When a smile is genuine, which muscle is primarily affected?

When a smile is genuine, the muscle that is primarily affected is the zygomatic muscle. This muscle runs from the cheekbone to the corners of the mouth and is responsible for lifting the corners of the mouth upward, creating a smile. A genuine smile, often referred to as a Duchenne smile, not only involves the zygomatic muscles but also typically engages the orbicularis oculi muscles around the eyes, leading to a more authentic expression that includes the "crow's feet" around the eyes.

In contrast, the orbicularis oris is responsible for movements of the lips, such as closing the mouth, rather than the expression of a genuine smile. The frontalis muscle is involved in raising the eyebrows and does not contribute to the upward motion of the mouth. The buccinator muscle assists with actions like blowing and chewing but does not directly engage in forming a smile. Understanding the specific roles of these muscles helps in recognizing the anatomy behind different facial expressions.