- A nerve fibre cannot be stimulated during the absolute refractory period of a previous stimulation because
(1) Sodium permeability remains high.
(2) Sodium-potassium pump does not operate.
(3) Voltage-gated calcium channels remain closed
(4) Potassium conductance remains low.
Introduction
The absolute refractory period is a fundamental characteristic of nerve fibers during which no new action potential can be initiated regardless of stimulus strength. This period ensures the unidirectional propagation of nerve impulses and proper intervals between action potentials. Understanding the ionic mechanism behind this refractory state explains why stimulation during this phase is ineffective.
Mechanism Behind the Absolute Refractory Period
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During an action potential, voltage-gated sodium (Na⁺) channels open, causing a rapid influx of sodium ions and depolarization of the membrane.wikipedia+1
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These sodium channels then enter an inactivated state shortly after opening. In this state, they cannot reopen immediately, regardless of membrane potential changes.pubmed.ncbi.nlm.nih+1
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The absolute refractory period spans from the initiation of the action potential through most of the repolarization phase until these sodium channels recover from inactivation to a closed but activatable state.
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During this time, despite any external stimulation, the neuron cannot generate a new action potential because these key sodium channels are non-functional.
Explanation of Options
Option Explanation Correctness (1) Sodium permeability remains high Sodium channels are inactivated, not persistently open Incorrect (2) Sodium-potassium pump does not operate Pump operates continuously but slower than action potential Incorrect (3) Voltage-gated calcium channels remain closed Calcium channels not primarily responsible for action potentials Incorrect (4) Potassium conductance remains low Potassium conductance increases during repolarization phase Incorrect The primary reason for the absolute refractory period is the inactivation of voltage-gated sodium channels, not any of the options listed verbatim. The closest corresponding concept is that sodium permeability is effectively non-functioning due to channel inactivation, not high permeability.
Correct Conceptual Understanding
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During the absolute refractory period, voltage-gated sodium channels are inactivated and cannot be reopened immediately, preventing the initiation of another action potential.
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This inactivation state is different from the closed but activatable resting state.
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Potassium channels open later in the action potential, affecting repolarization, but they do not cause the absolute refractory period.
Summary Table
Stage Ion Channel Status Ability to Fire Action Potential Depolarization (AP firing) Sodium channels open Yes Absolute refractory period Sodium channels inactivated No Relative refractory period Some sodium channels recovered; potassium conductance high Possible with strong stimulus Resting state Sodium channels closed but activatable Yes
Conclusion
A nerve fiber cannot be stimulated during the absolute refractory period because most voltage-gated sodium channels remain in an inactivated state and cannot open, preventing new action potential generation, irrespective of stimulus strength.
None of the given options exactly describe this mechanism, but the best understanding relates to sodium channel inactivation, which is not synonymous with sodium permeability remaining high.
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