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An animal's response to a stimulus is coordinated by its central nervous system (CNS). A simple reflex is an involuntary response to a stimulus. Humans use reflex actions in only some of their behaviour, for example controlling the eye's pupil size.
Responding to a stimulus

A stimulus is a change in the environment of an organism.
Animals respond to a stimulus in order to keep themselves in favourable conditions.
Examples of this include:
Moving to somewhere warmer if they are too cold
Moving towards food if they are hungry
Moving away from danger to protect themselves

Eyes respond to stimuli
Animals that do not respond to a stimulus do not survive for long.
Receptors respond to a stimulus and send impulses along sensory neurons to the CNS.
Some receptors are found in the skin. Other receptors can form part of complex organs, such as:
Light receptor cells in the retina of the eye
Hormone-secreting cells in a gland
Muscle cells
Position receptors in the inner ear
Sound receptors in the ear
Touch, pressure, temperature and pain receptors in skin
Chemical receptors in the nose and tongue
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Certain innate behaviors can be produced in all individuals of the same species. Usually, these behaviors are triggered by a key stimulus (KS). The key stimulus triggers an innate releasing mechanism (IRM), a sort of middleman, which produces a fixed action pattern (FAP), a definite, constant response. The IRM can be any visual, hormonal, or muscular mechanism that results in the FAP. An important feature of the FAP is that once started it cannot be stopped until the entire action sequence is completed.An example of an innate behavior can be seen in Herring gull chicks, which were studied by Tinbergen. Adult females have a red spot on their beak. Chicks instinctually peck at this spot, which stimulates the female to regurgitate and feed the young chick. The red spot is the KS, and it releases the pecking FAP in the chick.

hope it helps
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