Occipitofrontalis

The occipitofrontalis is a long and wide muscle that stretches across the scalp, extending from the superior nuchal line of the occipital bone to the eyebrows. It is also known as the epicranius muscle.

The occipitofrontalis consists of two muscular bellies - the occipital belly (occipitalis muscle) and the frontal belly (frontalis muscle). Each belly contains a pair of quadrangular-shaped muscle heads.

Both bellies are connected by a thick fibrous tissue sheath called the epicranial aponeurosis (galea aponeurotica), which lies between the superficial and deep layers of the scalp. The superficial layer includes the skin and dense subcutaneous tissue containing blood vessels and nerves, while the deep layer is composed of loose connective tissue that allows the scalp to move over the calvaria and pericranium.

The frontal belly originates from the epicranial aponeurosis at the level of the coronal suture and courses anteriorly to end at the level of the eyebrows by inserting into the skin and blending with the surrounding muscles. In contrast, the occipital belly arises at the superior nuchal line of the occipital bone and extends superiorly to blend with the epicranial aponeurosis at the level of the lambdoid suture.

Since the occipitofrontalis muscle lacks bony insertions, it relies on the epicranial aponeurosis for coordinated function. This allows for subtle scalp movements and facial expressions, making it an essential component of nonverbal communication. Overall, the frontalis muscle elevates the eyebrows and wrinkles the skin of the forehead, while the occipitalis muscle retracts the scalp.

Like other facial muscles, both bellies are innervated by the facial nerve (CN VII). The frontal belly receives nerve supply from the temporal branches, while the occipital belly is innervated by the posterior auricular nerve.

The frontalis muscle is supplied by the ophthalmic artery and superficial temporal artery, branches of the internal carotid artery and external carotid artery, respectively. The occipitalis muscle receives arterial blood from the posterior auricular artery and occipital artery of the external carotid artery.

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.

Occipitalis muscle (occipital belly of occipitofrontalis)

The occipitalis muscle is a thin and flat muscle that covers the occipital region of the skull. While it may appear to be an individual muscle with two heads, it is actually a component of a larger structure.

The occipitalis muscle is a part of the occipitofrontalis muscle, forming its occipital belly. It connects to the frontalis muscle (the frontal belly of the occipitofrontalis muscle) via the epicranial aponeurosis (galea aponeurotica). The occipitalis draws the scalp backward.

Origin: lateral two-thirds of the superior nuchal line of the occipital bone; it may also extend to the mastoid process of the temporal bone.

Insertion: epicranial aponeurosis.

Action: retracts the scalp.

Innervation: posterior auricular nerve of the facial nerve (CN VII).

Arterial blood supply: occipital branches of the posterior auricular artery and occipital artery (both are branches of the external carotid artery).

Venous drainage: occipital vein (drains into the internal jugular vein or the posterior auricular vein).