Phasic receptors respond chiefly to what aspect of stimuli?

• A. Changes in stimuli (onset and offset)
• B. Constant stimulus intensity
• C. Temperature only
• D. Chemical changes over time

Answer: (A)

Phasic receptors are fast-adapting sensors that signal changes in the environment rather than steady conditions. They fire strongly at the moment a stimulus starts and again when it ends, but their activity drops during a constant stimulus. This makes them best suited to detect when something changes—the onset and offset of a stimulus or a shift in its intensity—while ignoring unchanging background input. For example, Pacinian and Meissner’s corpuscles in the skin respond to rapid changes like vibration or light touch and quickly adapt. By contrast, tonic receptors maintain a steady response as long as the stimulus is present, providing information about ongoing conditions. So the idea that phasic receptors chiefly respond to changes in stimuli aligns with how they detect new or evolving sensory events, not a constant level.