Irritability or
sensitivity is a characteristic feature of all living organisms and refers to
their ability to respond to a stimulus. Any thing that causes activity or
change in activity in an organism is called stimulus (plural: stimuli) or
anything that starts a nerve impulse.
Reaction to the
stimulus is known as response. The stimulus is received by a receptor. The
structures that give responses, when they are stimulated by nerve impulse or
hormones are called effectors. The main effectors are muscles and glands.
Animals have two different but related system of co-ordination, the nervous
system and endocrine system.
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