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Stimulus is a change that takes place internal or external to our body.  The external changes are detected by sensory receptors in the sensory organs.  The receptors get information from the stimulus.  A reflexive reaction is expected after a stimulus is provided to the body.  A stimuli transduction (through transducers) happens in which a  signal for the nervous system is generated in terms of a small potential, which is more than a threshold.

The main function of the stimuli is to alert the body of a condition and inform it to take a reflexive reaction.

Blood pressure and heart beat rate are continuously monitored by stretch receptors in carotid arteries in the heart.  These receptors detect stretching of the arteries and generate potential on the nervous system.  Then the central nervous system responds accordingly.

Sensory organs respond to stimuli given through touch, taste, vision etc.

Thermoreceptors are the sensors in the skin which receive the external stimuli of temperature changes.  The play part in initiation of the regulation of flow of liquids from sweat glands to the outer skin.

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In simple stimulus is a kind of a message that is sent to our medulla oblangata[a part of brain]. stimulus mainly happens in our five sensory organs that are touch ,taste, smell, sight and hearing. eg. when you touch a very hot thing you immediately withdraw your hand. this is because our receptor cell immediately send a message to medulla oblangata that we have touched a hot thing. the medulla respond by giving a command to the hand to move away. this process happens in a matter of a few micro-seconds. the receptor cells send message medulla oblangata because the medulla oblangata is the main control house of the the quick reaction to certain things.
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