Polarized light micrograph of crystals of endorphins. Endorphins ("endogenous" morphines) are a group of peptides (small proteins) produced by body cells that occur in the brain. Endorphins occur in the nervous system and other body sites. Their effects on nerve endings are analgesic (pain killing), while they may also control the body's response to stress as well as determining mood and pleasure. Magnification 4x.