Sensory receptor

A sensory receptor, or sense organ, is the part which responds to a stimulus in the internal or external environment of an organism. It is the input to the sensory system. In response to stimuli the sensory receptor fires off a nerve fibre which goes to the central nervous system.
In order to detect changes in the environment organisms need receptors. Organisms have various receptors, ranging from simple light or touch sensitive cells to complex nervous systems. 
Receptors are generally specialised for the detection of a specific type of stimulus.