Which statement correctly distinguishes tonic and phasic receptors?

• A. Phasic receptors respond only when the stimulus changes
• B. Phasic receptors maintain signaling for the duration of the stimulus
• C. Both types respond equally to all stimuli
• D. Phasic receptors respond continuously to ongoing stimuli

Answer: (A)

Phasic receptors are rapidly adapting; they fire most strongly at the moment a stimulus starts or changes and then their activity declines even if the stimulus continues. This makes them excellent at signaling changes in the environment (onset, offset, or a change in intensity) rather than the constant presence of a stimulus. In contrast, tonic receptors respond more steadily with little adaptation, providing ongoing information about the duration and intensity of a stimulus. So the statement that phasic receptors respond only when the stimulus changes captures this key behavior.

For context, a classic example of a phasic receptor is the Pacinian corpuscle, which roars to life when vibration begins and then quiets down despite the vibration continuing. A tonic example would be Merkel discs, which keep signaling as long as there is sustained pressure. The other options describe continuous signaling or equal responses in a way that doesn’t fit the rapid adaptation pattern of phasic receptors.