Endorphins ("endogenous morphine") are endogenous opioid peptides that function as neurotransmitters. They are produced by the pituitary gland and the hypothalamus invertebrates during exercise, excitement, pain, consumption of spicy food, love and orgasm, and they resemble the opiates in their abilities to produce analgesia and a feeling of well-being.
The term "endorphin" implies a pharmacological activity (analogous to the activity of the corticosteroid category of biochemicals) as opposed to a specific chemical formulation. It consists of two parts: endo- and -orphin; these are short forms of the words endogenous and morphine, intended to mean "a morphine-like substance originating from within the body."
The term endorphin rush has been adopted in popular speech to refer to feelings of exhilaration brought on by pain, danger, or other forms of stress, supposedly due to the influence of endorphins. When a nerve impulse reaches the spinal cord, endorphins are released which prevent nerve cells from releasing more pain signals. Immediately after injury, endorphins allow animals to feel a sense of power and control over themselves that allows them to persist with activity for an extended time.
Another widely publicized effect of endorphin production is the so-called "runner's high", which is said to occur when strenuous exercise takes a person over a threshold that activates endorphin production. Endorphins are released during long, continuous workouts, when the level of intensity is between moderate and high, and breathing is difficult. This also corresponds with the time that muscles use up their stored glycogen. During a release of endorphins the person may be exposed to bodily harm from strenuous bodily functions after going past his or her body's physical limit. This means that runners can keep running despite pain, continuously surpassing what they once considered to be their limit.
Studies have shown that exercise releases endorphins in the brain, which in turn cause us to have what is commonly know as a "natural high" or a "runners high". You may not realize what caused it, but most of us have felt it. Whether we're engaged in a leisurely swim or an adrenaline-charged rock climb, there is that moment when suddenly pain or discomfort drops away and we are filled with a sense of euphoria. We have endorphins to thank for these moments of bliss.
Perhaps the most unusual activity believed to be able to stimulate the body's secretion of endorphins is laughter.