MS Active Source

MS The Disease

• Introduction
• History of MS
• What is MS?
• Who gets MS?
• Symptoms of MS
• Course of the disease
• Diagnosis
• Treatment of MS

What is MS – Pathology

MS is an autoimmune disease in which cells of the immune system attack the nerve cells or neurones of the brain and spinal cord leading to disruption of the electrical signals by which the brain communicates with the rest of body and vice versa. This results in the characteristic symptoms of MS.

Disease animation

This is a short animation that explains the impact of MS on the body.

Neurones and the CNS

The brain and the spinal cord make up the central nervous system (CNS). This consists of specialised cells called neurones or nerve cells that are involved in transporting messages between the brain and the rest of the body. These messages are transported in the form of electrical pulses that move along the neurones. Thus when you want to move your arm, an electrical signal is sent from your brain to the relevant muscle of your arm, and when you touch something cold with your finger, a message is sent from the sense receptor in your finger to your brain, which makes you aware of the sensation.

Neurones have four main parts (figure 1):

• the cell body which contains the nucleus, the mitochondria and other organelles required to maintain the cell,
• the dendrites which form connections (known as synapses) with other neurons and are involved in receiving electrical signals from other neurones,
• the axon or nerve fibre which is like a wire connecting different parts of the body to the brain and along which the electrical messages are passed, and
• the axon terminal where the axon connects with other neurones or muscles to pass on the electrical message.

Figure 1. Parts of a neuron

The axons are surrounded by an insulating phospholipid layer, known as the myelin sheath (figure 2). This is derived from cells known as oligodendrocytes.

By insulating the axon, the myelin sheath speeds up the transmission of the electrical signals along the axon. When the myelin sheath is damaged or destroyed, transmission of electrical signals along the axon is disrupted or lost all together.

Figure 2. Diagram showing the axon of a neuron (red) in cross-section, surrounded by the myelin sheath (blue)

The immune system

The immune system consists of a wide range of different cells which work together to protect the body from foreign organisms such as viruses and bacteria. The cells of the immune system also act to protect the body against abnormal cells and tissues that may form in the body such as cancer cells and tumours.

The main types of immune cells include:

• B-lymphocytes (also known as B-cells), which produce antibodies. These are proteins that bind to foreign bodies and promote their removal from the body.
• T-lymphocytes (also know as T-cells) which bind to foreign cells and may kill them directly or release proteins called cytokines which stimulate other cells of the immune system to kill and remove the foreign cells.
• Macrophages, which engulf foreign cells or bodies leading to their removal from the body.

These cells are present in the blood stream and in fluids in parts of the body that come into contact with the outside world, e.g. the mucous membranes of the nose and lungs. When a foreign body is detected, the release of cytokines stimulates cells of the immune system to migrate to the relevant part of the body and mount an attack against the foreign body.

Pathophysiology of MS

In autoimmune diseases such as MS, the cells of the immune system mistakenly recognise a part of the body as being foreign and mount an attack that can have disastrous consequences. In the case of MS, it is believed that cells of the immune system attack and destroy the myelin sheath around axons of the CNS. This is known as demyelination, (see figure 3). As a result, conduction of electrical signals along the affected axons is slowed down and this gives rise to symptoms.

Figure 3. Demyelination

In addition, without the surrounding myelin sheath, the demyelinated axons are vulnerable to damage. If the axon becomes so badly damaged that it breaks – known as axonal transection – transmission of electrical signals along that particular axon is permanently lost. While the brain is able to compensate for the loss of some axons, this process also contributes to the progression of MS and the resulting increased disability.

Immune attacks on the myelin sheath tend to be episodic as is typical of immune attacks, also known as inflammatory reactions. After a while, the inflammation subsides and the damage to the myelin sheath is repaired. This process – known as remyelination – results in restoration of the myelin sheath. However, the repair is often incomplete – the myelin layer may be thinner – making the region vulnerable to further attacks in the future. With repeated attacks, remyelination becomes less effective and a scar plaque forms around the damaged axon. Known as a plaque or area of sclerosis, scars can be detected on an MRI scan of the CNS, which is one of the techniques use to diagnose MS.

Although many of the processes involved in demyelination and remyelination are reasonably well understood, much less is known about why the immune attack occurs. In order for the attack to happen, immune cells from the blood stream need to cross the blood-brain barrier – a layer of cells that normally prevents cells from the blood entering the brain. It is unclear what changes occur to allow immune cells to cross the blood-brain barrier and what factors trigger this change. We don't fully understand why certain individuals develop MS, as is discussed further in Who gets MS.

Understanding pathology of MS is important for understanding both:

• the nature of MS and the course of the disease (as discussed in Symptoms of MS and Course of the Disease)
  
• the effects of the available treatments (as discussed in Treatment of MS)

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