- The membrane potential in a giant squid axon recorded intracellularly at the resting
condition (-70 mV) was reversed at the peak of action potential (+35 mV) after stimulation of the nerve fibre with a threshold electrical stimulus. This overshoot of the membrane potential has been explained in the following proposed statements:
A. The rapid increase in Na+-conductance during early phase of action potential uses membrane potential to move toward the equilibrium potential of Na+ (+45m V).
B. The Na+-conductance quickly decreases toward resting level after peak in the early phase and Na+-ions are not able to attain its equilibrium potential within this short time.
C. The conductance of K+ at the early phase of action potential is increased and that leads to the reversal of membrane potential.
D. The increase of K+-conductance due to stimulation of nerve occurs before the changes of Na+-conductance is initiated and thus causes overshoot at the peak of action potential.
Which one of the following is correct?
(1) A only (2) A and B
(3) C only (4) C and D
Introduction
The generation and propagation of action potentials are crucial for neuronal communication. The giant squid axon, due to its large size, has historically served as an important model for studying the biophysical properties of action potentials. One hallmark of the squid axon’s action potential is the characteristic overshoot phase, where the membrane potential reverses from resting (-70 mV) to positive values (+35 mV) after stimulation. Researchers have proposed several explanations for this phenomenon focusing on changes in ionic conductances. This article elaborates on these key mechanisms.
Overview of the Action Potential Phases
The typical action potential involves several phases:
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Depolarization: Rapid opening of voltage-gated sodium channels causing Na⁺ influx
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Overshoot: Membrane potential reverses and becomes positive briefly
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Repolarization: Opening of potassium channels allowing K⁺ efflux, restoring negativity
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Hyperpolarization (Undershoot): Membrane potential briefly becomes more negative than resting
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Resting Membrane Potential Restoration
Proposed Explanations for the Overshoot (Statements Analysis)
A. The rapid increase in Na⁺-conductance during early phase of action potential uses membrane potential to move toward the equilibrium potential of Na⁺ (+45 mV).
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Correct.
During the initial phase, voltage-gated Na⁺ channels open quickly, increasing Na⁺ conductance drastically. The membrane potential moves rapidly toward the equilibrium potential of Na⁺ (approx +45 mV), causing the overshoot phase.wikipedia+1
B. The Na⁺-conductance quickly decreases toward resting level after peak in the early phase and Na⁺ ions are not able to attain its equilibrium potential within this short time.
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Correct.
Sodium channels inactivate quickly after opening, reducing Na⁺ conductance as the membrane potential peaks. This causes the membrane potential to begin repolarization before reaching the exact Na⁺ equilibrium potential, explaining why the overshoot usually peaks slightly below +45 mV.nichd.nih+1
C. The conductance of K⁺ at the early phase of action potential is increased and that leads to the reversal of membrane potential.
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Incorrect.
Potassium conductance increases later during the repolarization phase, not at the early depolarization or overshoot phase.
D. The increase of K⁺-conductance due to stimulation of nerve occurs before the changes of Na⁺-conductance is initiated and thus causes overshoot at the peak of action potential.
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Incorrect.
Potassium channels open after the Na⁺ channels; they do not cause, but help terminate the overshoot by repolarizing the membrane.
Mechanistic Summary
Statement Calcium Conductance Increase Timing and Role in Overshoot Correct/Incorrect A Na⁺ conductance increases rapidly Na⁺ influx drives membrane potential toward +45 mV Correct B Na⁺ conductance quickly decreases Na⁺ channels inactivate, preventing membrane from reaching full equilibrium potential Correct C K⁺ conductance increases K⁺ conductance increases only after overshoot Incorrect D K⁺ conductance precedes Na⁺ conductance False; K⁺ conductance is delayed Incorrect
Conclusion
The correct explanation of the overshoot phase in the squid giant axon action potential is:
(2) A and B
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