The occipitofrontalis action is a fundamental component of facial expression, responsible for raising the eyebrows and creating the horizontal wrinkles on the forehead. This muscle consists of two distinct bellies, the frontal and the occipital, connected by a broad tendon known as the epicranial aponeurosis, or galea. Understanding this specific action requires looking at how these structures work together to move the skin of the scalp and forehead without moving the underlying skull.
Anatomy of the Epicranius Muscle
The occipitofrontalis is technically a single muscle with two functional units, often referred to as the epicranius. The frontal belly originates from the galea itself and inserts into the skin and subcutaneous tissue of the eyebrows and forehead. The occipital belly, located at the back of the head, originates on the superior nuchal line of the occipital bone and mastoid process, inserting into the same galea tendon. This unique structure allows for isolated movement of the forehead skin.
The Mechanics of the Occipitofrontalis Action
When the frontal belly contracts, it pulls the epicranial aponeurosis forward and downward. This tension lifts the eyebrows upward and toward the midline, resulting in the characteristic raised forehead and frown lines. The action is purely skin-moving; if the epicranial aponeurosis is fixed, contraction of the frontal belly can slightly elevate the scalp itself. The occipital belly performs the opposite motion, pulling the aponeurosis backward to raise the back of the head.
Neurological Control and Pathways
The occipitofrontalis action is governed by the facial nerve, specifically the temporal and frontal branches that emerge from the parotid gland. These motor fibers provide precise control over the muscle, allowing for subtle adjustments in eyebrow position and forehead wrinkling. Because this nerve controls the muscles of facial expression, damage to it can result in a loss of forehead movement, a key diagnostic indicator in neurological exams.
Functional Roles in Expression and Physiology
While the occipitofrontalis action is most famous for its role in non-verbal communication, it serves practical purposes beyond aesthetics. Raising the eyebrows increases the palpebral fissure, improving vision and allowing more light into the eyes when scanning the environment. The wrinkling of the forehead is theorized to redirect sweat away from the eyes, acting as a functional drainage system during periods of exertion or stress.
Clinical Significance and Assessment
Testing the occipitofrontalis action is a standard part of a neurological examination. Clinicians ask patients to raise their eyebrows tightly or wrinkle their foreheads to assess facial symmetry and nerve function. A lack of movement on one side may indicate a lesion or palsy affecting the facial nerve. Additionally, excessive or involuntary action of this muscle is associated with conditions like forehead hyperhidrosis (excessive sweating) or dynamic rhytids (wrinkles).
Comparison with Other Facial Muscles
Unlike the circular muscles that control the eyes and mouth, the occipitofrontalis action is unique in its ability to move large areas of skin rather than just altering the shape of an orifice. While the orbicularis oculi closes the eye, the occipitofrontalis opens the visual field vertically. It works in opposition to the corrugator supercilii, which pulls the eyebrows downward and inward, creating a frown, whereas the occipitofrontalis creates the surprised or angry look.
