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 saltation." Myelin decreases capacitance across the cell membrane, and increases electrical resistance. Thus, myelination helps prevent the electrical current from leaving the axon. When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. Unfortunately, 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 (PNS) die without re-growth. 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 reveal that optic nerve fibers can be regenerated in postnatal rats. This optic nerve 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. This study may lead to further understanding of nerve fiber regeneration in the central nervous system.