The scars - or scleroses - form on nerve fibers in the brain, spinal cord, and optic nerves, which are the basic components of the central nervous system (CNS). Also known as plaques, these scars are the result of lesions that destroy the protective material that surrounds the nerve fibers. This material is called the myelin sheath.
Much like the insulation on an electrical wire, an intact myelin sheath keeps nerve impulses traveling rapidly and accurately along the nerve fiber. These impulses are essential to normal movement and sensation throughout the body.
MS lesions eat away at the myelin sheath, eventually healing into hardened scar tissue. This process is called demyelination.
The scar tissue "short circuits" or interferes with the proper transmission of nerve impulses to various systems in the body. The result is a broad array of motor and sensory disabilities.
If the sheath can regenerate itself - known as remyelination -- normal nerve function may return. If not, the nerve will eventually die, and the disability will be permanent.
As the extent of nerve damage increases, the level of disability can grow progressively worse over time.
Researchers believe the damaging lesions are caused by an autoimmune reaction, where the body's defense system mistakenly attacks its own tissue. What triggers this abnormal immune response is presently unknown, although viral infection and/or environmental factors are suspect. Genetics may also play a role in susceptibility to the disorder.
The immune system's assault causes inflammation of CNS tissues. The inflamed nerves then develop the destructive lesions that adversely affect a variety of functions, depending upon the location and extent of the lesion damage.
These functions include: