Correct Answer: D. Loss of myelin sheath
Multiple sclerosis (MS) is a demyelinating disease where the immune system attacks oligodendrocytes and myelin sheaths in the central nervous system. The slowing of conduction velocity is fundamentally due to loss of myelin sheath. Myelin acts as an insulator around axons, enabling saltatory conduction—where action potentials "jump" between nodes of Ranvier at speeds of 100+ m/s. When myelin is destroyed, conduction becomes continuous and unmyelinated, reducing velocity to 0.5–2 m/s. This explains the clinical manifestations of MS: optic neuritis (demyelination of optic nerve), internuclear ophthalmoplegia (demyelination of medial longitudinal fasciculus), and progressive weakness. The demyelinated axons may eventually undergo secondary degeneration, but the primary pathophysiological mechanism causing slowed conduction is the loss of the myelin insulator itself. In Indian MS cohorts, demyelinating plaques are commonly seen on MRI brain/spine, confirming this pathology. The conduction block occurs because depolarization must now spread along the entire axon membrane rather than jumping between nodes.
Why the other options are wrong
A. Defect at node of Ranvier — While nodes of Ranvier are critical for saltatory conduction, MS does not primarily affect the node structure itself. The nodes remain intact; the problem is the loss of insulation (myelin) between them. A true nodal defect would cause conduction block at specific points, not the generalized slowing seen in demyelination. This is a distractor that confuses the anatomy of conduction with the pathology of MS. B. Gliosis — Gliosis (proliferation of astrocytes and microglia) is a secondary response to demyelination and axonal injury in MS, not the primary cause of conduction slowing. While gliosis contributes to scar formation and chronic inflammation, it does not directly impair action potential propagation. This option represents a consequence rather than the mechanism of slowed conduction. C. Leaky Na+ channel — Sodium channels are not inherently defective in MS. Although demyelinated axons may have altered sodium channel distribution (increased along the entire axon rather than concentrated at nodes), this is a consequence of demyelination, not its cause. The primary problem is loss of myelin insulation, not channel dysfunction. This represents confusion between secondary compensatory mechanisms and the primary pathology.
High-Yield Facts
- Saltatory conduction requires intact myelin; loss of myelin reduces conduction velocity from >100 m/s to <2 m/s.
- Demyelinating plaques in MS are the pathological hallmark—oligodendrocyte destruction leads to myelin loss in CNS.
- Conduction block in demyelinated segments occurs because action potentials must depolarize the entire axon membrane sequentially.
- Remyelination (by oligodendrocyte precursor cells) can partially restore conduction velocity in early MS, explaining relapsing-remitting course.
- Optic neuritis and internuclear ophthalmoplegia are classic demyelinating syndromes in Indian MS cohorts, confirming CNS myelin loss.
Mnemonics
MYELIN = Conduction Speed Myelin = Multiple m/s (fast); No myelin = No speed (slow). Loss of myelin sheath → loss of saltatory conduction → slowed velocity. Saltatory = Jumping (needs insulation) Saltatory conduction 'jumps' between nodes only when myelin insulates the internodal segment. Demyelination forces continuous conduction → slow.
NBE Trap
NBE pairs "node of Ranvier" with MS to trap students who memorize that nodes are important for conduction without understanding that MS destroys the myelin between nodes, not the nodes themselves. The trap is anatomical confusion: nodes are intact, but the insulation around them is gone.
Clinical Pearl
In Indian MS patients presenting with optic neuritis or progressive weakness, delayed visual evoked potentials (VEPs) and prolonged latencies on electrophysiology directly reflect demyelination-induced conduction slowing—a bedside correlate of myelin loss that guides diagnosis and prognosis.
_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 28 (Nervous System); Harrison's Principles of Internal Medicine, Ch. 380 (Multiple Sclerosis)_