1.1 Stimulus and Response

Stimulus - Change in an animal's surroundings

Response - Reaction to the change

Imagine while you are walking, some one throw a ball at you. If you have a good coordination system. You will definitely try to avoid the ball. In this example ball coming towards you is the stimulus and the action you take to avoid the ball is the response.  

Humans need a receptor to detect the stimulus. The receptor can be ear, eye, skin, tongue or norse. On the other hand response to a certain stimulus will be completed with the help of a effector (organ). 

Simply the coordination is linking the relevant receptor organ with effector. In humans it is done by nervous system. Nerve cells in the nervous system, transmit information in the form of electrical signal called nerve impulses

1.2 Receptors

Receptors are biological transducers that convert energy within the stimulus that come from both external and internal environments into electrical energy of the nerve impulse.