- The conduction velocity of action potential in a myelinated nerve fibre was much greater than that of an unmyelinated fibre of the same diameter. The following statements were proposed to explain this observation:
A. The speed of conduction in a nerve fibre is determined by the plasma membrane resistance and axial resistance of axonal cytoplasm.
B. The electrical properties of myelinated and unmyelinated nerve fibres are not similar.
C. The myelin sheath decreases the effective membrane resistance.
D. The magnitude of an electrotonic potential decreases more with distance along the axon in myelinated nerve fibres than that of unmyelinated fibres.
E. The voltage-gated Na+ channels are highly concentrated at the nodes of Ranvier.
Choose one of the following combinations with both INCORRECT statements.
(1) A and B (2) B and C
(3) C and D (4) D and E
Introduction
Myelinated nerve fibers exhibit significantly higher conduction velocities than unmyelinated fibers of the same diameter. This critical feature of nervous system design enables rapid, efficient communication between neurons and their targets. Several factors, from membrane properties to channel distribution, contribute to this phenomenon. This article clarifies which of the proposed statements about conduction velocity in myelinated vs. unmyelinated fibers are incorrect and explains the underlying mechanisms.
Key Concepts in Nerve Conduction Velocity
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Membrane resistance and axial resistance:
The speed of conduction depends heavily on the membrane resistance (R_m) and axial resistance (R_a) of the axon cytoplasm. Increased membrane resistance slows leak currents, promoting rapid passive current spread.pubmed.ncbi.nlm.nih -
Myelin’s electrical properties:
Myelin acts as an insulator, dramatically increasing the membrane resistance and decreasing capacitance. This prevents charge dissipation and permits faster saltatory conduction between nodes of Ranvier.nature+1 -
Voltage-gated Na⁺ channels concentration:
In myelinated fibers, voltage-gated sodium channels are highly concentrated at the nodes of Ranvier, enabling rapid regeneration of the action potential.sciencedirect+1 -
Electrotonic potentials:
Electrotonic (passive) potentials decay less over distance in myelinated fibers due to insulation from myelin, unlike unmyelinated fibers where these potentials drop more sharply.sciencedirect
Analysis of the Proposed Statements
Statement Explanation Correct/Incorrect A Speed depends on plasma membrane and axial resistance Correct B Electrical properties differ between myelinated and unmyelinated fibers Correct C Myelin sheath decreases the effective membrane resistance Incorrect: myelin increases membrane resistance by insulating the membrane D Electrotonic potentials decrease more rapidly in myelinated fibers Incorrect: Electrotonic potentials decay less in myelinated fibers due to insulation E Voltage-gated Na⁺ channels concentrated at nodes of Ranvier Correct
Identification of Incorrect Statements
The incorrect statements pairing are:
(3) C and D
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Statement C wrongly claims myelin decreases effective membrane resistance; in reality, it increases resistance, reducing leak currents.
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Statement D incorrectly states that electrotonic potential decreases more in myelinated fibers, but insulation actually preserves signal strength over distance.
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1 Comment
Bhawna Choudhary
September 25, 2025C and D option