Which structure insulates nerve fibers and speeds transmission?

• A. Node of Ranvier
• B. Neurilemma
• C. Myelin Sheath
• D. Axon

Answer: (C)

The insulation of nerve fibers by a myelin sheath is what speeds transmission. The myelin wraps around the axon, created by Schwann cells in the peripheral nervous system (and by oligodendrocytes in the central nervous system). This wrapping acts as an electrical insulator, which makes the membrane less permeable to ion flow and increases the membrane’s resistance while lowering its capacitance. Because of that insulation, the depolarizing current leaks less across the membrane, allowing the signal to travel much faster along the axon. Transmission occurs in a saltatory fashion, jumping from one Node of Ranvier—gaps rich in voltage-gated channels—to the next, where the action potential is re-triggered. The neurilemma is the outer layer of Schwann cells surrounding the myelin sheath and is more about protection and regeneration than speeding conduction, while the axon is the nerve fiber itself that conducts the impulse.