- The action potential was recorded intracellularly from a squid giant axon bathed in two types of fluid such as sea water and artificial sea water having lower concentration of sodium ions while maintaining the same osmotic pressure with choline chloride. The nature of action potential was different in the two bathing fluids. Which of the following results is most likely?
(1) The resting transmembrane potential was not changed but the amplitude of action potential was increased with lower sodium concentration in the bathing fluid.
(2) The amplitude of action potential was gradually decreased with reduction of sodium
concentration in bathing fluid but the duration of action potential was prolonged.
(3) The resting transmembrane potential was decreased and the amplitude of action potential
was also decreased with lower sodium concentration in the bathing fluid.
(4) The amplitude of action potential was not changed with reduction of sodium concentration in the bathing fluid but the duration of action potential was prolonged.
Introduction
The squid giant axon is a classic model system for studying the physiology of nerve impulse generation due to its large size and accessibility. One fundamental aspect of action potential generation involves the external sodium (Na⁺) concentration. Experimentally altering the Na⁺ concentration in the bathing fluid while maintaining osmotic pressure (e.g., substituting NaCl with choline chloride) changes the properties of the action potential. This article discusses the most likely effects when the external sodium concentration is decreased.
Sodium Concentration’s Role in Action Potential Amplitude and Duration
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The sodium equilibrium potential (E_Na) is governed by the Nernst equation, depending on the ratio of external to internal sodium ion concentrations.
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Reducing external sodium concentration lowers the driving force for Na⁺ influx during the depolarization phase of the action potential.
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Consequently, the amplitude of the action potential decreases because the peak membrane potential is closer to the diminished sodium equilibrium potential.rupress+1
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Lower external Na⁺ also affects the kinetics of the channels, potentially altering the duration of the action potential. Often, the duration is prolonged because the repolarization mechanisms remain intact and the reduced inward sodium current slows down depolarization.pubmed.ncbi.nlm.nih
Experimental Observations from Squid Giant Axon Studies
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Studies replacing seawater with artificial sea water of lower Na⁺ concentration see a gradual reduction in action potential amplitude.
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These experiments maintain osmotic pressure using choline chloride or sucrose, isolating the effects of sodium concentration changes.pmc.ncbi.nlm.nih
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Additionally, the duration of the action potential typically increases, attributed to slower depolarization and altered ionic current dynamics.
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Despite these changes, the resting membrane potential often remains unchanged because it is primarily influenced by potassium gradients and membrane permeability.rupress
Analysis of the Provided Options
Option Description Likelihood 1 Resting potential unchanged but amplitude increased with lower sodium Unlikely 2 Amplitude decreased gradually and duration prolonged with reduced sodium Most likely correct 3 Resting potential decreased and amplitude decreased Less likely (resting potential relatively stable) 4 Amplitude unchanged but duration prolonged with reduced sodium Unlikely
Scientific Explanation
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The amplitude of the action potential depends on the electrochemical gradient for sodium ions which is diminished by reducing external sodium concentration.
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As sodium conductance begins to increase during depolarization, less external sodium means less influx, resulting in a smaller peak membrane potential, decreasing amplitude.
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Prolongation of the action potential occurs as the balance between inward sodium and outward potassium currents shifts, resulting in slower repolarization kinetics.
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The resting membrane potential, governed mainly by potassium permeability and gradients, is generally unaffected by changes in external sodium concentrations.sciencedirect+1
Conclusion
The most accurate description of the effect of reducing sodium concentration in the bathing fluid of squid giant axons on action potentials is:
(2) The amplitude of action potential was gradually decreased with reduction of sodium concentration in bathing fluid but the duration of action potential was prolonged.
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