What are tonic receptors?

• A. Slowly adapting receptors that respond for the duration of a stimulus
• B. Rapidly adapting receptors that respond only at stimulus onset
• C. Receptors that do not adapt
• D. Receptors that respond to chemical signals only

Answer: (A)

Tonic receptors are defined by their slow adaptation: they continue to respond for the entire duration of a stimulus, providing a persistent signal about both presence and duration (and to some extent intensity) of the stimulus. This sustained firing lets you perceive ongoing contact, pressure, or stretch. Classic examples in the skin are Merkel’s disks and Ruffini endings, which keep signaling as long as the stimulus is applied.

In contrast, rapidly adapting receptors fire mainly at the onset (and sometimes offset) of a stimulus and then stop, which makes them good at detecting changes or movement but not sustained stimuli. The idea of receptors that “do not adapt” isn’t the same as tonic signaling; tonic specifically refers to slow adaptation with ongoing response, whereas chemical signaling is a separate modality and isn’t what defines tonic versus phasic.