Multiple sclerosis (MS) is a chronic, often disabling disease of the central nervous system (CNS). Converging lines of evidence suggest that the disease is caused by a disturbance in immune function. This disturbance permits cells of the immune system to attack myelin, the insulating sheath that surrounds the nerve fibers (axons) located in the CNS (i.e., the brain and spinal cord). When myelin is damaged, electrical impulses cannot travel quickly along nerve fiber pathways in the brain and spinal cord. Disruption of electrical conductivity results in fatigue and disturbances of vision, strength, coordination, balance, sensations, and bladder and bowel function.
Why was this disease called multiple sclerosis? Physicians during the 19th century noticed that the brains and spinal cords of patients with MS contained many areas where the nervous tissue was hard to the touch and appeared scarred. The Latin word for scar is sclerosis. Thus, the term multiple sclerosis was chosen to describe the appearance of the brain in patients who died with this illness. Pathologists call these scars plaques. When observed microscopically, plaques consist of inflammatory cells, astroglial cells, increased water (edema), and destroyed myelin fragments. Larger plaques may be seen on magnetic resonance imaging (MRI) scans of the brain and spinal cord.
The loss of normal myelin is called demyelination. Demyelination produces a situation analogous to that resulting from cracks or tears in the insulator surrounding an electrical lamp cord. When the insulating surface is disrupted, the lamp will short-circuit and the light bulb will flicker or no longer illuminate. Similarly, loss of myelin surrounding nerve fibers results in short-circuits in nerves traversing the brain and spinal cord that result in symptoms of MS.
In contrast to a single wire pathway in a lamp cord, there are thousands of nerve pathways in the brain and spinal cord, the two components of the CNS. The symptoms of multiple sclerosis depend largely on which particular nerve fiber pathway is involved in the CNS. Tingling, numbness, sensations of tightness, or weakness may result when loss of myelin occurs in the spinal cord. If the nerve fibers to the bladder are affected, urinary incontinence may follow. If the cerebellum of the brain is affected, imbalance or incoordination may result. Since the plaques of MS can arise in any location of the CNS, it is easy to understand why no two MS patients have exactly the same symptoms.
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