How Smiles Control Us All

The neuronal signals for smiles usually start in the cortex of our brain. From there they travel to the deeper part, the brainstem -- which, in terms of evolution, also happens to be one of the oldest parts of our brain. From there, a nerve that's large enough to be visible to the naked eye, called the seventh cranial nerve, carries the signal in front of the ear to the more central part of the face, where it reaches the smile muscle.

The smile muscle is attached from the mouth to the cheekbone. When this nerve fires, the muscle is activated, the corners of our mouth are pulled up, and we look happy. And if it is a true smile, one that signifies real enjoyment by its wearer, then a branch of the facial nerve also activates little muscles around the eyes, leading to wrinkling around the eyes in addition to a mouth smile.

The scientific analysis of the smile really began with the French anatomist Guillaume-Benjamin-Amand Duchenne de Boulogne. In the 1860s, he used electrical currents to make his subjects' "facial muscles contract to speak the language of the emotions and the sentiments." Duchenne believed that one could gain insight into the ways the face expresses emotions by studying the muscles underlying facial movement. He recorded the expressions produced by the electrical stimulation by taking photographs and was the first to use photography to prove a scientific theory. Duchenne showed that in the particular smile he called the "smile of joy," the muscle that is just to the side of the eyes (called orbicularis oculi) is activated. When this muscle contracts, it creates creases, sometimes called crow's feet. He called this the "true smile," the pure smile of enjoyment. Duchenne concluded that the mouth smile obeys the will, but the eye smile does not. He said, "The muscle around the eye ... is only brought into play by a true feeling, an agreeable emotion. Its inertia in smiling unmasks a false friend."

In the image at right, Duchenne applies electrodes to his cooperative subject in order to stimulate the contraction of the smile muscle (zygomaticus major) that pulls the corners of the mouth up when we smile. His subject is clearly enjoying this. Duchenne's jolt of electricity activates the smile muscle around the mouth, but the gentleman's eyes are also smiling, as seen by the creases formed just to the side of his eyes so the happiness comes from inside. When Duchenne first applied the electricity, only the smile muscle around the mouth was activated; he realized that it didn't look like a true, natural smile, so he told this gentleman a joke to make his eyes smile as well.

more here: TheAtlantic