Organs, which may consist of one cell or a small group of cells, which respond to different forms of external or internal stimuli by conveying impulses down nerves to the CENTRAL NERVOUS SYSTEM, alerting it to changes in the internal or external environment.

A small, discrete area on the cell membrane or within the cell with which molecules or molecular complexes (e.g. hormones, drugs, and other chemical messengers) interact. When this interaction takes place it initiates a change in the working of the cell.