The immune system, the body’s defence mechanism, is a complex network of cells that helps our bodies fight off infection.
Crucial to the normal functioning of the immune system is the ability of immune system cells to recognize the difference between healthy cells that are part of the body and unhealthy invaders such as bacteria and viruses.
In autoimmune diseases, the immune system loses this ability to distinguish between host and invader. Certain cells in the immune system start attacking healthy tissue as if it were an infection, turning on parts of the body instead of attacking enemies from outside, such as viruses or bacteria that cause illness. That is what ‘auto’ means. This is why both MS and the rheumatic diseases are known as auto-immune diseases.
An argument in favour of seeing MS as an auto-immune disease is the fact that it is possible to cause a condition resembling MS in laboratory animals by giving them a solution of nerve tissue that activates their immune system. Another argument is that auto-immune diseases are more common in women than in men (just like MS). Women, in fact, have two to three times more chance of developing MS than men do
- The Blood-Brain Barrier (BBB) is disrupted
In MS, the brain’s “gatekeeper”, the Blood-Brain Barrier (BBB), is damaged and repeatedly breached by activated immune cells.1,2
- Inflammatory cytokines flood the brain
In MS, activated immune cells release proinflammatory cytokines and free radicals, and subject axons to demyelination, damage, and death.1-3
- Neurorepair is compromised
In MS, the natural process of remyelination, regeneration, and repair is impaired. 7-14
Low levels of the neurotrophin Nerve Growth Factor (NGF) correlate with axonal loss.4
- Hartung H-P, Bar-Or A, Zoukos Y. J Nuerol. 2004;251(suppl 5): V/12-V/29.
- Noseworthy JH, Lucchinetti C, Rodriguez M, Weinshenker BG. N Engl J. Med. 2000;343:938-952.
- Bruck W. J Neurol. 2005;252(supp 5):V/3-V/9.
- Biernacki K, Antel JP, Blain M, Narayanan S, Arnold DL, Prat A. Arch Neurol. 2005;62:563-568.
- Dressel A, Mirowska-Guzel D, Gerlach C, Weber F. Acta Neurol Scand. 2007;116:164-168.
- Rudick RA, Ransohoff RM, Lee J-C, et al. Neurology. 1998;50:1294-1300.
- Villoslada P, Genain C. Prog in Brain Res. 2004;146:403-414.
- Boutros T, Croze E, Yong VW. J Neurochem. 1997;69:939-946.
CNS function & “white matter”
The central nervous system (CNS) is made up of the brain and the spinal cord. The CNS is a kind of computer that keeps us in touch with the world around us. Through the central nervous system we can pick up signals from the outside world – such as sound, and we can see, smell, taste and feel. The CNS also coordinates all our conscious and unconscious activities – such things as moving, talking, thinking, remembering and the reflex actions that happen without us thinking about them.
There are nerve cells in the CNS. When looked at with the naked eye, tissues containing many nerve cells seem to be grey. For this reason they are known as “grey matter”. These nerve cells are joined together and to the rest of the body by nerve fibres, also known as axons.
Very weak electrical signals are sent along the axons so that information is passed between the nerve cells. Axons are lighter in colour than the nerve cells because they are enclosed in what is known as “white matter” or “myelin”.
Myelin is a fatty substance which is wrapped around the nerve fibres. Myelin works like a kind of insulation around the nerve fibres, enabling information (electrical signals or electrical impulses) to be sent more quickly from the brain to the rest of the body or from a particular part of the body and back to the brain.
The white matter contains blood vessels that supply the nervous system with oxygen and food. It would seem that the inflammation in MS often occurs around these blood vessels.
Nerve Damage & Demyelination
You may have heard that multiple sclerosis involves something called demyelination? Demyelination is damage that occurs to the protective sheath, called myelin, which surrounds nerve fibres.
Nerve fibres (axons) connect the muscles and sensory fibres to the brain and the spinal cord. Electrical impulses travel along these axons, bringing information back and forth across the brain, the spinal cord and the nerves. This communication over the axons enables movement, speech, thinking, vision and the other senses.
Surrounding these axons at regular intervals is a sheath made of fat and protein called myelin. Myelin protects the axons and also helps speed up the signals travelling along the spinal cord and in the brain. For example, when you decide to take a walk, myelin helps ensure that the signal from your brain that tells your leg to move gets there immediately and intact.
In MS, cells that normally fight infection start attacking the myelin around nerve fibres as if it were an invader, (known as an inflammatory reaction).
One of the indicators of MS is an abnormally high level of certain immune cells in the spinal fluid. Lesions (plaques) form where myelin is damaged causing the electrical impulses travelling to and from the brain and the spinal cord to be interrupted or slowed down. When these electrical impulses can’t get to their destination efficiently or can’t get there at all, symptoms develop.
Over time, this process, known as “demyelination” can cause the myelin layer to become thinner and thinner, making it increasingly difficult for the nerve fibres to transmit information. This then will result in a differing diagnosis (See types of MS and disease progression)
Multiple sclerosis can damage the myelin in many places throughout the central nervous system. This is why people with MS present varied symptoms in various places throughout their bodies.(See section on MS symptoms).
Recently, another factor contributing to the progress of MS was discovered. This factor is called axonal injury. It has been shown that nerves or axons of people with MS may be damaged or destroyed. Axonal injury might occur very early in the course of the disease. Axonal injury means that the neuronal transmission of electrical signals is completely blocked and cannot be restored. Therefore, experts believe that permanent disability in MS could be the result of a combination of demyelination and axonal injury.
Remyelination & Plaques
After a few weeks, once the inflammation has died down, the nervous system can recover. And it is also possible that the myelin grows again (this is called ‘remyelination’ or restoration of the myelin). But when the inflammation covers a large area it can often leave a scar in the nervous system. The scar is known as a ‘plaque’.