Dominic Stone
The human face is composed of about 20 muscles that work together to control facial movements and expressions. Smiling is one of the most well-known and universal expressions, transcending cultural and linguistic boundaries. Interestingly, the muscles involved in smiling are quite similar to those involved in frowning, with the direction of the mouth's corners determining the type of expression. The zygomaticus major and minor muscles, the risorius muscle, the depressor anguli oris, and the levator labii superioris muscle are all key players in shaping our smiles. These muscles interact with the orbicularis oris, the circular muscle around the mouth, to create the upward curve of a smile.
| Characteristics | Values |
|---|---|
| Number of muscles that control facial expressions | 20 |
| Muscles involved in smiling | Zygomaticus major, Zygomaticus minor, Risorius, Levator labii superioris, Depressor anguli oris, Buccinator muscle |
| Function of Zygomaticus major muscle | Lifts the corners of the mouth upward |
| Function of Zygomaticus minor muscle | Elevates the upper lip |
| Function of Risorius muscle | Pulls the corners of the mouth back, mimicking a smile |
| Function of Levator labii superioris muscle | Elevates the upper lip |
| Function of Depressor anguli oris muscle | Pulls the corners of the mouth downward, producing a frown |
| Function of Buccinator muscle | Located in the cheeks and plays a role in actions such as blowing, whistling, and sucking |
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What You'll Learn
Zygomaticus major and minor muscles
The human face has about 20 facial muscles that work together to enable us to chew, make facial expressions, and perform other tasks. One of the most recognizable facial expressions is smiling, which is controlled by the zygomaticus major and minor muscles.
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 innervated by the zygomatic and buccal branches of the facial nerve (CN VII) and supplied by the superior labial artery, which branches off the facial artery. When the zygomaticus major contracts, it pulls the angle of the mouth superolaterally, lifting the corners of the mouth and producing a smile. Acting in conjunction with other muscles of facial expression, it also raises the upper lip to bare the upper teeth and produces other facial expressions such as disdain, contempt, or smugness.
The zygomaticus minor muscle is a muscle of facial expression that originates from the zygomatic bone, lateral to the levator labii superioris muscle, and inserts into the outer part of the upper lip. It draws the upper lip backward, upward, and outward, elevating the upper lip and contributing to the expression of a smile. The zygomaticus minor muscle is innervated by the facial nerve (VII) and receives motor innervation from the zygomatic and buccal branches of the facial nerve (CN VII).
Together, the zygomaticus major and minor muscles play a crucial role in producing the expression of a smile, working in synergy with other facial muscles such as the risorius muscle to create the upward curvature of the lips and the distinctive appearance of happiness and joy that we recognize in a smile.
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Risorius muscle
The risorius muscle is a slender and narrow muscle that is part of the buccolabial group of muscles. It is highly variable and inconsistent, and it may be absent in a significant minority of people. When contracted, the risorius pulls the corners of the mouth back, mimicking a smile. However, it does not affect the skin around the smile, and the expression is often interpreted as insincere.
The risorius muscle is located in the superficial layer of the facial muscles, on either side of the lips, overlying the buccinator muscle. It is deemed the 'smiling muscle' due to its function in producing smiles and other facial expressions. The risorius has multiple origin points, including the zygomatic arch, parotid fascia, and fascia enclosing the platysma. From these origin points, the muscle fibres of the risorius converge medially and move horizontally towards the angle of the mouth.
The risorius muscle is innervated by the facial nerve, specifically by one of its branches, either the buccal or the marginal mandibular branch. The muscle receives its arterial blood supply from the facial artery and the transverse facial artery. The facial and transverse facial veins drain deoxygenated blood from the risorius.
The risorius muscle is involved in producing facial expressions such as smiles, grins, and laughs. It contracts in an outward and upward motion, pulling the angle of the mouth laterally. This movement, in conjunction with other facial muscles, helps create a range of expressions, including smiles and frowns.
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Levator labii superioris muscle
The levator labii superioris (LLS) muscle is a muscle in the human body that contributes to facial expressions and mouth and
The LLS is a short, paired triangular muscle of the face and belongs to the buccolabial group of muscles of facial expression. This group includes muscles such as the levator labii superioris alaeque nasi, zygomaticus major, zygomaticus minor, levator anguli oris, risorius, depressor labii inferioris, and orbicularis oris. The buccolabial muscles work together to control the shape, posture, and movements of the lips.
The LLS originates from the zygomatic process of the maxilla and the maxillary process of the zygomatic bone, just superior to the infraorbital foramen. It courses inferomedially toward the upper lip, gradually tapering and becoming triangular in shape. The LLS inserts into the upper lip directly, acting upon it without the use of an intermediary. By contracting, the LLS helps other buccolabial muscles elevate and invert the upper lip.
The LLS is innervated by the zygomatic and buccal branches of the facial nerve (CN VII). Its vascular supply is provided by the facial artery and the infraorbital branch of the maxillary artery. Impairment of the LLS can result from damage to the facial nerve or its zygomatic or buccal branches. This impairment can lead to mild paresis or complete paralysis of the muscle.
The LLS plays a crucial role in producing facial expressions, specifically by elevating the upper lip. This elevation is essential for expressions such as smiling, where it works in conjunction with other muscles to create the characteristic smile shape.
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Orbicularis oris muscle
The human face consists of numerous muscles that work together to control fine movements and produce facial expressions. One of these muscles is the orbicularis oris muscle, which is responsible for various facial expressions and speech.
The orbicularis oris muscle, also known as musculus orbicularis oris, is a complex, multi-layered muscle that surrounds the mouth and forms the majority of the lips. It is composed of two parts: a larger peripheral part and a smaller marginal part. The peripheral portion, or the extrinsic portion, originates from the modiolus and consists of fibres from five buccolabial muscles: the levator anguli oris, depressor anguli oris, buccinator, risorius, and zygomaticus major muscles. The marginal portion, or the 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 muscle is not a true sphincter muscle, as previously thought. Instead, it consists of four independent quadrants that interlace and give the appearance of circularity. It is composed of numerous strata of muscular fibres surrounding the mouth, with fibres derived from other facial muscles inserted into the lips and fibres proper to the lips. The deep fibres of the orbicularis oris muscle act as constrictors, contributing to the sphincteric action of the mouth, while the superficial fibres divide into upper and lower bundles. The upper bundle, or the nasal bundle, is the common insertion point for various muscles, including the zygomaticus major and minor, levator labii superioris, and others. The lower bundle, or the nasolabial bundle, receives fibres from the depressor anguli oris muscle on each side and forms the ridges at the philtrum.
The orbicularis oris muscle is a critical structure in cleft lip cases, as it is responsible for movements of the orofacial region and contributes to the contour of the lip region. Its proper function and orientation are crucial for normal lip function. In rare cases, newborns may be born with an absence of this muscle on one side of the face, resulting in a partial droop on the affected side. Treatment for this condition may include botulinum toxin injections directly into the muscle.
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Buccinator muscle
The buccinator muscle is a thin, quadrilateral-shaped muscle that occupies the space between the maxilla and mandible at the side of the face. It is the main component of the cheek and provides it with structure and tightness. It compresses the cheek against the molar teeth, which is important for keeping food in the centre of the oral cavity and preventing the cheeks from being bitten during mastication.
The buccinator muscle is also called the 'musculus buccae' or 'musculus buccalis' in Latin, and the 'muscle of the cheek' in English. It has a number of origin points, which is the basis for subdividing the muscle fibres into superior, inferior, and posterior parts. The superior part arises from the outer surface of the alveolar process of the maxilla, opposite to the three maxillary molar teeth. The inferior part arises from the alveolar part of the mandible, opposite to the three mandibular molar teeth. The posterior part of the buccinator originates from the anterior margin of the pterygomandibular raphe, a tendinous band behind the third molar, spanning from the pterygoid hamulus to the posterior end of the mylohyoid line.
The buccinator is an oral muscle of facial expression and is involved in actions such as blowing, whistling, and sucking. It is also the main muscle involved in playing wind instruments, as it expels air from the distended cheeks. The fibres of the buccinator converge towards the angle of the mouth, where they interlace with other muscles that attach at the same site, including the orbicularis oris, risorius, depressor anguli oris, and zygomaticus major. These muscles blend to form a dense fibromuscular mass called the modiolus.
After forming the modiolus, the fibres of the buccinator continue towards the upper and lower lips to comprise the peripheral part of the orbicularis oris muscle. The uppermost and lowermost fibres of the buccinator diverge and enter the superior and inferior lips, respectively. The central fibres of the buccinator split into two and decussate so that the inferior half enters the superior lip, and the superior half enters the inferior lip.
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Frequently asked questions
The zygomaticus major and minor muscles are involved in smiling. The major muscle lifts the corners of the mouth upward, while the minor helps elevate the upper lip.
The risorius muscle pulls the mouth back, mimicking a smile, but this does not affect the skin around the smile. The depressor anguli oris, the levator labii superioris, and the buccinator muscle are also involved in smiling.
The human face has about 20 facial muscles that are essential for chewing and making facial expressions. They originate from bone or fascia and attach to the skin of your face, allowing it to be manipulated.
Smiling is considered a universal expression of happiness, joy, and goodwill. Interestingly, activating the muscles associated with smiling can trick your brain into thinking you are happy, releasing endorphins into your bloodstream.
