What are the three types of stimuli?

• A. Mechanical, electrical, thermal
• B. Chemical, electrical, thermal
• C. Mechanical, heat, chemical
• D. Mechanical, chemical, light

Answer: (C)

Stimuli that receptors respond to fall into three broad kinds: mechanical energy (such as pressure, touch, or stretch), chemical energy (changes in chemical substances, ions, pH, or other molecules), and thermal energy (temperature changes, i.e., heat). Mechanoreceptors, chemoreceptors, and thermoreceptors are the sensor types corresponding to these categories. So listing mechanical, heat, and chemical matches the standard triad, since heat is just a thermal stimulus. Other options bring in electrical energy or light, which aren’t part of this basic three-category framework in most physiology contexts.