<P> In physiology, a stimulus (plural stimuli) is a detectable change in the internal or external environment . The ability of an organism or organ to respond to external stimuli is called sensitivity . When a stimulus is applied to a sensory receptor, it normally elicits or influences a reflex via stimulus transduction . These sensory receptors can receive information from outside the body, as in touch receptors found in the skin or light receptors in the eye, as well as from inside the body, as in chemoreceptors and mechanoreceptors . An internal stimulus is often the first component of a homeostatic control system . External stimuli are capable of producing systemic responses throughout the body, as in the fight - or - flight response . In order for a stimulus to be detected with high probability, its level 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 . </P> <P> Homeostatic imbalances are the main driving force for changes in the body . These stimuli are monitored closely by receptors and sensors in different parts of the body . These sensors are mechanoreceptors, chemoreceptors and thermoreceptors that, respectively, respond to pressure or stretching, chemical changes, or temperature changes . Examples of mechanoreceptors include baroreceptors which detect changes in blood pressure, Merkel's discs which detect sustained touch and pressure, and hair cells which detect sound stimuli . Homeostatic imbalances that can serve as internal stimuli include nutrient and ion levels in the blood, oxygen levels, and water levels . Deviations from the homeostatic ideal may generate a homeostatic emotion, such as pain, thirst or fatigue, that motivates behavior that will restore the body to stasis (such as withdrawal, drinking or resting). </P> <P> Blood pressure, heart rate, and cardiac output are measured by stretch receptors found in the carotid arteries . Nerves embed themselves within these receptors and when they detect stretching, they are stimulated and fire action potentials to the central nervous system . These impulses inhibit the constriction of blood vessels and lower the heart rate . If these nerves do not detect stretching, the body determines perceives low blood pressure as a dangerous stimulus and signals are not sent, preventing the inhibition CNS action; blood vessels constrict and the heart rate increases, causing an increase in blood pressure in the body . </P>

Each type of nerve receptor can detect only one type of