansion
Structure: concentric spirals of glial cell plasma membrane, about 70 to 80 per cent lipid, wrapped around the axon. Interrupted at the nodes of Ranvier, where voltage-gated sodium channels are concentrated.
Formed by
- Oligodendrocytes centrally, each myelinating many axons
- Schwann cells peripherally, each forming one internode on a single axon
Function: increases conduction velocity by saltatory conduction, with the impulse jumping node to node, and by acting as an insulator reducing capacitance.
Histological appearance: because myelin is largely lipid, it is dissolved by routine alcohol and xylene processing, appearing as clear space around the axon. It is demonstrated by Luxol fast blue or by osmium in resin sections.
Demyelinating disease maps onto the two cell types:
- Central: multiple sclerosis, with perivenular plaques showing loss of Luxol fast blue staining but relative axonal preservation; progressive multifocal leukoencephalopathy; central pontine myelinolysis
- Peripheral: Guillain-Barre syndrome, chronic inflammatory demyelinating polyneuropathy, Charcot-Marie-Tooth disease
Demyelination slows conduction, which is the basis of nerve conduction studies showing reduced velocity in demyelination and reduced amplitude in axonal loss.