What causes symptoms of multiple sclerosis (MS)?

Multiple sclerosis (MS) symptoms are caused by loss of myelin, axons and nerve cells.

The brain in multiple sclerosis:

• The immune system attacks the myelin covering the axons of nerve cells.
• It destroys patches or sections of myelin, leaving behind bare axons.

Nerve signals fail to reach the intended target:

• A nerve signal begins at the cell body of the nerve cell.
• It moves down the axon but can't jump an area of myelin damage.
• Unable to jump across the section of bare axon, the signal is blocked.
• The signal fails to reach its intended target, resulting in symptoms.

MS symptoms depend on the affected nerve pathway:

• For the nerve-sending signals from the eye to the brain, vision may be affected.
• A nerve sending signals for motor function may result in weakness.

Myelin, axons and nerve cells are damaged in MS:

• Damage is done directly to the axons by the immune attack on the myelin.
• The axon may be unable to survive if enough myelin is lost.
• Loss of axons leaves fewer of them to carry the signal to the intended targets.
• It is unknown whether immune attacks with myelin damage are the only cause of loss of the nerve cell body.
• Nerve cells are eventually lost in MS.
• Symptoms reflect damage/loss of the nerve pathway (vision, motor, etc.).

Lost cells in a nerve pathway cause MS symptoms:

• An affected nerve pathway serves a specific function (vision, motor, etc.).
• Lost nerve cells decrease the number of nerve cells available to carry a signal.
• There is a reduced likelihood that signals in an injured nerve pathway will get through.