Step 1: Normal resting potential.
The neuron's resting membrane potential is approximately -70 mV, maintained by sodium-potassium pumps and ion channels. This signifies that the cell's interior is more negative than its exterior.
Step 2: Events during depolarization.
Upon reaching threshold, voltage-gated sodium channels open. Sodium ions rapidly enter the cell, increasing the membrane potential. The cell's interior becomes less negative, eventually reaching zero and then turning positive.
Step 3: Analysis of options.
- (A) Hyperpolarizing phase: This phase follows repolarization, where the membrane potential becomes more negative than the resting potential. Incorrect.
- (B) Repolarizing phase: During this phase, potassium ions exit the cell, restoring the negative potential. Incorrect.
- (C) Depolarizing phase: Correct. This is the phase where the negative membrane potential becomes less negative, reaches zero, and turns positive.
- (D) Action potential: This encompasses depolarization, repolarization, and hyperpolarization. The question specifically asks about the depolarization phase.
Step 4: Clinical correlation.
Depolarization is crucial for nerve signal transmission and muscle contraction. Abnormal depolarization can lead to arrhythmias, seizures, or muscle weakness.
Step 5: Conclusion.
Consequently, the depolarizing phase is the correct answer.