Definition and causes of Multiple Sclerosis – Disseminated Sclerosis
Multiple Sclerosis also called disseminated sclerosis is a chronic neurological disease that affects various parts of the central nervous system and gradually destroys the affected parts.
In the early years with multiple sclerosis there are often attacks with various symptoms which disappear completely or partially. Later in the progression these symptoms will no longer disappear entirely, leading to increasing disability (see article Symptoms of multiple sclerosis).
The central nervous system consists of the brain and the spinal cord. Each nerve cell, called a neuron, has a cell body and specialized extensions to receive and send nerve signals. A single neuron can have several extensions that can receive signals called dendrites but it only has one extension sending signals the so called axon. The central nervous system consists of the gray matter where all the neurons are located and the white matter that mainly consists of myelinated axons.
Moreover, there are special support cells which produce a fatty material of lipid (fat) and protein that forms a protective sheath around the axon. This is called myelin sheath and contributes to nerve signal transmission speed and protects the axon (as insulation around a wire).
In multiple sclerosis these myelin sheaths are attacked and destroyed by the immune system. The nerve signal transmission speed is slowed down and eventually the axon itself is destroyed shutting down the nerve signals totally.
The reason that the immune system is activated against the myelin is not known in detail. It is believed that some of the white blood cells might be activated against a virus or a bacterium resembling the myelin and then mistakenly attacks the myelin sheath as an invading threat. This leaves an area with axons without the protective myelin sheath and some scar tissue. The process is called demyelization and the affected area is called a plague and can be seen on an MRI scan of the brain helping to determine, very accurately, if there is multiple sclerosis. (see also Diagnosis of multiple sclerosis).
Since axons lie in the white substance, the myelin, is the mostly here the disease is most active. Not all of the white substance is attacked at the same time, which is why the disease can have very different symptoms (see Symptoms of multiple sclerosis).
The symptoms reflect the area which is attacked. This means that if the part controlling the movement of the right hand is attacked then this is where the symptoms will appear. If in contrast the area that is affected leads sensory signals to the left foot the symptoms will be reduced or disrupted tactile sense here.
In the beginning, many of the symptoms disappear again as an indication that the injury to the myelin sheath was not complete and likely new myelin has formed around the axon. In the later stages of the disease damage will be so severe that the symptoms are permanent and often more areas will become affected, leading to new symptoms.
Who gets multiple sclerosis?
Multiple sclerosis is fairly common. In western developed countries there is around 1 new case of sclerosis per 800 citizens and new cases arrive daily. The disease affects mostly young people aged 20-40 years and twice as many women than men are affected.
There is some hereditary predisposition for the disease because it is seen more frequently in families where there is a family member with multiple sclerosis. However, it is not actually a hereditary disease, since there is no known gene, which is solely responsible for the development of multiple sclerosis.
It is believed that the hereditary element is a predisposition for the development of the disease combined with so far unknown factors which lead to the disease.
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