In certain neurons, the axon may be covered by a fatty white substance called myelin which functions as an insulating layer

• Myelin is a mixture of protein and phospholipids that is produced by glial cells (Schwann cells in PNS; oligodendrocytes in CNS)
  

The myelin forms a sheath around the axon to increase the speed of electrical transmissions via saltatory conduction

• The myelin sheath is structured in segments with ion channels clustered at the gaps between each segment (called the nodes of Ranvier)

• In myelinated neurons, the action potentials ‘hop' between the nodes of Ranvier, resulting in an increase in transmission speed (up to 100 fold)
  

Myelinated neurons take up significant space and have increased energy demands, so not all neurons are myelinated

• Regions of the nervous system composed of myelinated axon tracts appear as white matter