Smiling is a fundamental human expression, a universal signal that transcends language and culture. This seemingly simple act plays a significant role in human communication, conveying emotions and fostering connections. While it appears effortless, the process of smiling involves a complex interplay of various muscles in the face.
Key Muscles Involved in Smiling
A basic smile, involving only the raising of the mouth corners, can activate as few as four muscles. A natural smile generally engages around 10 to 12 muscles. For a broad, genuine grin that lights up the entire face, the number of activated muscles can extend to as many as 43.
The zygomaticus major is a primary muscle in smiling, often referred to as the “smiling muscle.” This muscle originates from the cheekbone and extends to the corner of the mouth, pulling it upward and outward. Its contraction creates the upward curve associated with happiness. The zygomaticus minor also contributes by lifting the upper lip.
The orbicularis oculi encircles the eye. Its contraction around the eyes creates the characteristic “crow’s feet” wrinkles. This eye muscle is particularly active during genuine smiles. Other muscles, such as the risorius, can pull the mouth corners outward, especially in social smiles.
The levator labii superioris lifts the upper lip, while the levator anguli oris raises the angle of the mouth. The circular orbicularis oris muscle, which surrounds the mouth, also plays a role in shaping the lips during a smile. All these facial muscles are intricately controlled by branches of the facial nerve, also known as cranial nerve VII.
Distinguishing Different Smiles
Not all smiles convey the same emotion or involve the same muscle activation. The “Duchenne smile,” named after 19th-century French neurologist Guillaume Duchenne, is a genuine expression of happiness.
A Duchenne smile involves the coordinated contraction of both the zygomaticus major and the orbicularis oculi, which creates wrinkles around the eyes and slightly raises the cheeks. The involvement of the orbicularis oculi is an indicator of a genuine smile. This eye muscle is difficult to contract voluntarily, making a Duchenne smile often involuntary and a reflection of true enjoyment.
In contrast, a “non-Duchenne” or “social” smile primarily involves only the zygomaticus major. In these smiles, the muscles around the eyes remain relaxed, and the eyes may appear indifferent. These smiles are often used for politeness or as social tools rather than to express authentic positive emotion.
The neurological pathways for these different smiles also vary. Genuine Duchenne smiles originate from the limbic system, the brain’s emotional center, while non-Duchenne smiles are under conscious control from the motor cortex. Even in infants, early “social smiles” show the involvement of both eye and cheek movements, resembling Duchenne smiles, indicating their responsive nature.