The smile is a type of facial expression that results from a feeling of joy, happiness, or any feeling of satisfaction that the human being presents. From the anatomophysiological point of view, a smile can be mentioned as the product of the total contraction of 17 muscles of the face close to the oral and orbital cavity (eyes), in humans it mainly occurs when there is a satisfactory or pleasant feeling, without However, it can also be evidenced in states of anxiety, anger or sarcasm, being on these occasions an involuntary contraction.
Specialized studies have shown that the smile is not a voluntary or learned movement, being this clearly innate (typical of birth), this explains why people who are blind from birth know how to smile, however, the facial silhouette called smile is only For human use, on the contrary, when animals make a grimace similar to a smile, it is not due to a feeling of satisfaction, but is produced spontaneously due to threatening situations for the animal.
As time goes by, the smile movement can be applied for different occasions, to show appreciation, to greet people you know, also as a show of kindness, etc. Several scientists have described different types of smiles: firstly, the “Duchene” smile can be mentioned, it is produced involuntarily by the human, it exerts the contraction of the facial muscles such as the zygomatic muscles, and the muscles close to the eyes, denominated like genuine smile; "Professional" smile, this is the smile applied to demonstrate cordiality towards people who are in a common room; lastly, the "sardonic" smile can be mentioned This is a type of involuntary contraction caused by contamination with a pathogen called clostridium tetani, this is responsible for the pathology known as tetanus which is characterized by producing a contraction of muscles of the back, neck and face, making it impossible for the patients to achieve closure of the mouth.
Smiling not only modifies the contraction of muscles, thus changing the facial expressions of a person, but also induces an endocrinological change, when smiling in the diencephalon (area between the brain and brainstem) a hormone called endorphins is secreted, which generates the sensation of well-being or pleasure, reducing emotional or physical pain that the individual presents.
