Another aspect of the nervous system covered in the last module was the axons that are part of neurons. Some of these are coated in myelin sheaths, which increases the rate at which information is passed along the axon. In some people, this myelin sheath starts to break down, which in turn slows down the functioning of the nervous system. This is multiple sclerosis (MS), an autoimmune condition with no known cause that affects over 100,000 people in the UK.

There are three main forms of MS:

1. Relapsing-remitting, in which there are disease exacerbations during which symptoms flare up (relapses) followed by recovery and periods of stability (remissions)

2. Secondary progressive, in which the disease starts as relapsing-remitting but the damage done to the nervous system means that the remissions are not complete and instead the patient experiences a deterioration in their general condition

3. Primary progressive, in which the disease steadily worsens without any remissions. MS tends to be diagnosed between the ages of 20 and 30 years, and affects more women than men.

Because almost any part of the central nervous system can be affected by MS, the signs and symptoms can vary hugely. Some of the most common are fatigue; numbness or tingling; muscle stiffness or weakness, which can impair mobility; and problems with the bladder, bowels and eyesight. Sexual dysfunction can also occur, plus pain, difficulties with thinking and planning, speech and swallowing issues, and mental health conditions such as depression. Relapses also vary hugely. They can be mild to very severe and may last days, weeks or months.