From Seed:
You’re at breakfast enjoying a mouthful of milk when it happens: the zygomatic muscles, anchored at each cheekbone, tug the corners of your mouth backwards and up. Orbicular muscles encircling your eyeballs slowly squeeze tight beneath wrinkling skin. A 310-millisecond-long noise explodes from your throat, extending to a frequency of 10,000 Hertz. Five shorter pulses of the “h” sound follow, five times per second, hovering around 6 Hertz, each lasting a fifteenth of a second. Your heart reaches 115 beats per minute. Blood vessels relax. Muscle tone softens. Abdominal muscles clench. The soft tissue lining your upper larynx vibrates 120 times per second as air blasts past. The milk spews forth. You are laughing.
Laughter, real laugh-till-you-cry laughter, is one of many human emotional expressions. Arguably, laughing and its tearful counterpart, crying, are the loudest, most intrusive non-linguistic expressions of our species. But for all of that familiarity, they are little-understood behavioral mysteries parading in the light of everyday experience. Though evolutionary biologists have long explored the mammalian origins of emotional expression, human laughs and cries only rarely become subjects of cognitive neuroscience. But that may not stay the case. Laughing and crying, being live demonstrations of emotion and its social expression, provide new entryways into the tangled pathways of the brain.
More here.