Definition

Multiple sclerosis (MS; pronounced multiple skluh-RO-siss) is a chronic autoimmune disorder (see autoimmune disorders entry) that affects the nerves. "Chronic" means that it develops slowly over time; "autoimmune" means that the body's immune system becomes confused about some part of the body it is designed to protect. It attacks that part of the body as if it were a foreign invader. MS affects a person's ability to move, to feel, and to control his or her body functions.

Description

Nerve messages consist of electrical impulses that travel through the body by means of nerve cells. Nerve cells are also called neurons. Neurons are covered by a thin layer of tissue known as myelin (pronounced MY-uh-lin) that acts as an insulator. It prevents the electrical currents that pass through neurons from leaking away.

MS occurs when the myelin that surrounds neurons in the brain and spinal...