The frontalis muscle is a large, flat, quadrilateral muscle that spans the entire forehead, residing just beneath the skin. It is the anterior part, or belly, of the occipitofrontalis muscle complex, which is part of the epicranius covering the top and sides of the skull. The frontalis plays a central role in conveying human emotion and is responsible for specific movements of the upper face, impacting both clinical medicine and cosmetic procedures.
Anatomical Location and Structure
The frontalis muscle occupies the space from the eyebrows up to the hairline. Unlike most skeletal muscles, it has no direct bony attachment at its upper boundary. Instead, its superior fibers originate from the galea aponeurotica, a dense, fibrous sheet of connective tissue covering the top of the skull.
The galea aponeurotica connects the frontalis to the occipitalis muscle at the back of the head. The frontalis fibers extend downward, inserting into the skin and soft tissues around the eyebrows and the root of the nose. This attachment to the skin, rather than to bone, allows the frontalis to directly manipulate the skin’s surface.
Primary Function: Facial Expression and Movement
The frontalis muscle is the sole elevator of the eyebrows. When the muscle contracts, its vertical fibers pull the skin of the forehead and eyebrows upward and slightly backward. This action is engaged when expressing emotions such as surprise, attention, or worry.
The upward pull of the muscle causes the skin to bunch up, resulting in characteristic horizontal creases across the forehead. It also assists in slightly raising the upper eyelids. The frontalis works in opposition to brow depressors, including the procerus, corrugator supercilii, and orbicularis oculi.
Clinical and Cosmetic Significance
Repeated contraction of the frontalis muscle is the direct cause of horizontal forehead lines. These lines are initially dynamic, appearing only with movement, but eventually become static wrinkles etched into the skin. Chronic tension or overuse, often due to squinting or prolonged concentration, can contribute to tension headaches across the forehead or scalp.
The frontalis is a target for cosmetic treatments using Botulinum Toxin Type A (BoNT-A), or Botox. This neurotoxin is injected directly into the muscle to temporarily block acetylcholine, the chemical messenger responsible for muscle contraction. This localized reduction in muscle activity smooths the appearance of horizontal forehead lines.
Treatment typically involves distributing 10 to 30 units of the toxin across multiple injection points in the forehead, with the effect lasting three to six months. Motor control for the frontalis is provided by the temporal branch of the facial nerve (Cranial Nerve VII).
Damage to this nerve, such as in Bell’s Palsy, can result in paralysis of the frontalis on the affected side, causing noticeable asymmetry in the forehead and eyebrow position. In these cases, BoNT-A may be strategically injected into the unaffected side of the frontalis to reduce its hyperactivity. This technique helps temporarily restore facial symmetry, improving both appearance and functional balance.