Victor Forge
The upper and lower lips are essential to human facial expressions, phonation, sensation, mastication, physical attraction, and intimacy. The upper lip, in particular, plays a critical role in facial expressions, such as smiling, grimacing, and expressing sadness or seriousness. Several muscles are involved in elevating the upper lip, including the levator labii superioris, levator labii superioris alaeque nasi, risorius, levator anguli oris, zygomaticus major, and zygomaticus minor muscles. These muscles work in conjunction with each other and the upper lip's other muscles to elevate and evert the upper lip, contributing to various facial expressions.
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What You'll Learn
Levator labii superioris
The levator labii superioris (LLS) muscle, also known as the quadratus labii, is a muscle in the human body used for facial expression and mouth and
The LLS receives blood from the angular artery branches inferiorly and the infraorbital artery superiorly. The angular artery is a terminal facial artery branch, while the infraorbital artery diverges from the maxillary artery's third part. Both the facial and maxillary arteries are external carotid artery branches. The LLS is innervated by the zygomatic and buccal branches of the facial nerve (CN VII). Vascular supply to this muscle is provided by the facial artery and the infraorbital branch of the maxillary artery.
The LLS elevates and everts the upper lip, particularly during smiling. This muscle also broadens the nostril by elevating the nasal ala and upper lip, pulling the alar facial crease and skin outward and upward. The LLS and surrounding perioral muscles facilitate complex mouth and nose movements for humans' varied facial expressions and speech. The LLS helps to produce various facial expressions, such as smiling or disdain. The LLS is a direct tractor of the upper lip, meaning that it inserts into the 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. This action exposes the maxillary teeth and deepens the nasolabial lines, an important function in several facial expressions, including smiling, smugness, and contempt.
The LLS is a muscle of the buccolabial group of muscles of facial expression. This is a broad group of muscles that, besides the LLS, also contains the levator labii superioris alaeque nasi, zygomaticus major, zygomaticus minor, levator anguli oris, risorius, depressor labii inferioris, depressor anguli oris, mentalis, orbicularis oris, incisivus superior and inferior, and buccinator muscles.
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Levator labii superioris alaeque nasi
The levator labii superioris alaeque nasi is one of eleven facial muscles that create movement in the mouth. It is a direct tractor of the upper lip, inserting into the lip directly and acting upon it without the use of an intermediary. By contracting, it helps other buccolabial muscles to elevate and invert the upper lip. This exposes the maxillary teeth and deepens the nasolabial lines, which is important for several facial expressions, such as smiling, smugness, and contempt.
The muscle 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 towards the upper lip, gradually tapering and becoming triangular in shape. It fills the bottom of a triangular space bounded laterally by the zygomaticus minor and medially by the levator labii superioris alaeque nasi. The levator labii superioris alaeque nasi is innervated by the zygomatic and buccal branches of the facial nerve (CN VII).
The infraorbital branch of the maxillary division of the trigeminal nerve (cranial nerve V2) supplies the upper lip. The mental nerve, derived from the mandibular division of the trigeminal nerve (cranial nerve V3), allows for sensation from the lower lip. The muscles acting on the lips are derived from the second pharyngeal arch mesoderm and are part of the panniculus carnosus, which forms attachments with the dermis, resulting in dimpling of the overlying skin.
The levator labii superioris alaeque nasi works in conjunction with the levator labii superioris, zygomaticus minor, and other buccolabial muscles. The levator labii superioris originates from the lateral nasal aspect, extending laterally toward the zygomatic bone. It receives blood from the angular artery branches inferiorly and the infraorbital artery superiorly. The angular artery is a terminal facial artery branch, while the infraorbital artery diverges from the maxillary artery's third part.
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Risorius
The risorius is a narrow and slender facial muscle that belongs to the buccolabial group of muscles. It is highly variable and inconsistent, and it may be absent in a significant minority of people. The risorius muscle elevates the angle of the mouth, drawing it upwards and outwards, and is therefore known as the 'smiling muscle'. It contributes to various facial expressions, such as smiling, smirking, grinning, laughing, and frowning, either on its own or in conjunction with other muscles.
The risorius is found in the superficial layer of the facial muscles, on either side of the lips, and it overlies the masseter muscle and the buccinator muscle. It has a variable form, ranging from a slender bundle to a wide, thin fan shape. The muscle fibres converge medially, taking a horizontal course towards the angle of the mouth, where they interlace with other muscles to form the modiolus, a dense, mobile, fibromuscular mass.
The risorius has multiple origin points, which are sometimes inconsistent. These include the zygomatic arch, the parotid fascia, the fascia enclosing the platysma, and the fascia over the mastoid process and the masseter muscle. The risorius is innervated by the buccal branch of the facial nerve (CN VII) and is mainly supplied by the superior labial artery, a branch of the facial artery, which passes deep to the risorius and superficial to the buccinator muscle.
The risorius muscle is unique to homininae, including African great apes and humans. Its contraction pulls the angle of the mouth laterally and slightly superiorly, contributing to facial expressions. However, due to its proximity to the masseter muscle, it may be unintentionally affected during Botox injections, resulting in unnatural facial expressions.
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Levator anguli oris
The levator anguli oris (LAO) is a muscle of facial expression that elevates the corners of the mouth. It works in tandem with the zygomaticus major muscle to raise and lateralize the oral commissure, moving it obliquely, superiorly, and laterally. This muscle is responsible for deepening the nasolabial folds and enabling smiling.
The LAO is a paired strap-like muscle of the face, located above the angles of the lips. It is part of a large group of muscles of facial expression called the buccolabial group. This group also includes the levator labii superioris alaeque nasi, levator labii superioris, zygomaticus major, zygomaticus minor, risorius, depressor labii inferioris, depressor anguli oris, mentalis, orbicularis oris, incisivus superior and inferior, and buccinator muscles.
The LAO is crucial in maintaining the oral commissure's resting position. It is a component of interest during lip repositioning or reconstructive surgeries, particularly in the context of flaccid facial paralysis management. The muscular and closely associated mucosal component makes the LAO and depressor anguli oris ideal tissue donors for lip defects, as the sphincter tone can be maintained by transferring the innervated muscle.
The LAO's fibres travel anteroinferiorly and insert into the modiolus, a fibromuscular structure in the oral commissure lateral to the orbicularis oris at the corner of the mouth. The LAO's primary function is to elevate the mouth's corners, and variations in the contributions of this muscle and the zygomaticus major to the oral commissure's movements define an individual's unique smile.
The upper and lower lips are essential for facial expression, phonation, sensation, mastication, physical attraction, and intimacy. The lips contain mucosal membranes, vermilion, and cutaneous surfaces. The upper lip, or labium superius oris, extends from the nasolabial folds to the inferior margin of the nose. The lower lip, or labium inferius oris, encompasses the region between the lateral commissures and the labiomental crease of the chin.
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Zygomaticus major
The zygomaticus major 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 carry out the functions of the cheeks and lips. The zygomaticus major muscle is responsible for pulling the angle of the mouth superolaterally, which facilitates speech and creates facial expressions.
The zygomaticus major muscle works in conjunction with other muscles of facial expression that elevate the lip, such as the risorius, levator labii superioris, and levator labii superioris alaeque nasi. Together, these muscles enable a range of facial expressions, including smiling, laughing, and forming expressions of disdain, contempt, or smugness.
The zygomaticus major muscle is also involved in deepening and raising the nasolabial furrow, which is the groove between the nose and the mouth. This action contributes to the formation of certain facial expressions. Additionally, the muscle may be used in reconstructive surgery to replace lost tissue in the lips.
The position of the zygomaticus major muscle can vary, and it may occur in a bifid form, with two fascicles that are partially or completely separate but adjacent. It is believed that cheek dimples are caused by this bifid variation of the zygomaticus major muscle.
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Frequently asked questions
The levator labii superioris, zygomaticus major, levator anguli oris, zygomaticus minor, and risorius muscles all elevate the upper lip.
The levator labii superioris is a superficial elevator muscle of the upper lip. It starts on the maxilla and inserts into the upper lip's skin.
The risorius muscle pulls the corner of the mouth laterally in facial activities such as grinning and laughing.
The zygomaticus minor is a superficial elevator muscle that sits beneath the orbicularis oculi. It elevates the upper lip outward and upward, allowing the maxillary teeth to show.
