Alec Titan
Smiling is a complex process that involves the coordination of many facial muscles. While there is ongoing debate about the exact number of muscles involved in a smile, with estimates ranging from 10 to 43, it is clear that several muscle groups are required to produce this ubiquitous expression. The zygomaticus muscles, including the zygomaticus major and minor, play a key role by pulling the corners of the mouth upwards and outward. Other muscles, such as the risorius, orbicularis oris, levator labii superioris, and depressor labii inferioris, also contribute to the formation of a smile by moving the lips and mouth. Understanding the muscles behind smiles is important, as it can provide insight into the underlying emotions and social interactions that smiles convey.
| Characteristics | Values |
|---|---|
| Number of muscles involved in a smile | 10 (minimum), 17, 26, 43 |
| Muscles involved in a smile | Zygomaticus major, zygomaticus minor, risorius, orbicularis oris, depressor anguli oris, levator labii superioris, depressor labii inferioris, mentalis, platysma |
| Muscle pairs involved in a smile | 5 |
| Facial nerve that innervates the muscles involved in a smile | Facial nerve (VII) |
| Functions of the facial muscles | Chewing, making facial expressions, talking |
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What You'll Learn
Zygomaticus major
The zygomaticus major muscle is a thin, paired facial muscle that extends diagonally from the zygomatic bone to the angle of the mouth. It is part of the buccolabial group of facial muscles, which form the structure and functions of the cheeks and lips.
The primary function of the zygomaticus major muscle is to pull the angle of the mouth upwards and outwards. In coordination with other muscles, it facilitates speech and creates facial expressions. When working in conjunction with the risorius muscle, it contributes to laughing, which is why they are often referred to as the "laughing muscles".
The zygomaticus major muscle is also responsible for raising the upper lip to bare the upper teeth. It deepens and raises the nasolabial furrow, producing facial expressions such as smiling, disdain, contempt, or smugness. The average zygomaticus major muscle can contract with a force of 200 grams.
This muscle may also have a role in reconstructive surgery. Due to its location and function, it can be used to replace lost tissue in injuries to the lips.
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Risorius
The risorius is a slender and narrow muscle that is part of the buccolabial group of muscles. It is unique to African apes and humans and is found in the superficial layer of the facial muscles on either side of the lips. It is commonly referred to as the "smile muscle" as its contraction pulls the corners of the mouth outwards and slightly upwards, resulting in a smile. The risorius also forms dimples in the cheeks and is not always active in all people.
The risorius muscle is highly variable in form and may be absent in some individuals. It can range from one or more slender bundles to a wide yet thin fan shape. The muscle fibres of the risorius originate from various points, including the zygomatic arch, parotid fascia, and the fascia over the masseter muscle. From these origin points, the fibres converge medially and move horizontally towards the angle of the mouth.
The risorius muscle plays a crucial role in producing facial expressions such as smiles, grins, laughs, and smirks. It works in conjunction with other muscles that converge at the angle of the mouth, forming a dense fibromuscular mass called the modiolus. The risorius is innervated by the buccal branch of the facial nerve (CN VII) and receives its blood supply primarily from the superior labial artery, a branch of the facial artery.
The risorius muscle can be unintentionally affected during botox injections, leading to unnatural facial expressions. Its proximity to the masseter muscle makes it susceptible to the spread of botulinum toxin, which can result in temporary paralysis of the risorius and altered facial movements.
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Orbicularis oris
The orbicularis oris is a complex circular muscle that surrounds the mouth and forms the majority of the lips. It is composed of four independent quadrants that interlace and give the appearance of circularity. It is not a simple sphincter muscle but consists of numerous layers of muscular fibres surrounding the mouth, with different directions.
The muscle is made up of two parts: a larger peripheral part and a smaller marginal part. The peripheral portion, also called the extrinsic portion, originates from the modiolus, arising from the fibres of the buccolabial muscles that attach to it. The marginal portion, or intrinsic portion, consists of a single or double band of muscle fibres that stem from the modiolus toward the midline, passing through the connective tissue of the lips.
The orbicularis oris is responsible for closing, protruding and compressing the lips, and it puckers the lips when contracted. It is one of the muscles used in playing all brass instruments and some woodwind instruments. It is also involved in speech and producing various facial expressions, such as anger and sadness.
The modiolar muscles that contribute to the orbicularis oris include the levator anguli oris, depressor anguli oris, buccinator, risorius, and zygomaticus major muscles. Most of the fibres that comprise the orbicularis oris originate from the buccinator muscle.
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Depressor anguli oris
The depressor anguli oris is a facial muscle that allows a person to move the corners of their mouth downwards. It is also referred to as the "sadness muscle" because its activation is associated with frowning. The depressor anguli oris is a paired triangular muscle situated on either side of the mouth, extending almost vertically upwards. It originates from the oblique line and mental tubercle of the mandible, located on the anterior aspect of the bone. The muscle fibres converge into a narrow fascicle that runs superiorly towards the angle of the mouth.
The depressor anguli oris muscle is supplied by the marginal mandibular branch of the facial nerve and receives motor innervation from the marginal mandibular and buccal branches of the facial nerve. It is also supplied by the inferior labial and mental arteries, which are branches of the facial artery and maxillary artery, respectively. The venous blood is drained through the facial vein into the internal jugular vein.
Inferiorly, the depressor anguli oris is continuous with the platysma muscle and cervical fascia. Along with other muscles that attach at the angle of the mouth, it forms a fibromuscular structure called the modiolus. It is not entirely clear which muscles attach to the modiolus, but some of the certain ones include the depressor anguli oris, buccinator, risorius, zygomaticus major, and orbicularis oris muscles.
The depressor anguli oris plays an important role in facial expression and helps express feelings of sadness or anger. Issues with this muscle can make it difficult to smile, resulting in a lopsided smile or excessive frowning. Some individuals may opt for cosmetic procedures such as Botox injections to manage the constant downward pull of this muscle and improve the appearance of an overly frown-like expression.
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Levator labii superioris
The levator labii superioris is a muscle of facial expression. It is responsible for elevating the upper lip, which makes it critical to the formation of normal facial expressions, as well as speech and nonverbal communication.
The levator labii superioris is also called the quadratis labii superioris or quadratus labii. It arises from the maxilla and zygomatic bones just below the orbit and inserts into the fleshy muscle and dermis of the upper lip. It is a thin, quadrilateral muscle that begins at the infraorbital margin of the maxilla and extends toward the upper lip. The levator labii superioris has three distinct heads: the angular head, the intermediate head, and the lateral head. The angular head emerges from the frontal process of the maxilla inferiorly and forms a medial and lateral section. The infraorbital head inserts into the skin of the upper lip, while the zygomatic head inserts into the nasolabial groove (smile line) in the skin of the upper lip.
The levator labii superioris is one of the muscles involved in smiling. It pulls the upper lip up and contributes to the depth and contour of the nasolabial fold. The constant pull on this muscle will deepen the nasolabial folds over time. It is also the target for treatment in the "gummy smile", where it is used to measurably reduce gingival display.
The levator labii superioris is also involved in other facial expressions, such as expressions of disgust, sadness, and disdain, as well as nasal flaring, retching (vomiting), and oral movements.
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Frequently asked questions
The zygomaticus muscles, also known as the smile muscle, are the main muscles involved in smiling. These muscles pull the orbicularis oris (the circular muscle of your mouth) upwards. Other muscles involved in smiling include the risorius, depressor anguli oris, levator labii superioris, depressor labii inferioris, mentalis, and platysma.
It is difficult to determine the exact number of muscles involved in smiling as it depends on the type of smile and the individual's facial structure. However, it is estimated that a minimum of 10 muscles are used in a subtle, restrained smile that only raises the corners of the lips and the upper lip.
The facial nerve (VII) has various branches that innervate the muscles involved in smiling. When these muscles are activated, signals are sent to the brain indicating that you are smiling. This results in the release of endorphins into the bloodstream, leading to feelings of happiness and reduced stress levels.
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