Which of the following statements about the response of neurones to stimulus is correct?

Step 1: Analyze the question The question asks which statement correctly describes how neurones respond to stimuli. This is about nerve impulse transmission, specifically the threshold and the refractory period.

Step 2: Key concepts

• Threshold value: A neurone fires only when the stimulus is strong enough to depolarise the membrane to a critical level (about -55 mV, from a resting potential of about -70 mV). Below this, nothing happens.
• All-or-none law: Once the threshold is reached, the neurone fires a full impulse. A stronger stimulus does not make a bigger impulse. It only makes the impulses more frequent.
• Refractory period: Right after firing, the neurone cannot respond for a short time. During the absolute refractory period, no stimulus, however strong, can trigger another impulse. This is because the sodium channels are inactivated.

Step 3: Evaluate each option

• A. Neurons respond to stimulus of any strength. Wrong. Sub-threshold stimuli produce no impulse.
• B. Neurons respond to all stimuli applied in rapid succession. Wrong. Stimuli that arrive during the refractory period get no response.
• C. Neurons respond more rapidly during the absolute refractory period. Wrong. During this period the neurone does not respond at all.
• D. Intensity of stimulus must reach a threshold value before the neurones can be excited. Correct. This is the threshold principle behind the all-or-none law.

Step 4: Answer

Exam tip: Remember the phrase "Threshold, All-or-none, Refractory": "TAR the road, no impulse until the threshold is reached." In any question on nerve response, wrong options usually claim that the neurone responds to "any" or "all" stimuli, and these words are red flags.