By Dr Ananya Mandal, MD
The myelin sheath is a protective covering that surrounds fibres called axons, the long thin projections that extend from the main body of a nerve cell or neuron.
Axons vary in length from 1 millimetre to up to 1 metre or more and carry nerve signals away from the main neuronal body to other nerve cells, muscles and glands. When axons are bundled together, they form nerves which create the network for the passage of electrical nerve impulses across the body. The main functions of myelin is to protect and insulate these axons and enhance their transmission of electrical impulses.
The myelin sheath is made up of multiple segments of myelin. Between the segments lie small, uncovered areas called the nodes of Ranvier. Each segment of the myelin sheath are extensions of oligodendrocytes and one oligodendrocyte can cover up to 50 different axons.
As an electrical impulse travels along an excited axon fibre, it cannot flow through the resistant myelin sheath and travels along the axon to reach the next node of Ranvier. This ensures a linear flow of the impulse along the nerve.
One way the space and energy saved by the myelin sheath can be illustrated is through the comparison of squid and frog axons. In the squid, a giant axon can span a diameter of 500μm but is unmyelinated while a frog axon is only 12 μm in diameter and myelinated. Calculations show that when both nerves conduct an impulse at a speed of 25m/s and a temperature of 20˚C, the unmyelinated squid axon uses up 5000 times more energy and 1500 times more space than the frog axon.
Reviewed by Sally Robertson, BSc