The muscles that move the eyebrows play a crucial role in facial expression, conveying a wide range of emotions from surprise to anger. These muscles work in concert to elevate, depress, and furrow the brows, contributing significantly to nonverbal communication. This article delves into the anatomy, function, and clinical significance of these vital facial muscles.
Facial muscles are located throughout the face, including the ears, mouth, forehead, nose, and eyes. These muscles run underneath the skin from the scalp down to the neck, positioned around facial openings and stretching across the skull and neck. Typically, these muscles are paired, with one on the left side of the face and one on the right. These muscles are part of the skeletal system (musculoskeletal system) and contain elastic fibers that allow them to contract.
The facial muscles involved in chewing, known as the muscles of mastication, include:
The muscles that control facial expression can be divided into groups based on their location on the face.
The auricular muscles are located around the ears and allow some people to move their ears. They include:
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The buccolabial muscles are in and around the mouth and include:
The epicranial muscles are around the forehead, skull, and neck and include:
The nasal muscles are around the nose and include:
The orbital muscles surround the eyes and include:
The frontalis muscle plays a significant role in social interactions. As the only muscle that raises the eyebrows, its function extends beyond simply keeping the brows out of one’s visual field; it is also necessary for conveying emotions and nonverbal communication.
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The frontalis muscle is on the front of the head and is part of a larger structure referred to as the occipitofrontalis muscle or epicranius. The occipitofrontalis is composed of two muscle bellies: frontalis and occipitalis, which are attached and encased by dense connective tissue called the epicranial aponeurosis or galea aponeurotica. The occipital part of the occipitofrontalis muscle moves the scalp forwards, and the frontalis part lifts the brows and moves the anterior scalp backward. When the frontalis muscle contracts, the vertical fibers pull the skin of the eyebrows upward.
The superficial musculoaponeurotic system (SMAS) continues above the zygomatic arch and includes the temporoparietal fascia (as it blends into the galea) and the frontalis muscle as part of the SMAS. The antagonist muscles to the frontalis muscle are the orbicularis oculi, corrugated, and procerus muscles. The frontalis muscle has no bony attachments. Frontalis muscle action produces horizontal forehead lines.
The frontalis muscle receives its blood supply from branches of both the internal and external carotid arteries. The supratrochlear and supraorbital arteries supply the muscle from the inferior margin after they exit the orbit and travel up the forehead. The supraorbital artery can exit from the supraorbital notch/foramen, while the supratrochlear artery exits more medially from the orbit. They are both branches of the ophthalmic artery, which is a branch of the internal carotid artery. The arteries form an anastomosis with one another to form a highly vascularized network.
The lymphatic drainage of the forehead is complicated and not well understood, but it appears the forehead drains mainly into the preauricular nodes and parotid nodes. The main venous drainage occurs between three veins: the supratrochlear vein being most medial, then the intermediate supraorbital vein, and finally, the lateral frontal vein.
The muscles of facial expression receive nerve supply from cranial nerve VII (the facial nerve), which separates into five main branches: temporal, zygomatic, buccal, marginal mandibular, and cervical. The facial nerve exits the skull at the stylomastoid foramen, and its temporal branch crosses over the zygomatic arch, passes through the areolar tissue on the surface of the temporal fascia, and subsequently enters into the frontalis muscle to provide its deep innervation. The supratrochlear and supraorbital nerves that run along their respective arteries penetrate through the frontalis to reach the superficial skin.
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The lateral-most extent of the frontalis muscle where it interdigitates with the orbital orbicularis muscle varies. The lateral brow will have less support from the frontalis in subjects that have a smaller horizontal frontalis muscle, thereby causing more significant brow ptosis and lateral brow ptosis with secondary dermatochalasis in particular. The lateral-most extent of the frontalis muscle where it interdigitates with the orbital orbicularis muscle may be asymmetric in 20%. The right belly of the frontalis muscle is significantly larger than the left side, although electromyographic studies have shown the right and left frontalis muscles generate the same amount of muscle activity. The frontalis muscle may be confluent from right to left with no bifurcation (up to 45% of subjects). Gross asymmetry between the right and left frontalis muscle belly may be found in a third of subjects.
An understanding of frontalis anatomy is essential for reconstructive and cosmetic procedures. In cases where children are born with an absent or dysfunctional levator palpebrae superioris, such as in bilateral congenital ptosis, the frontalis-orbicularis muscle advancement is possible. Another option for ptosis correction utilizing the frontalis is referred to as a frontalis sling or frontalis suspension. The forehead is a site that many people seek to rejuvenate to maintain a youthful appearance. Relaxed skin tension lines, also called wrinkles, form perpendicular to the underlying muscles as we age. Botulinum toxin is an effective tool for treating rhytides and wrinkles and can be injected directly into the frontalis muscle. Fillers are also often placed into the forehead to create volume and fullness in the horizontal wrinkles.
The corrugator supercilii muscle is located at the medial end of the eyebrow. It draws the eyebrow downward and medially, producing the vertical "frowning" wrinkles of the forehead and is considered the principal muscle in the facial expression of suffering.
The corrugator supercilii muscle is located at the medial end of the eyebrow and inserts between the palpebral and orbital portions of the orbicularis oculi muscle. The muscle acts in tandem with the orbicularis oculi muscle. It is a small, pyramidal muscle that lies deep to the frontal portion of the occipitofrontalis and the orbicularis oculi muscles. It arises from the medial supraorbital ridge of the frontal bone and inserts into the dermis of the medial aspect of the eyebrow above the supraorbital margin.
The paired corrugator supercilii muscles have two heads: a transverse head and an oblique head. The transverse head arises from the superomedial part of the orbital rim that serves to pull the brows medially, while the smaller oblique head runs parallel to the depressor supercilii and serves to depress the medial brow. Along with the glabellar muscles and procerus, the corrugator supercilii muscles also function as accessory protractors of the upper eyelid that aid to reduce glare under bright sunlight. The corrugator supercilii muscles serve as mimetic muscles that contract when one is angered or perplexed.
The corrugator supercilii muscle receives blood supply from the supraorbital and supratrochlear arteries of the ophthalmic artery, which is a branch of the internal carotid artery. The supratrochlear artery, which first traverses underneath the frontalis and corrugator supercilii muscles, gradually appears more superficial. The supratrochlear neuromuscular bundle passes through the corrugator supercilii muscles and provides sensation and vascularity to the forehead.
The corrugator supercilii muscle has a dual nerve supply from branches of the temporal, zygomatic, and buccal branches of the facial nerve. The motor nerve supply of the corrugator supercilii muscle originates from the frontal branch of the temporal division of the facial nerve, while the zygomatic branch appears to innervate the oblique head. The supratrochlear nerve (STN) and supraorbital nerve (SON), which are some terminal branches of the frontal nerve, are also closely associated with the corrugator supercilii muscle.
The lateral margin of the corrugator supercilii muscle may differ among different individuals, sometimes extending to the lateral third of the brow. Anatomical variations require consideration during surgical procedures involving the corrugator supercilii muscle.
Complete comprehension of the anatomy of the corrugator supercilii muscle and its associated nerves and blood supply is essential for corrugator supercilii muscle resections during forehead rejuvenation and supraorbital nerve/supratrochlear nerve decompression in migraine headache treatment. Cosmetic physicians may inject neurotoxins into the glabellar complex to maintain an aesthetically pleasing brow position and shape.
As a significant component of the glabellar complex, the corrugator supercilii muscle plays an essential role in maintaining the position of the brow. Complete resection of the corrugator supercilii muscle has been a recommendation for forehead rejuvenation. Complete resection of the corrugator supercilii muscle has also been a recommended approach for the treatment of migraine headaches. Patients with thyroid eye disease with upper eyelid retraction often have overactivity of the accessory muscles that play a role in eyelid closure and the glabellar muscles. Among patients with abnormal Parkinson disease, the “procerus sign” is a clinical sign that is often observed, particularly in patients with progressive supranuclear palsy (PSP).
The orbicularis oculi is a circular muscle that surrounds the eye and is responsible for closing the eyelids and creating crow’s feet wrinkles at the outer corners of the eyes when you smile or squint. It also pulls the brow down.
Injecting neuromodulators into the orbicularis oculi softens crow’s feet wrinkles and reduces the muscle’s ability to pull the brow down, which can create a subtle brow lift when done correctly.
The procerus is a small muscle located at the bridge of the nose, just above the nasal bone. It pulls the skin between the eyebrows down, contributing to horizontal lines at the top of the nose, often called the “bunny lines.”
Injecting the procerus reduces the muscle’s downward pull, smoothing the lines at the root of the nose and softening the expression of anger or frustration.
The frontalis muscle is the sole elevator of the eyebrows, while the orbicularis oculi, corrugators, and procerus muscles all contribute to pulling the brows down. When you inject neuromodulators into the forehead, it’s important to consider the balance between these opposing muscle groups.
A well-balanced forehead injection involves precise and strategic dosing across all the involved muscle groups:
Injecting the corrugator muscle requires particular attention because of its overlap with the frontalis. If you inadvertently inject into the frontalis while treating the corrugators, you may weaken the ability of the frontalis to lift the brows, leading to brow heaviness or droop.
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