Myelin sheath is the lipid covering of the axons, which protect the axon and speed up the process of impulse conduction. Neurons with myelin sheath are called as myelinated neurons and neurons without myelin sheath are called as non myelinated neurons.
The gap junctions found at regular intervals in the myelin sheath are called as Nodes of Ranveir. Myelin sheath does not allow the exchange of ions through it and exchange of ions occurs at Nodes of Ranveir only. Myelin sheath helps in the faster conduction of impulses through the axons by saltatory conduction (impulses jump from one node to another).