What is the minimal point at which a stimulus causes an action potential to occur?

What is the minimal point at which a stimulus causes an action potential to occur?

• A. Stimulus
• B. Threshold
• C. Sub-threshold
• D. Response

Answer: (B)

The minimal point is the threshold—the membrane potential level that must be reached to trigger an action potential. When depolarization reaches this threshold, voltage-gated Na+ channels open, creating a rapid positive feedback surge of Na+ into the cell and the characteristic upstroke of the spike. If the depolarization never reaches threshold, the neuron does not fire and simply returns toward resting potential. Once threshold is crossed, the spike has a nearly fixed size due to the all-or-nothing nature of action potentials; stronger stimuli don’t make the peak higher, though they can increase firing rate by producing more spikes over time. In many neurons, threshold is around −55 to −50 mV, but the exact value varies.

The minimal point is the threshold—the membrane potential level that must be reached to trigger an action potential. When depolarization reaches this threshold, voltage-gated Na+ channels open, creating a rapid positive feedback surge of Na+ into the cell and the characteristic upstroke of the spike. If the depolarization never reaches threshold, the neuron does not fire and simply returns toward resting potential. Once threshold is crossed, the spike has a nearly fixed size due to the all-or-nothing nature of action potentials; stronger stimuli don’t make the peak higher, though they can increase firing rate by producing more spikes over time. In many neurons, threshold is around −55 to −50 mV, but the exact value varies.