Introduction

The procerus muscle is a pyramidal-shaped muscle of facial expression part of the glabellar complex. The procerus muscle is derived from the second pharyngeal arch, innervated by branches of the facial nerve, and supplied by the facial artery via the external carotid artery. The structure and functionality of the procerus muscle and the associated glabellar complex are of clinical importance in cosmetic and surgical procedures, including botulinum toxin injection, injectable fillers, and blepharoplasty. 

Structure and Function

The procerus muscle is a pyramidal-shaped muscle arising from the fascia of the superior nasal region, near the junction of the nasal bones, and the superolateral nasal cartilage.[1] The procerus muscle fibers run superiorly and merge with the frontalis muscle. Muscle fibers insert into the skin between the eyebrows. 

The procerus muscle is located inferomedial to the frontalis muscles bilaterally. The corrugator supercilii, depressor supercilii, and orbicularis oculi muscles are in close proximity, located just lateral to the procerus muscle bilaterally. The nasalis muscle is inferior to the procerus muscle.

The procerus muscle works with other muscles part of the glabellar complex. The glabellar complex includes bilateral fibers of the procerus, frontalis, corrugator supercilii, depressor supercilii, and orbicularis oculi muscles. These muscles depress the medial eyebrow, producing transverse cutaneous furrows, and also assist nostril flaring.[1] 

Embryology

The neural crest cells, endoderm, mesoderm, and ectoderm form structures of the head and neck. During the fourth week of embryonic development, neural crest cells migrate into the pharyngeal arches.[2] The mesoderm of the second pharyngeal arch differentiates into a core of muscle primordium that eventually becomes the muscles of facial expression. The facial nerve penetrates the core of muscle primordium of the second pharyngeal arch, thus innervating the muscles of facial expression.[3]

Blood Supply and Lymphatics

The procerus muscle receives its blood supply via branches of the facial artery. The facial artery is a branch of the external carotid artery that supplies the superficial face. Lymphatics from the face, scalp, and neck drain into the superficial ring of lymph nodes; these lymph nodes are known as the pericervical collar. This ring of lymph nodes includes the submental, submandibular, parotid, mastoid, and occipital lymph nodes.

Nerves

The procerus muscle receives nervous innervation from branches of the facial nerve. Recall that the facial nerve innervates the muscles of facial expression (derived from the second pharyngeal arch). Not all sources agree on the exact branches innervating the procerus muscle. Research has found the buccal, temporal, and zygomatic branches of the facial nerve innervate the procerus muscle.[4] Another study described the angular nerve, arising from both the buccal and zygomatic branches of the facial nerve, as the primary innervation of the procerus muscle.[5] 

Muscles

The procerus muscle works with other muscles part of the glabellar complex. The glabellar complex includes bilateral fibers of the procerus, frontalis, corrugator supercilii, depressor supercilii, and orbicularis oculi.[1]

Physiologic Variants

The exact size and position of the procerus muscle may vary between individuals. Neurovascular innervation may also differ. As mentioned previously, the precise branches of the facial nerve innervating the procerus are not exact but may include the buccal, temporal, and/or zygomatic branches.[4][5] Likewise, variations in the facial artery may exist.

Surgical Considerations

The arterial supply of the procerus and glabellar complex is vital for surgical procedures, including botulinum injections and injectable fillers. Disruption of the normal vasculature can cause necrosis of the supplied tissues. Similarly, disruption of the normal nervous innervation of the procerus muscle and glabellar complex may cause unwanted facial drooping of the medial eyebrow region. 

Clinical Significance

The procerus muscle is of importance in cosmetic and surgical procedures. The procerus and the larger glabellar complex are a frequent target for botulinum toxin injection to ease or prevent skin wrinkles. Botulinum toxin is a protein that inhibits the release of acetylcholine into the neuromuscular junction, thus preventing muscle contraction. Therefore, injection of botulinum toxin into the glabella (including the procerus muscle) will paralyze its function, preventing cutaneous furrows and associated wrinkling. The anatomy of the facial arterial supply is of greater importance regarding the use of fillers. Fillers are injectable products comprising hyaluronic acid, poly-L lactic acid, calcium hydroxylapatite, or polymethylmethacrylate that help fill in wrinkles or "plump up" skin for cosmetic purposes. The injection of these fillers directly into an artery can cause injection necrosis. Injection necrosis is uncommon, but the glabella is one of the most common sites where it will occur.[6] Blepharoplasty is a surgical procedure used to treat eyebrow ptosis or forehead wrinkling. Knowledge of the structure and function of the procerus and associated glabellar complex muscles is vital for a surgeon to correct eyebrow ptosis or forehead wrinkling associated with the medial eyebrow depressors.[7]

The function of the procerus muscle and associated glabellar complex is a useful tool during a routine physical exam. Physicians can test the muscles of facial expression to ensure the facial nerve is grossly intact. Instructing the patient to pull their eyebrows down as if making an expression of anger will show intact facial nerve innervation and procerus muscle function. In pediatrics, examination of the glabella can help to recognize dysmorphic facies, such as those seen in Wolf-Hirschhorn syndrome. Wolf-Hirschhorn syndrome is a developmental disorder involving chromosome 4 with dysmorphic facial features described as a "Greek warrior helmet" that includes a prominent glabella.[8][9] In neurology, the "procerus sign" is a term used as a clinical tool to diagnose early progressive supranuclear palsy. Progressive supranuclear palsy is an uncommon neurodegenerative disorder that is often misdiagnosed as Parkinson's disease. Researchers argue the procerus sign is due to a dystonic reaction causing horizontal forehead wrinkling. However, the "procerus sign" may be a misnomer as the corrugator supercilii muscles may be the primary muscles involved.[10] The clinical significance of the procerus sign is controversial as many patients may have physiologic forehead wrinkling associated with age. The glabellar tap sign, or glabellar reflex, is another examination tool that involves repetitive tapping of the glabella to elicit a blink response. A normal response includes blinking only after the first few taps. Abnormal responses involve continued blinking with repetitive tapping. Clinicians once used this diagnostic tool to diagnose Parkinson's disease, but researchers have found the glabella reflex associated with other neurodegenerative disorders and normal aging.[11][12]