What is the myelin sheath, and why does it speed up nerve transmissions?

Myelin sheaths refer to the covering of fatty tissue, which safeguards the nerve cells. These cells are the components of CNS that conduct messages back and forth between the brain and the remaining parts of the body.  

The majority of the nerve fibers are enveloped by an insulating, fatty sheath known as myelin that functions to accelerate the impulses. The myelin sheath comprises periodic breaks known as nodes of Ranvier. Hopping from nodes to nodes, the impulse can pass much more briskly than if it had to move along the whole length of the nerve fiber.  

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