An overview of the muscles of facial expression.

15708980021582942881451611215293Lateral view of the facial muscles

We’ve already looked at the innervation of the facial muscles and in particular the consequences of upper motor neurone and lower motor neurone lesions of the facial nerve here. This article will focus on the overall functional anatomy of the muscles of facial expression, the article is not an exhaustive list of the muscles of facial expression.

The facial muscles have a particular feature which makes them different to most other groups of muscles in the body. Whereas most muscles have their origins and insertions on bone and are covered in layers of deep fascia; the facial muscles originate on the bones of the skull, but insert into the skin rather than bone*. Except for the deep fascia of the temporal fossa, parotid gland and neck, there is no deep fascia on the rest of the face. Deep to the skin and superficial layer of adipose tissue, is a layer called the superficial musculoaponeurotic system consisting of muscle and fibrous tissue. However, there is no consensus regarding the exact extent and nature of this layer. The inferior aspect of the SMAS appears to be continuous with the platysma, laterally it appears to be superficial to the parotid-masseteric fascia. Its superior margin is disputed, some studies suggesting it ends just below the zygomatic arch, others stating it is continuous with the temporoparietal fascia. The facial nerve and its branches are deep to the SMAS, while sensory branches of the trigeminal nerve are superficial to the SMAS. During a rhytidectomy (face lift), the platysma and the layer continuous with it on the face (SMAS) are pulled back and up and secured under tension. Interestingly during facial transplant, the facial flap is incised in the sub-SMAS and sub-platysmal plane, to ensure the facial musculature remains intact.

It is the connection of the facial muscles to the skin which permits the muscles to alter the shape and appearance of our face, giving rise to the variety of facial expressions that allow the expression of emotion, hence the term mimetic muscles. Arguably more importantly, from an evolutionary perspective is the ability to protect our special sense organs, help in the ingestion of food & drink and altering the flow of air through our mouth and nose.

The facial muscles arise from the second pharyngeal arch and are innervated by branches of the facial nerve, with the exception of levator palpebrae superiors, which can be functionally considered as a muscle of facial expression, even though it is innervated by the oculomotor nerve and sympathetics.

The muscles of facial expression can be divided functionally into four major groups:

  • Epicranial
  • Circumorbital & Palpebral
  • Nasal
  • Buccolabial

Epicranial group

The epicranial group consists of occipitofrontalis and temporoparietalis. Occipitofrontalis consists of occipitalis, the galea aponeurotica and frontalis. Occiptalis originates from the superior nuchal line and is innervated by the posterior auricular branch of the facial nerve. Occiptalis inserts into the galea aponeurotica, this aponeurosis covers much of the scalp and forms the third layer of the scalp, the anterior end of this aponeurosis forms the origin for the frontalis muscle, forming a v shaped muscle over the forehead. Frontalis is innervated by the temporal branch of the facial nerve. The fibres of frontslis insert into the skin of the forehead and interdigitate with fibres of orbicularis oculi, procerus and the paired corrigator supercilli, helping to lift the upper eye lids and wrinkle our forehead. It is this anterior attachment into the skin of the forehead and the top of the eyes that cause subgaleal haematomas to spread anteriorly, causing black eyes. In our closest evolutionary cousins (bonobos, chimpanzees, gorillas) occipitofrontalis helps to extend the head during quadrapedal gait, allowing them to look forwards. As bipeds, this muscle functionally contributes more to our range of facial expression than any significant extension of the head.

Circumorbital & Palpebral

This group consists of orbicularis oculi, corrugator supercilli and levator palpebral superioris. Orbicularis oculi has three parts, orbital, palpebral and lacrimal. The orbital component consists of the large circular muscle around each orbit, with its palpebral parts extending into the eyelids. The orbital part allows us to squeeze our eyes tightly shut, the palpebral part permits gentle voluntary closure of the lids, or reflex closure when blinking. The palpebral parts attach on the medial and lateral sides of the palpebral fissure to the medial palpebral ligament and lateral palpebral raphe respectively. The lacrimal component of orbicularis oculi attaches to the posterior lacrimal crest and some of its fibres attach to the lacrimal sac, which lays in front in the fossa for the lacrimal sac. The lacrimal component of orbicularis oculi also sends slips of muscle to both tarsal plates. Anterior to the lacrimal sac is the medial palpebral ligament, to which the palpebral part of orbicular oris is attached. It’s thought that contraction of the palpebral and lacrimal parts of orbicularis oculi helps the passage of tears by causing the puncta to dip into the lacus lacrimalis (lacrimal lake) and the lacrimal part dilating the lacrimal sac, causing tears to be sucked into the sac from the lacrimal canaliculi.

Gray896

Lacrimal system

Levator palpebrae superioris, is an interesting muscle, as it may be considered functionally a muscle of facial expression, however unlike the other facial muscles innervated by the facial nerve, levator palpebrae superioris is innervated by both the superior division of the oculomotor nerve and the sympathetics. The muscle can be thought of as having two components, a voluntary (skeletal) component innervated by the superior division of the oculomotor nerve and an involuntary component, known as the superior tarsal muscle which is innervated by the sympathetics. In Horner’s syndrome, the impairment of the sympathetics results in dysfunction of the superior tarsal component of levator palpebrae superioris, resulting in a partial ptosis only, as the voluntary component remains functional.

Nasal

This functional group consists of procerus, levator labii alaeque superioris, nasalis and depressor septi (the latter is often considered as part of nasalis). Procerus draws the medial aspect of the eyebrows together, protecting the eyes from the glare of sunlight. The rest of the group are thought to aid the flow of air through the anterior aspect of the nasal cavity, during inspiration. Levator labii alaeque superioris is responsible for lifting the upper lip and ala of the nose. Nasalis, comprises two parts a compressor and dilator element, thus can both open and close the nostrils. Nasal flaring is a feature visible during intense exercise or those experiencing respiratory distress, which is most likely caused by both nasalis and levator labii alaeque superioris.

Buccolabial

This group consists of orbicularis oris, buccinator, mentalis, risorius, and all of the levator and depressor muscles which attach either to the modulus or the lips themselves. Orbicularis oris was historically thought of as one muscle, however, research shows that it is made up of multiple muscles, with fibres originating from buccinator, the levator and depressor muscles. The fibres from both buccinator muscles contribute to the upper and lower components of orbicularis oris. The medial fibres of buccinator decussate at the corner of orbicularis oris, while its lateral fibres continue across without crossing. The muscle is responsible for closing the oral fissure and puckering the lips. Depressor anguli oris, a component muscle of orbicularis oris is innervated by the marginal mandibular branch of the facial nerve and is particularly susceptible to injury along the lower edge of the mandible. Injury due to iatrogenic trauma or other injury can result in paralysis of this muscle and drooping of the corner of the mouth and persistent drooling.

Another muscle of note is buccinator, despite developing from the second pharyngeal arch and being innervated by the facial nerve, the muscle plays an important role in mastication. When food is being chewed, buccinator is responsible for driving food from the vestibule (the space between the cheek and teeth) back into the oral cavity proper.

* Both cremaster and dartos muscles, neither originate or insert on bone.

References

Netter’s Atlas of Human Anatomy, Frank Netter
Gray’s Anatomy: The Anatomical Basis of Clinical Practice, Susan Standring