The function of the myelin sheath is to improve the safety and speed of the neural impulse transmission along the axon. The myelin sheath serves as an electrical insulator preventing the dispersion of the impulse to other adjacent structures. Since the myelin sheath has gaps called Ranviers’ nodes in its length, the neural impulse “jumps” from one node to another thus increasing the speed of the neural transmission.
Not all neurons have a myelin sheath. There are myelinated axonal fibers and unmyelinated ones.