Question

What is stimulus?

Answer 1

Hint: Changes in the internal or external environment or surroundings evoke responses in the organisms. Stimuli may come in a range of formats including physical, audio or visual.

Complete answer:
A detectable change in the chemical or physical structure of an organism's external or internal environment is called a stimulus. Sensitivity is the ability of an organ or organism to detect external stimuli, so that an appropriate reaction can be produced. Sensory receptors receive information from outside the body, as in light receptors in the eye or touch receptors found in the skin, as well as from inside our body, as in mechanoreceptors and chemoreceptors.
A sensory receptor can elicit a reflex when a stimulus is detected via stimulus transduction.. The stimulus can be either internal or external. An internal stimulus is the first component of the homeostatic control system. Systemic responses throughout the body are produced by external stimuli. In order for stimulus to be detected with high probability, its level of strength should exceed the absolute threshold; if a signal reaches the threshold value, the information is transmitted to the CNS (central nervous system), where it is integrated and a decision is made. Although stimulus commonly causes the body to respond, it is the central nervous system that finally determines whether the signal causes reaction or not.

Note: An example of an external stimulus is the response of the iris to bright light. Bright light could damage the retina, and here light is the stimulus. The light sensitive cells of the retina are the receptors. The brain acts as coordinator and decides that the light is too bright and it stimulates the effector, here the circular muscles of the iris. The response is, contraction of eyes making the pupil narrower and reducing the amount of light entering the eye.
An example of an internal stimulus is the temperature control of the body. The temperature of the blood is the stimulus here. The brain is the coordinator and the hypothalamus of the brain is the receptor. A number of effectors are present in this case. The sweat glands, the blood vessels of the skin and the erector muscles are just a few.