Q.50 The propagation of action potential in myelinated nerve fiber is faster because of
1. Continuous conduction
2. Saltatory conduction
3. Spontaneous conduction
4. Intermittent conduction
Myelinated nerve fibers propagate action potentials faster through saltatory conduction, where the impulse jumps between nodes of Ranvier. This mechanism dramatically increases speed compared to continuous conduction in unmyelinated fibers.
Action Potential Basics
Action potentials travel along axons via local current flow depolarizing adjacent membrane.
Myelin sheaths (from Schwann cells in PNS, oligodendrocytes in CNS) insulate internodes, exposing nodes of Ranvier rich in voltage-gated Na⁺ channels.
This setup enables “jumping” conduction, boosting velocity 10-100x (up to 150 m/s).
Correct Answer: 2. Saltatory Conduction
Saltatory conduction (“leaping”) occurs as depolarization passively spreads under myelin to the next node, where Na⁺ influx regenerates the full spike.
Myelin lowers capacitance and raises resistance, extending current reach while conserving energy (fewer ion pumps needed).
Unmyelinated fibers use slower continuous regeneration every micrometer.
Option Breakdown
| Option | Explanation | Correct? |
|---|---|---|
| 1. Continuous conduction | Seen in unmyelinated axons; slow (~1 m/s) as AP regenerates along entire membrane. | No |
| 2. Saltatory conduction | “Jumps” node-to-node via insulated internodes; fastest (~150 m/s). | Yes |
| 3. Spontaneous conduction | Not a recognized term; implies untriggered firing (e.g., ectopic beats), not propagation speed. | No |
| 4. Intermittent conduction | Not standard; conduction block or decremental applies to pathology, not normal myelinated speed. | No |
Mechanism Advantages
-
Speed: Current travels farther passively; regenerates only at nodes.
-
Efficiency: Less Na⁺/K⁺ ATPase activity.
-
Pathology link: Demyelination (MS, Guillain-Barré) reverts to continuous/slow/blocked conduction.
