Question

In multiple sclerosis, the slow conduction of motor and sensory pathways is due to?

• A. Defect in the node of Ranvier
• B. Loss of myelin sheath
• C. Leaking of sodium channels
• D. Leaking of calcium channels

Hint:

What insulating layer must be intact for fast saltatory conduction?

Answer 1

The Correct Option is B

Step 1: Recall the basic lesion. Multiple sclerosis is an immune-mediated demyelinating disease of the central nervous system. The hallmark is destruction of the myelin sheath around CNS axons, with later scar (sclerotic plaque) formation.
Step 2: Link demyelination to slow conduction. Myelin allows fast saltatory conduction, where the impulse jumps from one node of Ranvier to the next. Losing myelin disrupts saltatory conduction, so the impulse spreads slowly and continuously instead of jumping, giving slowed conduction velocity and conduction block. So the answer is loss of the myelin sheath, option (b).
Step 3: Why not the others. The nodes of Ranvier are not the primary structural target, and channel leaks are downstream consequences rather than the core defect that explains the slow conduction in MS.
Ref: Standard CNS physiology of demyelination.