Answer: Immune cells that improperly target myelin for destruction.
Myelin is a protective sheath that surrounds axons. Myelin also serves to increase conduction speed of signals through saltatory conduction. Myelin is produced by oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system.
In multiple sclerosis, some mechanism causes the immune cells to believe that myelin is an invader. The immune system then destroys the beneficial myelin. Therefore, multiple sclerosis is considered an autoimmune disease.
Patients with multiple sclerosis often experience numbness or tingling in the limbs, muscle weakness, or tremor. These symptoms are a result of a loss of nerve conduction due to demyelination.