Skip to content
Explore atlas
PH-NE03

Multiple sclerosis

How an immune attack on central myelin slows or blocks nerve conduction, why plaques favour the optic nerves, periventricular white matter and spinal cord, and why the disease can relapse or progress.

By the end, you can

  • Explain how myelin and nodes of Ranvier allow fast saltatory conduction.
  • Describe the immune-mediated injury to myelin and oligodendrocytes in the central nervous system.
  • Explain why a demyelinated axon conducts slowly or not at all, and why symptoms can worsen with heat.
  • Relate typical plaque locations to symptoms such as optic neuritis, double vision and sensory or motor loss.
  • Distinguish relapsing and progressive courses in terms of inflammation and axonal loss.
  • Recognise the classic plaque appearances, disease courses, diagnostic criteria and tests of MS, and explain how relapse, disease-modifying and symptom treatments act on its mechanism.

What it is

Multiple sclerosis is a chronic immune-mediated disease of the central nervous system in which patches of inflammation strip myelin from axons, forming plaques of demyelination scattered in place and time, with variable axonal loss. It does not affect peripheral nerves, whose myelin is made by a different cell.

3 types to tell apart
Relapsing-remitting
The commonest pattern at onset: episodes of new or worse symptoms (relapses) from new inflammatory plaques, followed by partial or full recovery.
Secondary progressive
After a relapsing phase, disability gradually increases independent of relapses, as axonal loss and neurodegeneration accumulate.
Primary progressive
Gradual worsening from the start without distinct relapses, often as a slowly progressive spinal cord syndrome.

US spelling: fibres → fibers.

Normal function

Myelin and saltatory conduction

An action potential travels along an as sodium channels open in sequence. In a bare axon each patch of membrane must be depolarised in turn, which is slow.

A is many layers of cell membrane wrapped round the axon. It insulates the internode, reducing the leak of current and the charge needed to depolarise it. Sodium channels are concentrated at the short bare gaps between segments, the , so the impulse jumps from node to node (saltatory conduction). Large myelinated fibres conduct at up to about 100 m/s, many times faster than unmyelinated fibres of similar size.

In peripheral nerves, as in this model, Schwann cells make the myelin. In the brain, optic nerves and spinal cord it is made by oligodendrocytes, each wrapping segments of many axons. This difference is why multiple sclerosis affects only the central nervous system.

Point to it on the model