Which statement correctly identifies the threshold for triggering an action potential?

Which statement correctly identifies the threshold for triggering an action potential?

• A. -20 mV
• B. -30 mV
• C. -50 mV
• D. -40 mV

Answer: (D)

The threshold is the depolarization level at which voltage-gated Na+ channels open enough to create a positive feedback loop that drives the neuron from a resting state into an all-or-nothing action potential. When the membrane potential reaches about -40 mV, the inward Na+ current becomes large enough to overcome the outward leak and K+ currents, causing rapid, self-sustaining depolarization. That point marks the switch from a graded response to an action potential. So -40 mV is the triggering threshold because it represents the critical depolarization where the regenerative Na+ influx dominates, initiating the spike. Potentials around -50 mV are still too hyperpolarized to reliably start the spike, while -30 mV or -20 mV are more depolarized than the threshold and typically occur during the rising phase of the spike, not at its initiation.

The threshold is the depolarization level at which voltage-gated Na+ channels open enough to create a positive feedback loop that drives the neuron from a resting state into an all-or-nothing action potential. When the membrane potential reaches about -40 mV, the inward Na+ current becomes large enough to overcome the outward leak and K+ currents, causing rapid, self-sustaining depolarization. That point marks the switch from a graded response to an action potential.

So -40 mV is the triggering threshold because it represents the critical depolarization where the regenerative Na+ influx dominates, initiating the spike. Potentials around -50 mV are still too hyperpolarized to reliably start the spike, while -30 mV or -20 mV are more depolarized than the threshold and typically occur during the rising phase of the spike, not at its initiation.