Smiling is a universal human expression. It serves as a fundamental signal of emotion, connection, and social intent. Understanding the muscular and neurological processes that create a smile reveals the intricate biological mechanisms behind this common human display.
The Primary Muscles of a Smile
Smiling involves several facial muscles. The zygomaticus major muscle is largely responsible for raising the corners of the mouth upward and outward. This muscle originates from the zygomatic bone, or cheekbone, and extends to insert into the skin at the corner of the mouth, contributing significantly to the visual appearance of a smile.
Another muscle, the risorius, pulls the corners of the mouth laterally, or sideways, contributing to a wider grin. The risorius is located in the cheek, originating from the fascia covering the parotid gland and masseter muscle, and inserts into the skin at the angle of the mouth.
The levator labii superioris also elevates the upper lip and exposing the upper teeth. This muscle originates from parts of the maxilla and zygomatic bone and inserts directly into the upper lip. These muscles work in concert to create the diverse range of expressions associated with smiling.
How Different Smiles Are Formed
The interplay of facial muscles distinguishes different types of smiles, particularly between genuine and polite expressions. A genuine smile, a Duchenne smile, involves the contraction of both the zygomaticus major and the orbicularis oculi muscles. The orbicularis oculi, a circular muscle around the eye, causes the skin around the eyes to crinkle, forming “crow’s feet.” This combined activation is involuntary and reflects true enjoyment. In contrast, a polite or non-Duchenne smile primarily engages only the zygomaticus major, lifting the corners of the mouth without eye involvement. Research by psychologist Paul Ekman has identified numerous distinct smile types, with only one being considered a truly genuine expression involving the eye muscles. The presence or absence of the orbicularis oculi activation differentiates a spontaneous smile from a consciously controlled one.
The Brain’s Role in Orchestrating a Smile
The brain orchestrates smiling through distinct neural pathways, depending on whether the smile is voluntary or involuntary. Voluntary smiles, such as those posed for a photograph, are initiated in the motor cortex. Signals through the pyramidal tract to the facial nerve activate the necessary facial muscles. Conversely, genuine, involuntary smiles arise from the limbic system, the brain’s emotional center. This pathway involves the extrapyramidal system, bypassing the motor cortex and directly signaling the facial nerve to produce a spontaneous expression. The facial nerve, also known as Cranial Nerve VII, serves as the primary conduit, transmitting these signals from the brain to all the muscles responsible for facial expressions.