<P> Myelin comprises different cell types and varies in chemical composition and configuration but performs the same insulating function . Myelinated axons are white; hence, the "white matter" of the brain . Myelin insulates axons from electrically charged atoms and molecules . These charged particles (ions) are found in the fluid surrounding the entire nervous system . Under a microscope, myelin looks like strings of sausages . </P> <P> Cholesterol is an essential component of myelin, which comprises about 40% water; the dry mass comprises between 60% and 75% lipids and between 15% and 25% proteins . Myelin basic protein (MBP) constitutes ~ 23% of myelin protein, myelin oligodendrocyte glycoprotein, and proteolipid protein (PLP, which makes up ~ 50% of myelin protein). The primary lipid of myelin is a glycolipid called galactocerebroside . The intertwining hydrocarbon chains of sphingomyelin strengthen the myelin sheath . In brain, the myelin sheath covers the fibers of the corpus callosum, which constitute the inner part of the cerebral hemisphere . </P> <P> The main purpose of a myelin sheath is to increase the speed at which impulses propagate along the myelinated fiber . Along unmyelinated fibers, impulses continuously move as waves, but, in myelinated fibers, they "hop" or propagate by saltatory conduction . Myelin decreases capacitance and increases electrical resistance across the cell membrane (the axolemma). Thus, myelination prevents the electric current from leaving the axon . It has been suggested that myelin permits larger body size by maintaining agile communication between distant body parts . </P> <P> Myelinated fibers lack voltage - gated ion channels (approximately 25 μm) along the myelinated internodes, exposing them only at the nodes of Ranvier . Here, they are found far more abundantly (between 2,000 and 12,000 μm). Myelinated fibers succeed in reducing sodium leakage into the extracellular fluid (ECF), maintaining a strong separation of charge between the intracellular fluid (ICF) and the ECF . This increases sodium's ability to travel along the axon more freely . However, the sodium diffuses along the axolemma rapidly, but is decremental by nature . The sodium cannot trigger the opening of the voltage - gated sodium channels as it becomes weaker . The nodes of Ranvier, being exposed to the ECF every 1 mm or so, contain large amounts of voltage - gated sodium channels, and allow enough sodium into the axon to regenerate the action potential . Each time the action potential reaches a node of Ranvier, it is restored to its original action potential (+ 35 mV). </P>

What is the functional advantage of the myelin sheath
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