The frontalis muscle is a flat, thin, quadrilateral muscle located across the forehead, playing a visible role in facial expression. It is the only muscle capable of elevating the eyebrows, conveying surprise, attention, or curiosity. Understanding its anatomy, particularly its origin, helps explain how it produces these movements of the scalp and forehead skin.
Defining the Frontalis Muscle Complex
The frontalis is not a solitary structure but functions as the anterior portion of a larger unit known as the occipitofrontalis muscle, or the epicranius muscle. This complex covers the dome of the skull, connecting the facial muscles in the front with the muscles at the back of the head. The entire epicranius is composed of three main parts that work together to move the scalp.
The occipitofrontalis includes the frontalis belly, the occipitalis belly, and the dense, fibrous tissue that links them. This connecting structure is a broad, tendon-like sheet called the galea aponeurotica, also known as the epicranial aponeurosis. The galea aponeurotica covers the upper part of the skull and acts as a shared tendon for both muscle bellies.
The occipitalis belly is located at the back of the skull, near the occipital bone, while the frontalis belly is situated in the forehead area. This structural relationship dictates that the frontalis muscle does not attach directly to a bone at its superior end, unlike most skeletal muscles.
Precise Location of the Muscle’s Origin
The specific origin point of the frontalis muscle is the galea aponeurotica, a thick, tough layer of connective tissue within the scalp. It arises from the anterior margin of this fibrous sheet, generally near the coronal suture of the skull, providing a non-bony attachment surface. The muscle fibers then extend inferiorly, or downward, across the forehead.
This unique origin allows the frontalis muscle to move the entire scalp when it contracts. The galea aponeurotica is stabilized posteriorly by the occipitalis muscle, which originates from the superior nuchal line of the occipital bone. The frontalis muscle fibers ultimately terminate by inserting into the skin and superficial fascia around the eyebrows and the root of the nose.
Primary Function and Action
The main action of the frontalis muscle is directly related to its unique origin and insertion points. When the muscle contracts, it pulls the galea aponeurotica and the scalp forward. Because its lower end is anchored to the skin of the eyebrows, this pulling action causes the eyebrows to elevate.
This upward pull is the only mechanism that allows a person to raise their eyebrows, creating expressions such as surprise. Simultaneously, the contraction of the vertically oriented muscle fibers causes the skin of the forehead to wrinkle horizontally. These transverse forehead lines result from the frontalis muscle shortening and bunching the skin.
The frontalis muscle’s insertion into the skin, rather than directly onto bone, allows it to manipulate the soft tissues of the face effectively. The muscle also interdigitates with other facial muscles at its lower border, including the procerus and the orbicularis oculi muscles. These surrounding muscles often act in opposition to the frontalis, helping to regulate eyebrow position and shape.