Myelin sheath – insulation for axons
This is best thought of as insulation for axons. It’s a fatty white sheath around the axons which provides electrical insulation and protection, and actually helps increase the speed of impulses.
My violin (myelin) had to be played in a well-insulated room.
The myelin sheath is wrapped around the axon in a spiral fashion. It’s like a rubber coating on a copper wire, protecting the axon as well as speeding up impulses.
Cross-sectional view of myelin surrounding axons, as seen under an electron microscope.
Not all axons are myelin-coated (or myelinated). Some shorter axons, and some that send messages that are not particularly time-critical, do not have myelin sheaths.