A stimulus is a noticeable change in the internal or external environment. This provokes a response within an organism. A stimulus may not only be physiological, but also psychological, whereby an action of another individual directly affects the behaviour of another.
Types of receptors in the human body.
Chemoreceptors, detect the presence of chemicals. Thermoreceptors on the other hand detect changes in temperature. Mechanoreceptors detect mechanical forces. Photoreceptors in the body, detect light during vision. 
- ↑ Sensory Receptors | Boundless Anatomy and Physiology [Internet]. Courses.lumenlearning.com. 2018 [cited 10 December 2018]. Available from: https://courses.lumenlearning.com/boundless-ap/chapter/sensory-receptors/