When voltage-gated sodium channels open due to the depolarization of a neuron or axon, which of the statements below best describes why sodium ions enter the neuron?

Question

BlackTom AI · Accepted Answer

Opening of voltage-gated Na+ channels produces Na+ influx because of the electrochemical gradient: the combined effect of the concentration difference and the negative membrane potential inside the neuron.

1) Relevant concepts
- Concentration gradient: Na+ is much more concentrated outside (≈145 mM) than inside (≈12 mM), so diffusion tends to move Na+ inward.
- Electrical gradient (membrane potential): the inside of a resting neuron (~–70 mV) is negative relative to outside, which attracts positively charged Na+ ions.
- Electrochemical driving force: net force on an ion equals the difference between the membrane potential (Vm) and the ion’s equilibrium (Nernst) potential (Eion). A negative value for Vm–Eion for a cation indicates inward driving force.

2) Step-by-step reasoning
- Depolarization opens voltage-gated Na+ channels; whether Na+ actually moves depends on the electrochemical gradient.
- Compute the Na+ equilibrium potential with the Nernst equation (37°C): ENa ≈ 61.5 mV × log10([Na+]out/[Na+]in).
- Using typical values: ENa ≈ 61.5 × log10(145/12) ≈ 61.5 × 1.082 ≈ +66.6 mV.
- Driving force = Vm – ENa = (–70 mV) – (+66.6 mV) = –136.6 mV. The large negative number indicates a strong inward electrochemical driving force on positive Na+ ions.

3) Why the correct answer is correct
- The inward movement of Na+ is not explained by concentration alone or by a simple inside-negative statement; it is the sum of both chemical (concentration) and electrical (voltage) gradients that determines net flux. The provided correct option explicitly states this combination.

4) Why the other choices are incorrect
- “There are more negatively charged ions inside than outside”: This is vague and incorrect as a direct cause of Na+ entry; it ignores the explicit concentration and voltage gradients relevant to Na+ movement.
- “There is more sodium outside the neurons than inside (a concentration gradient)”: True but incomplete—ignores the electrical attraction due to inside negativity.
- “The balance of sodium and chloride inside the cell is greater than that ratio outside the neuron.”: Irrelevant and misleading; Na+ movement is governed by its own electrochemical gradient, not a Na+/Cl– ratio.

Conclusion: Sodium enters because of the combination of the concentration gradient and the relatively negative potential inside the neuron, confirming the provided correct answer.

When voltage-gated sodium channels open due to the depolarization of a neuron or axon, which of the statements below best describes why sodium ions enter the neuron?Single choice

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