Frontalis muscle (frontal belly of occipitofrontalis)

The frontalis muscle is a thin and flat muscle that extends across the forehead of the head. While it may look like an individual muscle, it is actually a component of a larger structure. The frontalis muscle is a part of the occipitofrontalis muscle, forming its frontal belly. It is connected to the occipitalis muscle (occipital belly of the occipitofrontalis muscle) via the epicranial aponeurosis.

The frontalis muscle is the only muscle that raises the eyebrows. Therefore, it has a significant role in everyday social interactions during nonverbal communication as it helps convey emotions. The antagonist muscles to the frontalis muscle are the corrugator supercilii, procerus, and orbicularis oculi muscles.

The frontalis muscle is a superficial muscle and part of the superficial musculoaponeurotic system (SMAS). Overall, the frontalis muscle is thin. However, it is more thicker centrally while thinner laterally. This muscle is also very well vascularized. It has no bony attachments. Some research has shown that the right part of the frontalis muscle is larger than that of the left side.

The fibers of the frontalis muscle have a vertical fan-like distribution. Although most of them are inserted into the dermis at the eyebrows, they also blend with the muscles in the glabellar region. Inferomedially, the frontalis muscle blends with the fibers of the procerus muscle. Inferolaterally, it attaches to the orbicularis oculi and corrugator supercilii muscles. The lateral edge of the frontalis muscle usually extends to the temporal line of the frontal bone. However, it may extend beyond or may fall short of it.

In up to 45% of individuals, both heads of the frontalis muscle have fully merged, and there is no bifurcation between them. Up to 55% of individuals have variable central bifurcation containing connective tissue between both heads. The width between both heads varies from person to person, and some studies have also shown completely separate both heads of the frontalis muscle.

Besides raising the eyebrows, contractions of the frontalis muscle are also responsible for the formation of transverse lines on the forehead, producing the facial expression of astonishment.

Origin: epicranial aponeurosis near the coronal suture of the skull (approximately at the hairline).

Insertion: superficial fibers insert in the skin (dermis) and subcutaneous tissue of the eyebrows and the root of the nose; deep fibers blend with the fibers of the procerus, orbicularis oculi, and corrugator supercilii muscles.

Action: moves the anterior scalp backward and raises the eyebrows, producing horizontal skin wrinkles on the forehead. Contractions of the muscle produce such facial expressions as astonishment, surprise, and shock.

Innervation: temporal branches of the facial nerve (CN VII).

Arterial blood supply: frontal branch of the superficial temporal artery (terminal branch of the external carotid artery); supraorbital and supratrochlear arteries of the ophthalmic artery (branch of the internal carotid artery).

Venous drainage: supratrochlear (frontal) vein and supraorbital vein.