Questions: Action Potential Phases

Question 1

During repolarization of an action potential, what two events work together to return the membrane potential toward resting?

• A. Voltage-gated Na+ channels reopen and K+ channels close
• B. Voltage-gated Na+ channels inactivate and voltage-gated K+ channels open
• C. The Na+/K+ ATPase pump rapidly restores both ion gradients
• D. Ca2+ channels open while Na+ channels reset to the closed state

Answer: (B)

Repolarization is a two-part process: voltage-gated Na+ channels transition from their open state to an inactivated (blocked) state, stopping Na+ influx; simultaneously, voltage-gated K+ channels (which activate more slowly) open and K+ flows out down its electrochemical gradient. This K+ efflux drives the membrane back toward the K+ equilibrium potential (~-80 mV), causing afterhyperpolarization before slow K+ channel closure returns the membrane to resting potential.

Question 2

During the absolute refractory period immediately following an action potential, a second stimulus of twice the threshold strength can trigger another action potential in the same axon segment.

• T. True
• F. False

Answer: False

The absolute refractory period exists because Na+ channels are in their inactivated state — a conformation distinct from the closed resting state — and cannot be reopened by any stimulus regardless of strength. Only after the inactivation gate resets (during the relative refractory period) can a suprathreshold stimulus generate a new action potential. This property ensures unidirectional propagation and sets a ceiling on firing frequency.

Question 3

Explain why the action potential is described as 'all-or-none' and what determines the threshold that triggers it.

Model answer: Once membrane depolarization reaches threshold (~-55 mV), a positive feedback loop is triggered: Na+ channels open, Na+ influx further depolarizes the membrane, opening more Na+ channels. This regenerative process runs to completion regardless of the initial stimulus strength — the resulting action potential is always the same amplitude. Threshold is set by the density and kinetics of voltage-gated Na+ channels; a stimulus must depolarize enough membrane to open enough Na+ channels to initiate this self-sustaining cascade.

The all-or-none property is a direct consequence of the positive feedback between membrane depolarization and Na+ channel opening. Below threshold, K+ leak and Na+ channel closure can restore resting potential; at threshold, the inward Na+ current overwhelms the restoring forces and the spike propagates to full amplitude. This property is what makes neural signals reliable over long distances — the signal regenerates at each node rather than decrementing like a passive cable signal.