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. This sheath is composed of protein and lipids.
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 a network for the passage of electrical nerve impulses across the body.
The main function of myelin is to protect and insulate these axons and enhance their transmission of electrical impulses. If myelin is damaged, the transmission of these impulses is slowed down, which is seen in severe neurological conditions such as multiple sclerosis.
How does myelin enhance the transmission of electrical impulses?
Myelin surrounds and insulates the axon and builds specialized molecular structures at small, uncovered gaps in the sheath, which are referred to as the nodes of Ranvier. In the case of unmyelinated axons, the nerve impulse (action potential) moves along the axon continuously. By contrast, in a myelinated nerve fiber, currents can only occur where the axonal membrane is uncovered, at the nodes of Ranvier.
The lipid-rich myelin sheath therefore acts as an insulator, offering high transverse resistance and only allowing a current to flow along the segments that lie between these nodes of Ranvier.
Taking the most thoroughly myelinated axon as an example, which is 12 to 20μm in diameter, the speed at which an impulse is conducted along the axon is 70 to 120 metres per second (m/s), which is the speed of a race car.
Another example of how space and energy is saved by the myelin sheath can be illustrated 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 12μm 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 5,000 times more energy and 1,500 times more space than the frog axon.
Multiple sclerosis is an autoimmune condition where the body’s own immune cells attack this myelin sheath. T cells strip the myelin from the nerve fibers it protects, meaning the fibers are left exposed and uninsulated. These unprotected nerves are then less able to conduct electrical impulses from the brain to other parts of the body and the nerve signals sent to the brain are delayed and distorted. The damaged areas of the nerve where the myelin has been destroyed forms hard scar tissue (sclerosis) that further disrupts the conduction capacity of the nerve. These scarred areas are also referred to as plaques and they can be identified using magnetic resonance imaging, a technique that aids doctors in the diagnosis of multiple sclerosis.
As more myelin is destroyed, the less efficient the nerves are at transmitting nerve impulses. The severity of multiple sclerosis symptoms depends on whether the myelin has been partially or completely stripped from the nerve fibers. Determining the extent of damage to the myelin sheath may be key to predicting how severe symptoms will become.