The frontalis muscle contracts to elevate the eyebrows, a key component in expressions of surprise and attentiveness. Its location on the forehead makes it the primary muscle for this action.

The orbicularis oculi surrounds the eye and contracts to close the eyelids, playing a crucial role in protecting the eyes. This muscle is essential for actions such as blinking and squinting.

The orbicularis oris forms a circular sphincter around the mouth and is responsible for puckering the lips. This action is fundamental for whistling, kissing, and various speech movements.

The buccinator muscle compresses the cheeks against the teeth, which is essential during chewing. Its contraction helps keep food in place and aids in effective mastication.

The zygomaticus major muscle elevates the corners of the mouth to produce a smile. Its contraction is directly associated with expressions of happiness and amusement.

The corrugator supercilii muscle contracts to draw the eyebrows closer together, creating a frowning expression. This muscle's action is commonly associated with emotions such as worry or concentration.

The orbicularis oculi is responsible for closing the eyelids and plays an essential role in blinking. Its effective functioning is vital for protecting the eyes and maintaining proper tear film distribution.

The levator labii superioris muscle elevates the upper lip, producing expressions such as a sneer or subtle disdain. Its contraction changes the position of the lip, highlighting its role in nuanced facial expressions.

The masseter is a primary muscle involved in mastication and is not considered a muscle of facial expression. In contrast, the other options play key roles in conveying emotions through facial movements.

The platysma is a thin, broad muscle that covers the neck and lower face, helping to depress the jaw and lower lip. Its superficial location makes it readily identifiable in facial expressions involving tension.

The frontalis muscle, located across the forehead, contracts to create wrinkles. Its action is most visible when expressing surprise or deep focus.

The corrugator supercilii draws the eyebrows inward and downward, producing a frown. This muscle is integral to facial expressions of worry or mild anger.

When the zygomaticus major contracts, it elevates the corners of the mouth and deepens the nasolabial fold during a smile. This action is a key element of a natural, expressive smile.

The risorius muscle retracts the corners of the mouth sideways to produce a wide smile. Its specific lateral pull distinguishes its function from other muscles involved in smiling.

The depressor anguli oris muscle pulls the corners of the mouth downward, creating an expression commonly associated with sadness or discontent. Its action is distinct from muscles that elevate or retract the mouth during other expressions.

The frontalis muscle is located on the forehead and is best identified by the skin above the eyebrows. This landmark helps surgeons accurately locate the muscle during reconstructive procedures.

The orbicularis oculi is a key muscle of facial expression innervated by the facial nerve. An injury to this nerve typically results in diminished activity of muscles like the orbicularis oculi.

The frontalis muscle is directly responsible for wrinkling the forehead and lifting the eyebrows. Dysfunction of this muscle would result in an inability to perform these movements, as observed in the patient.

The zygomaticus major muscles are essential for elevating the corners of the mouth during a smile. Evaluating their function on both sides helps determine the symmetry and balance of the smile.

The orbicularis oris encircles the mouth and is integral to actions like pursing the lips. Dysfunction in this muscle can compromise the ability to maintain a tight lip seal, affecting both expression and function.