The senses are the media through which stimuli in the environment of an organism act on it (external senses); also, the internal senses which report on the internal state of the organism (through thirst and hunger, pain, etc). The organs of sense, the eye, ear, skin, etc., all contain specialized cells and nerve endings which communicate with centers in the nervous system. Sense organs my be stimulated by pressure (in touch, hearing, and balance), chemical stimulation (smell and taste), or electromagnetic radiation (vision; heat sensors).