stimulus
Sign in to savein physiology, a detectable change in the internal or external surroundings
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A stimulus is any detectable change in your surroundings—either inside your body or in the environment around you—like a sound, touch, temperature shift, or change in blood sugar. Your body detects these stimuli through sensory systems and responds to them, which is how you perceive the world and maintain health.
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Article
The light from the lamp (1.) functions as a detectable change in the plant's environment. As a result, the plant exhibits a reaction of phototropism—directional growth (2.) toward the light stimulus.
In physiology, a stimulus is a change in an organism's internal or external environment. This change, when detected by an organism or organ using sensitivity, can lead to a physiological reaction. Sensory receptors can receive stimuli from outside the body, as in touch receptors in skin or light receptors in the eye, as well as from inside the body, as in chemoreceptors and mechanoreceptors. When detected by a sensory receptor, a stimulus can elicit a reflex via stimulus transduction. An internal stimulus is often the first component of a homeostatic control system. External stimuli are capable of producing systemic responses throughout a body, as in the fight-or-flight response. For a stimulus to be detected with high probability, its level of strength must exceed the absolute threshold; if a signal does reach threshold, the information is transmitted to the central nervous system (CNS), where it is integrated and a decision on how to react is made. Although stimuli commonly cause the body to respond, it is the CNS that finally determines whether a signal causes a reaction or not.