The nervous system receives information from both the internal and external environment of the organism. This input comes from the receptors. Receptors are specialized organs or nerve endings that detect specific changes in the internal and external environments. Receptors may be classified based on their location. Exteroceptors are receptors located near the external and internal body surfaces.
Receptors differ in the type of stimuli to which they respond.
Mechanoreceptors are those that respond to displacement or vibration. Photoreceptors , responsible for sight, are sensitive to light. Finally, nociceptors , which consist of both mechoreceptors and chemoreceptors, detect pain sensation arising from injury, irritation or chemical change. Nondiscriminative touch, which may be felt by lightly touching skin surface with a ball of cotton, only conveys that a contact has been made with an object. Discriminative touch, a part of DGS, can be experienced by poking the blunt end of a pencil against the skin, allows us to pinpoint the exact location touched on the body surface as well as telling us something about the shape, size, and texture of the object touching us.