Myelin is an essential fatty membrane insulating every nerve in the brain and the spinal cord the same way a plastic sheath surrounds an electric wire. It is this protective envelope ensuring the normal transmission of messages via the nerves from one part of the body to another. Rapid conduction of nerve impulses along the nerve fibers is essential for motor, sensory and integrative functions in the central nervous (brain, cerebellum and spinal cord) and peripheral nervous (nerves) systems.
How does it work?
Oligodendrocytes form the myelin sheath which helps the transmission of nerve impulses. A single oligodendrocyte deploys up to 40 membrane layers which each twist tightly around an axon to form a myelin segment. The compacted membranes in the myelin sheath create a high resistance, a low capacity insulator, which enables electric impulses to be propagated. In a demyelinated region of an axon, this process is interrupted. The high capacity of the demyelinated axonal membrane and the low resistance of the extracellular space, compared to the axon, cause the impulse to be dissipated.
Demyelination is the term used to refer to an abnormal deterioration of the myelin. When this envelope is damaged, the consequences are clearly visible and can be dramatic. In this way, vision, hearing, speech, motor control or memory may be disturbed or paralysed. In many cases, there is a risk of total paralysis or premature death.
Depending on the disease, only one type of myelin may be affected:
- the myelin of the central nervous system, as in some leukodystrophies and multiple sclerosis,
- the myelin of the peripheral nervous system.
It is rarer for both types of myelin to be affected (some cases of adrenoleukodystrophy).
Each year, thousands of children or young adults are victims of myelin diseases, whether genetic conditions such as leukodystrophies, or illnesses such as multiple sclerosis, which selectively attacks the membrane.