External stimuli are changes outside the body, or knowledge that is passed to us through our senses. This could be cold or hot weather, light levels, or danger. Internal stimuli are changes to conditions inside the body, such as dangerous food in the stomach, viruses in the body, or lack of food or water. Receptors are groups of specialized cells. They detect a change in the environment and stimulate electrical impulses in response. Sense organs contain groups of receptors that respond to specific stimuli.