Laughing is a reaction to certain stimuli, fundamentally stress, which serves as an emotional balancing mechanism. Traditionally, it is considered a visual expression of happiness, or an inward feeling of joy. It may ensue from hearing a joke, being tickled, or other stimuli. It is in most cases a very pleasant sensation.
Laughter is found among various animals, as well as in humans, although it is more rare in most mammals and animals overall. Among the human species, it is a part of human behavior regulated by the brain, helping humans clarify their intentions in social interaction and providing an emotional context to conversations. Laughter is used as a signal for being part of a group—it signals acceptance and positive interactions with others.
Laughter is sometimes seen as contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback. This may account in part for the popularity of laugh tracks in situation comedy television shows. Those modern teens refer to laughing as laughing out loud or LOL which as of 2011 is in the Oxford English Dictionary.
Laughter is anatomically caused by the epiglottis constricting the larynx. The study of humor and laughter, and its psychological and physiological effects on the human body, is called gelotology. Recent investigations by Robert Provine suggest that laughter is a form of communication, probably the first one in the human race, which later evolved, with the liberation of voice from walking and breathing, into human language.
Laughter and health
A link between laughter and healthy function of blood vessels was first reported in 2005 by researchers at the University of Maryland Medical Center with the fact that laughter causes the dilatation of the inner lining of blood vessels, the endothelium, and increases blood flow. Drs. Michael Miller (University of Maryland) and William Fry (Stanford), theorize that beta-endorphin like compounds released by the hypothalamus activate receptors on the endothelial surface to release nitric oxide, thereby resulting in dilation of vessels. Other cardioprotective properties of nitric oxide include reduction of inflammation and decreased platelet aggregation.
