Smiling is a fundamental human expression, conveying emotions and fostering connections. Though seemingly effortless, forming a smile involves the coordinated action of numerous facial muscles. Understanding these muscles provides insight into this common nonverbal cue.
The Primary Smiling Muscle
The muscle most directly responsible for elevating the corners of the mouth into a smile is the Zygomaticus Major. This paired muscle originates from the zygomatic bone, commonly known as the cheekbone, and extends diagonally downward to insert into the angle of the mouth. Its primary action involves pulling the mouth’s corners superiorly and laterally, creating the upward curve characteristic of a smile. This muscle is consistently active in almost all types of smiles, from a subtle grin to a broad, toothy expression.
Beyond the Primary: Other Muscles Involved
While the Zygomaticus Major is central, other facial muscles contribute to the diverse range of smiles. The Risorius muscle, a slender and variable muscle, extends horizontally across the cheek from the parotid fascia to the corner of the mouth. Its contraction pulls the mouth angle laterally, aiding in broadening the smile.
The Levator Labii Superioris muscle also elevates the upper lip, exposing the upper teeth and deepening the nasolabial lines (folds from the nose to the mouth corners).
The Orbicularis Oculi, a circular muscle surrounding the eye, is another significant contributor to certain smiles. While not directly moving the mouth, its contraction raises the cheeks and creates the characteristic “crow’s feet” wrinkles at the outer corners of the eyes. This eye involvement is particularly indicative of genuine emotion.
Even the Depressor Anguli Oris, which pulls the mouth corners downward, plays an indirect role in a full smile by relaxing or allowing other muscles to dominate. These muscles, all innervated by branches of the facial nerve, allow for a wide array of human facial expressions.
The Science of a Genuine Smile
The distinction between a genuine smile and a voluntary or “social” smile lies in the involvement of specific muscle groups and their underlying neural control. A genuine smile, often referred to as a “Duchenne smile” after the 19th-century neurologist Guillaume Duchenne, involves the involuntary contraction of both the Zygomaticus Major and the Orbicularis Oculi. Activation of the Orbicularis Oculi, causing the eyes to crinkle and cheeks to rise, indicates true enjoyment and is difficult to consciously fake.
In contrast, a social smile primarily engages only the Zygomaticus Major, lifting the corners of the mouth without the characteristic eye involvement. The neural pathways governing these different types of smiles also differ.
Voluntary facial expressions, like a posed social smile, are primarily controlled by the cortical motor tract, originating from the brain’s cerebral cortex. These signals travel down the pyramidal tract to activate facial muscles. Genuine, spontaneous smiles, however, are thought to involve subcortical brain areas, particularly the extrapyramidal system, which is linked to emotional responses. This neurological distinction explains why individuals with certain brain injuries might be unable to produce a voluntary smile but can still smile genuinely in response to emotion, or vice versa.