What is myelin sheath?
A typical nerve consists of dendrites, axons, a cell body, terminal buttons and myelin sheaths. Dendrites gather information from the environment and the stimulus is gathered in the cell body. The axons of a nerve function to carry nerve impulses from the body to another nerve. Spaces in between myelin sheaths in axons, where electrical impulses travel, are called Nodes of Ranvier.
The myelin sheath, also called medullary sheath, is a modified plasma membrane formed by layers of Schwann’s cell membranes (in the peripheral nervous system) or oligodendrocyte membranes (in the central nervous system) wrapped around the axon of the nerve. It is made up of lipids and proteins. Lipids constitute approximately 70% to 85% present in different forms such as cholesterol, cerebroside, ethanol-containing plasmogens, lecithin and sphingomyelin. Proteins compose approximately 15%-30% of the myelin sheath in the form of myelin basic protein and proteolipid protein. The myelin sheath is the part of the nerve damaged by the immune system in Guillain-Barre Syndrome.
Figure 1.0 Structure of a typical myelinated neuron
What is the importance of myelin sheath in nerve conduction?
Myelin sheath is composed mainly of fat (lipids) which are good electric insulators. This property of lipids gives myelin sheath its major function, that is, insulation of electrical impulses along the axon. Because of the insulation function of the myelin sheath, electrical impulses can’t travel along the myelinated parts of the axon and jump only from one node of Ranvier to another instead of traveling the entire span of the axon. This causes electrical impulses to travel at a more rapid rate as fast as the speed of a race car (70-120 m/s). When the preexisting myelin sheaths are damaged or lost,(a condition called demyelination) the conduction of nerve impulses slows down or stops causing a remarkable damage in the information networks within the nervous system.
What is an autoimmune disorder?
An autoimmune disorder is a condition involving the attack of the immune system to the own body causing damage to healthy body tissues through antibodies. Typically, the white blood cells of the immune system protect the body from foreign substances, called antigens which may be harmful to the body. But in patients with autoimmune disorders, the immune system loses its ability to differentiate between healthy body tissues and antigens, resulting to an immune response destroying normal body tissues. There are many different types of autoimmune disorders causing damage to different tissues. Some of which are blood vessels, connective tissues, endocrine glands, joints, nerves, muscles, red blood cells and the skin. In Guillain-Barre syndrome, the autoimmune system specifically attacks the nerves particularly the myelin sheath of the nerves in the peripheral nervous system, causing demyelination and subsequently, nerve dysfunction.
Guillain-Barre Syndrome (GBS)