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Scientists think that MS is caused by the body's immune system mistakenly attacking and destroying myelin in the CNS (demyelination). Myelin is a fatty substance made by cells called oligodendrocytes in the CNS. It often appears white in color and so regions of myelin in the brain and spinal cord are called "white matter."

Once myelin is lost the CNS has difficulty repairing the damage, or remyelinating, especially after several attacks. This leads to symptoms that temporarily abate when the body is able to repair the damaged myelin, but that worsen with time as more and more myelin is destroyed and cannot be replaced.

Myelin provides insulation for the electrical signals sent by nerve cells, like the plastic insulation that surrounds an electrical cord. In MS, the myelin sheath is damaged, so electrical signals originating in the brain and traveling along a nerve can exit through gaps in the myelin, often failing to reach their destination in the body. This greatly slows or halts communication via the nerves affected, leading to the deficits in movement and cognition characteristic of MS.

Image: J. Dahms