Note in this magnified view that the axon is surrounded by specialized Schwann cells whose plasma membranes form a covering of the axon called the neurilemma (shown in brown on the model). These Schwann cells secrete a fatty myelin sheath (shown in yellow on the model) that protects and insulates nerve fibers from one another and increases the speed of transmission of nerve impulses.   Adjacent Schwann cells along an axon do not touch one another, leaving gaps in the sheath called nodes of Ranvier at regular intervals.