stimulus

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In physiology, a stimulus is a change within 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. The term stimulus represents a borrowing from the Latin word stimulus, meaning "a goad".