It is a common observation that an intense smile often involves the eyes closing or squinting. This intriguing connection between smiling and eye movement is a testament to the complex interplay of our facial muscles and neurological pathways.
The Muscles Behind the Movement
Eye closure during a smile involves two primary muscles: the orbicularis oculi and the zygomaticus major. The orbicularis oculi is a circular muscle that surrounds the eye, responsible for closing the eyelids and causing the skin around the eye to crinkle. The zygomaticus major muscle, on the other hand, originates from the cheekbone and extends to the corners of the mouth, primarily lifting them to form the characteristic upward curve of a smile.
The interaction between these two muscles often links smiling with eye closure. When the zygomaticus major contracts to create a broad smile, it pulls on the surrounding facial tissues, including the cheeks. This upward movement of the cheeks, in turn, presses against the orbicularis oculi, causing the eyes to narrow or “squint.” This coordinated action is a natural consequence of their anatomical arrangement and interconnectedness.
The Brain’s Role in Coordination
The neurological aspect of this phenomenon is largely controlled by the facial nerve. This nerve innervates all the muscles responsible for facial expression, including both the orbicularis oculi and the zygomaticus major. The facial nerve transmits signals from the brain to these muscles, orchestrating their movements.
The eye closure during a genuine smile is often an involuntary reflex rather than a deliberate action. The brain’s pathways link emotional expression to muscle activation. When true joy or amusement is experienced, the brain automatically triggers the contraction of both the smiling muscles and the muscles around the eyes. This involuntary nature highlights the deep connection between our emotional state and physical manifestations.
The Authenticity of a Smile
The involvement of eye muscles in a smile is often considered a sign of its authenticity, leading to the concept of a “Duchenne smile.” This genuine smile engages both the zygomaticus major (mouth corners) and the orbicularis oculi (eyes). The contraction of the orbicularis oculi causes the cheeks to rise, the eyes to narrow, and often creates “crow’s feet” wrinkles at the outer corners of the eyes.
In contrast, a “social smile” or “polite smile” typically involves only the mouth muscles, primarily the zygomaticus major, without significant engagement of the eye muscles. While a social smile can convey pleasantness, the Duchenne smile is widely recognized as a more authentic expression of true happiness, amusement, or enjoyment. The presence of eye crinkling makes the smile appear more sincere and reflects an internal emotional state.
Individual Variations
Not everyone’s eyes close to the same extent when they smile. Factors such as individual facial anatomy, including the structure of the cheekbones and the amount of subcutaneous tissue, can influence how much the cheeks push up against the eyes. The inherent strength and development of the orbicularis oculi and zygomaticus major muscles can also play a role, leading to differences in the intensity of eye narrowing.
The intensity of the emotion being expressed directly affects the degree of muscle activation. A mild amusement might result in a subtle eye crinkle, while profound joy could lead to a more pronounced eye closure. These natural differences contribute to the unique expressions of happiness seen in each individual.