- The explanations or increased conduction velocity of action potentials in a myelinated nerve fibre (MNF) as compared to that of a non-myelinated fibre (NNF) of same diameter are suggested below:
A. Much higher number of ions traverse a unit length of an MNF membrane compared to that of an NNF during conduction of action potential as the ionic currents are restricted to the membrane at the Nodes of Ranvier
B. The energy expenditure required to maintain ionic gradient after conduction of action potential in an MNF is higher than that of an NNF
C. The action potentials of MNF do not have a hyperpolarizing effect like that of an NNF
D. The relative refractory period is not extended in an MNF like that of an NNF
E. The axolemma at the nodes of Ranvier lacks K+ ion channels
F. MNFs are metabolically less efficient than NNFs
Choose all correct statements from the following options:
(1) A, B and C (2) B, C and D
(3) C, D and E (4) D, E and F
Introduction
Myelinated nerve fibers conduct action potentials much faster than unmyelinated fibers of the same diameter due to a range of structural and functional features. These include insulation by the myelin sheath, ion channel localization, and energy efficiency. This article evaluates various proposed explanations for this difference to identify the correct physiological mechanisms behind increased conduction velocity.
Analysis of Proposed Statements
A. Much higher number of ions traverse a unit length of an MNF membrane compared to that of an NNF during conduction of action potential as the ionic currents are restricted to the membrane at the Nodes of Ranvier
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Correct.
In myelinated fibers, ionic currents during an action potential occur mainly at the Nodes of Ranvier, where the membrane is exposed and sodium channels are concentrated. This restriction leads to higher localized ion flow when compared to continuous conduction in unmyelinated fibers.alevelbiology
B. The energy expenditure required to maintain ionic gradient after conduction of action potential in an MNF is higher than that of an NNF
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Incorrect.
Myelinated fibers are more energy-efficient because ion exchange is limited to the nodes, reducing the total number of ions crossing the membrane and thus reducing ATP consumption by the Na⁺/K⁺ pump.alevelbiology
C. The action potentials of MNF do not have a hyperpolarizing effect like that of an NNF
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Incorrect.
Both myelinated and unmyelinated fibers exhibit after-hyperpolarization phases, though the characteristics may vary.
D. The relative refractory period is not extended in an MNF like that of an NNF
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Correct.
The relative refractory period tends to be shorter in myelinated fibers, contributing to their higher firing rates and faster conduction.alevelbiology
E. The axolemma at the nodes of Ranvier lacks K+ ion channels
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Incorrect.
The nodes of Ranvier contain both sodium and potassium channels, essential for rapid depolarization and repolarization, respectively.alevelbiology
F. MNFs are metabolically less efficient than NNFs
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Incorrect.
Myelinated nerve fibers are more metabolically efficient due to reduced ion exchange.
Summary Table of Correctness
Statement Correctness A Correct B Incorrect (myelinated fibers are more energy efficient) C Incorrect (hyperpolarization occurs in both) D Correct E Incorrect (K⁺ channels present at nodes) F Incorrect (myelinated fibers are more efficient)
Correct Option Selection
The option containing all the correct statements is:
(4) D, E and F
However, since E and F are both incorrect, the correct statements are only A and D.
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