The zygomaticus major muscle is a paired muscle of the face, dedicated almost entirely to conveying emotion. Due to its specific action of pulling the mouth into an upward curve, this muscle is often called the “smiling muscle.” It plays a role in nonverbal communication, turning a neutral face into an expression of pleasure or mirth.
Anatomical Placement and Structure
The zygomaticus major muscle originates on the lateral surface of the zygomatic bone. From this origin, the muscle extends obliquely downward and forward across the cheek. It travels toward the corner of the mouth, where it terminates by inserting into the skin and connective tissue structure known as the modiolus.
The modiolus is a dense, mobile knot of muscle fibers where several facial muscles converge, acting as a central hub for mouth movement. The blending of the zygomaticus major fibers with other muscles, such as the orbicularis oris and levator anguli oris, allows for complex, coordinated movement of the mouth angle.
In some individuals, the muscle may split into two separate bundles before reaching the mouth, a variation known as a bifid or duplicated zygomaticus major. This structural anomaly is believed to be the underlying cause for the formation of cheek dimples when a person smiles.
The Core Function in Facial Movement
The primary function of the zygomaticus major is to elevate and retract the angle of the mouth. When the muscle contracts, it pulls the corner of the mouth upward and outward. This action forms the upward curve characteristic of a smile or laugh.
The muscle is a key player in distinguishing between different types of smiles. A true, involuntary smile, often called a Duchenne smile, requires the activation of the zygomaticus major working in conjunction with the orbicularis oculi muscle. The orbicularis oculi is responsible for raising the cheeks and creating the characteristic crinkling around the eyes. Smiles that involve only the mouth corners, without the eye involvement, are considered deliberate or non-Duchenne smiles.
The zygomaticus major’s action is distinct from its neighboring counterpart, the zygomaticus minor, which is located more anteriorly on the face. While both muscles originate on the cheekbone, the minor muscle inserts into the outer part of the upper lip itself, not the corner of the mouth. The zygomaticus minor primarily functions to elevate and deepen the nasolabial furrow, the crease running from the nose to the mouth, while the major muscle controls the angle of the mouth.
Clinical Importance and Aesthetic Role
The proper function of the zygomaticus major relies on the integrity of the facial nerve (Cranial Nerve VII). Motor signals are delivered to the muscle through the zygomatic and buccal branches of this nerve. Damage to the facial nerve from trauma, stroke, or conditions like Bell’s Palsy can result in paralysis of this muscle. This paralysis causes the corner of the mouth on the affected side to droop, preventing the formation of a symmetrical smile.
Understanding the muscle’s location and function is important for both reconstructive and aesthetic procedures. Surgeons may utilize the muscle in reconstructive surgeries to help restore lip function or symmetry following injury or cancer removal. Its precise anatomical course serves as a significant landmark during deep-plane facelifts and other midface surgeries.
In cosmetic procedures, the muscle’s activity is a consideration when injecting botulinum toxin to treat crow’s feet wrinkles around the eyes. Injecting too close to the major muscle can unintentionally weaken it, leading to a temporary, noticeable asymmetry in the patient’s smile. The muscle’s action also influences the appearance of the cheek and midface, making it a focus for treatments aiming to restore youthful facial contours.