How Multiple Sclerosis Works
Causes
The root causes of MS are still not known, but scientists have come up with several working theories. A leading theory is that MS is an auto-immune disease. That is, the body's immune system, which when working properly fights off germs and other foreign substances that would otherwise cause a person to become ill, somehow gets confused into mistaking portions of the person's own body as foreign substances. The immune system then starts attacking and doing damage to these healthy tissues--in the case of MS, those in the nervous system.
What might be setting the immune system off this way is not yet known either. Theories point to some kind of virus, some kind of genetic predisposition (a tendency to be more prone to a certain health problem than the norm, based on the genes one inherits from one's parents and ancestors), and some kind of environmental factor such as chemical pollutants. It is also possible that any or all of these factors could be operating in combination. For instance, a person might have a genetic predisposition to MS (as shown by a number of other family members having the disease), but not actually get MS themselves until some factor in the environment, perhaps a virus or a chemical pollutant, triggers the predisposition.
We do know that MS seems to be more common in women than in men, and more common in people of Northern European ancestry. It also has a tendency to run in families. All of this does hint at some genetic factors to the disease. There is also a good bit of data suggesting that geography somehow has something to do with MS. Occurrences of the disease drop the closer one gets to the Equator. Scientists continue to study all these factors and more for definitive answers.
Mechanics
What is fairly well known is the mechanism by which MS does its damage to the nervous system. To understand this mechanism, let's first go over some basics of how the nervous system works. The nervous system can be thought of as the body's command, control, and communication system. It carries signals from the brain (the nervous system's control center) via the nerves (the communications lines) to the rest of the body, to direct conscious movement such as walking, talking, picking up objects, and so on, plus such vital life functions as breathing and heartbeat. The nervous system also picks up inputs from the body's many data-gathering organs--these include eyes, ears, nose, tongue; the nerve-endings in the skin that detect sensations such as touch, heat, cold, pain; sensors within the body that detect balance and position; and so on--and carries all this data back to the brain for processing.
All of this information flow travels via long chains of nerve cells, or neurons. When a neuron is stimulated by a previous neuron in a chain it undergoes a chemical/electrical change, which it transmits down its long fiber-like axon to the next neuron in the chain. Then that neuron passes the signal to the next in turn, and so on along the many complex paths between the brain and the rest of the body, and within the brain itself.
All neurons, whether in the brain or elsewhere in the nervous system, are covered with a thin protective layer of a fatty substance known as myelin. The myelin sheath has an important role similar to the insulation around an electrical wire: it keeps the signals passing through the neurons safe from interference so they go through clear and strong. However, if a neuron's myelin sheath were to be broken or damaged in any way, the signals trying to make their way through that neuron would become weakened, garbled, or even cut off altogether.
It is the myelin sheath that comes under attack when a person comes down with MS. When an MS patient experiences a flare-up or exacerbation of the disease, some agent (still to be identified) attacks the myelin around several or many nerve cells, leaving the scar tissue, known as sclerosis, that gives the disease its name. This scar tissue does not insulate the nerve anywhere near as well as a healthy intact layer of myelin would, and so the damaged nerve begins to lose signals passed to it, leading to communication breakdowns.