The fatty tissue is called the
Myelin sheath. The myelin
sheath is a significantly extended and altered plasma membrane wrapped about
the nerve axon in a spiral form. The myelin membranes somes from and are a part
of the Schwann cells in the peripheral nervous system and the oligodendroglial
cells in the central nervous system. Each myelin-spawning cell delivers myelin
for only one segment of any given axon. The intermittent interruptions where
short portions of the axon are left uncovered by myelin are the nodes of
Ranvier, and they are critical to the functioning of myelin.
