The main purpose of a myelin layer (or sheath) is to increase the speed at which impulses propagate along the myelinated fiber. Along unmyelinated fibers, impulses move continuously 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 helps prevent the electric current from leaving the axon. It has been suggested that myelin permits larger body size by maintaining agile communication between distant body pas.Myelinated fibers lack voltage-gated ion channels (approximately 25 mm-2) 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 mm-2). 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).When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. However, the myelin layer does not ensure a perfect regeneration of the nerve fiber. Some regenerated nerve fibers do not find the correct muscle fibers, and some damaged motor neurons of the peripheral nervous system die without regrowth. Damage to the myelin sheath and nerve fiber is often associated with increased functional insufficiency.Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate.Some studies have revealed that optic nerve fibers can be regenerated in postnatal rats. This regeneration depends upon two conditions: axonal die-back has to be prevented with appropriate neurotrophic factors, and neurite growth inhibitory components have to be inactivated. These studies may lead to fuher understanding of nerve fiber regeneration in the central nervous system.Ref: Ganong&;s review of medical physiology, 23rd edition, Page no:82