Which term describes a stimulus that does not reach the threshold to trigger an action potential?

Which term describes a stimulus that does not reach the threshold to trigger an action potential?

• A. Stimulus below threshold
• B. Sub-threshold
• C. Threshold
• D. Supra-threshold

Answer: (B)

Sub-threshold describes a stimulus that fails to reach the level of depolarization needed to trigger an action potential. The neuron must reach a specific threshold voltage to open enough voltage-gated Na+ channels and produce the all-or-none spike. When a stimulus is sub-threshold, it may cause some depolarization, but it does not reach that critical threshold, so no action potential fires. If the stimulus were strong enough to surpass threshold, it would be supra-threshold and would reliably trigger an action potential. The term threshold refers to the critical value itself, not the stimulus, so sub-threshold is the precise way to describe stimuli that don’t reach that point.

Sub-threshold describes a stimulus that fails to reach the level of depolarization needed to trigger an action potential. The neuron must reach a specific threshold voltage to open enough voltage-gated Na+ channels and produce the all-or-none spike. When a stimulus is sub-threshold, it may cause some depolarization, but it does not reach that critical threshold, so no action potential fires. If the stimulus were strong enough to surpass threshold, it would be supra-threshold and would reliably trigger an action potential. The term threshold refers to the critical value itself, not the stimulus, so sub-threshold is the precise way to describe stimuli that don’t reach that point.