During the "Depolarization" phase of an action potential in a neuron, which ion movement occurs?

RML Lucknow - 2021

Applied Physiology

Easy

Appeared in: RML Lucknow - 2021

Explanation

Depolarization is the initial, rising phase of the action potential, where the neuron's internal charge rapidly becomes positive.
This phase is initiated when a stimulus causes voltage-gated sodium (Na+) channels in the neuron's membrane to open.
Due to a high concentration of Na+ outside the cell, these ions rush into the cell, down their electrochemical gradient.
The massive influx of positive sodium ions causes the membrane potential to shoot up from a resting level of about -70 mV to about +30 mV.

Option A: The outflow of potassium (K+) ions is the primary driver of repolarization, not depolarization. It makes the inside of the cell negative again after the action potential peaks.
Option C: Calcium (Ca2+) movement is not the primary event during the depolarization of the axon; its main role is in neurotransmitter release at the synapse. Furthermore, it typically flows into the cell, not out.
Option D: The influx of negatively charged chloride (Cl-) ions makes the neuron's interior more negative, a process called hyperpolarization. This is an inhibitory process.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Physiology of neuronal action potential and ion channel dynamics as background academic context rather than a clinical decision trigger.
Understanding ion channel function is fundamental to pharmacology. Local anesthetics (e.g., Lidocaine) work by blocking voltage-gated sodium channels, which prevents depolarization and stops nerve impulses that carry pain signals.
Electrolyte imbalances have significant clinical effects. For example, hyperkalemia (high blood potassium) raises the resting membrane potential closer to the threshold, making cells hyperexcitable and increasing the risk of cardiac arrhythmias.
What if? If a patient took a drug that blocked potassium channels, the repolarization phase would be prolonged. This would increase the duration of the action potential, a mechanism used by certain antiarrhythmic medications to treat heart rhythm disorders.

How to Approach the Question

First, identify the key concept in the question: the 'Depolarization' phase of a neuronal 'action potential'.
Recall the definition of depolarization: the process where the inside of the cell becomes less negative and rapidly positive.
To make the cell's interior positive, either a positive ion must enter or a negative ion must leave.
Evaluate the options based on the standard model of an action potential:
Sodium (Na+) is a positive ion concentrated outside the cell. Its influx would cause depolarization.
Potassium (K+) is a positive ion concentrated inside the cell. Its efflux (flowing out) would cause repolarization (making it more negative).

Concept Tested & Keywords

Concept Tested: Physiology of neuronal action potential and ion channel dynamics.
Stem keywords: Depolarization, action potential, neuron, ion movement
Lead-in keywords: which ion movement occurs
Negative lead-in flag: false

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