Stimulus (physiology)
In physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

, a stimulus (pl. stimuli) is a detectable change in the internal or external environment
Environment (systems)
In science and engineering, a system is the part of the universe that is being studied, while the environment is the remainder of the universe that lies outside the boundaries of the system. It is also known as the surroundings, and in thermodynamics, as the reservoir...

. The ability of an organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 or organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

 to respond to external stimuli is called sensitivity.

When a stimulus is applied to a sensory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

, it elicits or influences a reflex
Reflex action
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 via stimulus transduction
Transduction (physiology)
In physiology, transduction is the conversion of a stimulus from one form to another.Transduction in the nervous system typically refers to stimulus alerting events wherein a mechanical/physical/etc stimulus is converted into an action potential which is transmitted along axons towards the central...

. A stimulus is often the first component of a homeostatic control system
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

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