Immune and Nervous Systems Roles in Multiple Sclerosis
Even though multiple sclerosis (MS) is described as the most common neurologic disorder diagnosed in young adults, the problem doesn’t appear to originate with the nervous system. Instead, decades of research have pointed to the body’s immune system as the culprit.
Some kind of malfunction in the immune system interferes with the functioning of the body’s nervous system, resulting in the symptoms commonly associated with MS. The current thinking is that the glitch is an autoimmune problem, which basically means that your body is mistakenly destroying some of its own healthy tissues and cells. But, this thinking has yet to be proven.
The immune system and MS
The immune system — which is a complex network of glands, tissues, and circulating cells — is your body’s frontline defense in the fight against infection by viruses, bacteria, and other bad guys. When confronted with an infection, the immune system gears up to neutralize the foreign invader and make you healthy again.
In order for your immune system to do its job properly, it has to be able to distinguish between the good guys (the cells, tissues, and organs that make up your body) and the bad guys (any foreign invader, such as a virus or bacteria that doesn’t share your genes).
And get this: The immune system is so powerful that it could reject a pregnant woman’s developing fetus (which shares only some of her genes) if the hormones of pregnancy didn’t suppress her immune system.
The nervous system and MS
The nervous system, which controls all bodily functions, is made up of neurons, each of which consists of a cell body and its long extension — the axon. And many axons are covered by a protective or insulating coating called myelin. The neurons are gathered into small- and large-sized bundles called nerves.
The system is basically divided into two parts: The central nervous system (CNS), which consists of the brain, spinal cord, and optic nerves, is the target of the damage done in MS. The peripheral nervous system (PNS) includes the branching network of nerves and axons that connects the CNS to muscles, sensory organs, and glands in the rest of the body.
The nervous system conducts four basic kinds of electrical signals throughout the body:
Motor signals: These signals, which move from the CNS, through the PNS, and to muscles and other organs, control movement, strength, and other bodily functions.
Sensory signals: These signals go back to the CNS from the eyes, ears, skin, and other sensory organs, and they provide information about the environment from those organs.
Autonomic signals: These signals go to your GI system, your bladder, your sexual organs, and other parts of the body that often act without conscious control.
Integrative signals: These signals travel from nerve cell to nerve cell within the nervous system and are thought to be responsible for many cognitive functions, such as thinking and memory.
These electrical signals are like the current in an electrical wire: When everything is working fine, they travel long distances along the myelin-covered axons in the CNS, jumping from one axon to another as needed. The myelin (like the rubber or plastic insulation around an electrical wire) is what helps speed the electrical signals on their journey and smoothes out any bumps along the way.
What happens in MS
In autoimmune diseases like MS (and rheumatoid arthritis, myasthenia gravis, and Type I diabetes, among others), the immune system loses the ability to distinguish the good guys from the bad guys, and so it starts attacking the normal tissues in the body. In MS, this autoimmune response targets the myelin coating around the axons in the CNS, the axons themselves, and the cells that produce the myelin — called oligodendrocytes.
The autoimmune attack happens because of a breakdown in the blood brain barrier (BBB), which allows immune cells that have been living harmlessly in your blood to travel into your CNS to attack the myelin and axons, resulting in the symptoms associated with MS. The autoimmune process in MS follows these steps, as shown in the figure:
The inflammation that occurs during an MS relapse (also called an attack or exacerbation) damages the BBB, allowing the movement of immune cells into the CNS.
Toxic substances are released into the CNS, which can increase inflammation and result in the breakdown of myelin (in a process called demyelination) and the axons, sometimes even affecting the nerve cell bodies.
Nervous system cells called astrocytes move into the locations where the damage has occurred, and they form scar tissue (giving rise to the name multiple sclerosis, which means multiple scars).
The results of the autoimmune process aren’t all that pretty: The inflammation can cause swelling, which interferes with the conduction of signals in the nervous system. The demyelination results in a loss of insulation around the neurons’ axons, which slows or interrupts nervous system conduction. And finally, the axons can be broken (a process referred to as axonal loss), which breaks the connections between the nervous system and parts of the body. This image shows the steps involved in demyelination and axonal loss: