The human face is a complex and highly expressive region, containing a dense network of specialized skeletal muscles located just beneath the skin. Unlike most other skeletal muscles, many facial muscles do not connect bone to bone; instead, they attach bone to soft tissue or skin, allowing for subtle and varied movements. The face and neck region contain well over 40 distinct muscles that work in pairs. Their primary function is to facilitate communication through non-verbal expression and to manage the mechanical processes required for eating and speaking.
Functional Groupings of Facial Muscles
The facial muscles are not a single, uniform group but are functionally divided into two distinct categories based on their primary purpose and the nerve that controls them. This division reflects a separation between the muscles that manage communication and those that manage survival functions. The first and largest group is the muscles of facial expression, which are responsible for generating the vast array of human emotions displayed on the face.
The second group is the muscles of mastication, which are powerful muscles for moving the lower jaw. These robust structures connect the skull to the mandible, unlike the muscles of expression which primarily move the skin and soft tissues. This dual system allows for the simultaneous display of emotion and the execution of mechanical functions like chewing.
Muscles Responsible for Facial Expression
This extensive group of muscles allows humans to convey emotions by pulling and stretching the skin. The muscles are generally organized around the three major openings of the face: the eyes, the nose, and the mouth. For example, the Occipitofrontalis muscle extends across the top of the head, raising the eyebrows and creating the horizontal wrinkles of the forehead that communicate surprise.
Around the eyes, the Orbicularis Oculi muscle acts as a sphincter, surrounding the eye opening to control the eyelids. The palpebral portion of this muscle gently closes the eye for blinking, while the orbital part closes the eye forcefully, such as when squinting against bright light. The Corrugator Supercilii muscle, located near the eyebrow, draws the eyebrows medially and inferiorly, producing the vertical creases associated with frowning or worry.
The area around the mouth and nose contains the most complex arrangement of these expressive muscles, controlling smiling, pouting, and speech. The Zygomaticus Major muscle is famously known as the “laughing muscle” because it pulls the corners of the mouth upward and outward to form a smile. Conversely, the Depressor Anguli Oris pulls the corners of the mouth downward, forming a frown. The circular Orbicularis Oris muscle surrounds the lips, enabling actions like puckering, closing the mouth, and preventing food from spilling during chewing.
Muscles Responsible for Mastication
The muscles of mastication are built for strength to manage the forces required for biting and chewing. There are four main paired muscles that control the precise and powerful movements of the mandible. These muscles are responsible for closing the jaw, grinding food, and moving the jaw forward, backward, and side-to-side.
The Masseter is one of the strongest muscles relative to its size and is easily felt on the side of the cheek as the jaw clenches. Its primary action is to elevate the mandible forcefully, closing the jaw during biting. The Temporalis muscle is a large, fan-shaped muscle on the side of the head that assists the masseter in elevation and also helps to retract the jaw, pulling it backward.
The two Pterygoid muscles, the medial and lateral, are located deeper within the skull and control complex jaw movements. The Medial Pterygoid works with the masseter to elevate the jaw for closing, forming a powerful sling. The Lateral Pterygoid is unique because it is the main muscle responsible for depressing the jaw to open the mouth and for protruding the mandible forward. Working together, the pterygoid muscles also facilitate the side-to-side grinding motions that are fundamental to chewing.
The Cranial Nerves That Govern Facial Movement
The precise control of the facial muscles is managed by two separate cranial nerves originating from the brainstem. This division of labor reinforces the functional separation between expression and chewing. The Facial Nerve (CN VII) provides the motor input for every muscle of facial expression.
The Facial Nerve branches out extensively across the face, dictating movements from the forehead down to the neck muscles. Damage to this nerve prevents the ability to smile, frown, or close the eye on the affected side, demonstrating its importance to non-verbal communication. Conversely, the Trigeminal Nerve (CN V) controls the motor function of the four muscles of mastication and carries sensory information from the face, including touch and pain. This dual innervation system ensures that the mechanics of eating can proceed independently from the rapid, subtle contractions required for social interaction.