During the absolute refractory period of a nerve action potential, a second stimulus of any strength cannot generate another action potential. This is primarily because:
- A Voltage-gated Na+ channels are in the inactivated (closed) state ✓
- B The Na+/K+ ATPase pump is maximally active
- C The membrane is hyperpolarized beyond the threshold
- D Voltage-gated K+ channels are closed
Correct answer: A. Voltage-gated Na+ channels are in the inactivated (closed) state
Explanation
Following depolarization, voltage-gated sodium channels transition from the open state to the inactivated state, in which the inactivation gate (h gate) occludes the channel regardless of membrane voltage. During this absolute refractory period (approximately 1 ms in large myelinated fibers), no stimulus can reopen these channels. The channels must first recover to the resting closed state before another action potential is possible, which occurs progressively during repolarization (relative refractory period).
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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