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The risorius muscle is a muscle of facial expression located laterally to the opening of the oral cavity. It is also known as the 'smiling muscle' as its bilateral contraction pulls the angles of the mouth sideways, producing a smile. The risorius is a highly variable muscle, ranging from a fleshy slender fascicle to a broad thin superficial fan, and is reported to be unique to humans and African apes. It is also believed to be absent in most people.
| Characteristics | Values |
|---|---|
| Type | Muscle of facial expression |
| Location | Lateral to the opening of the oral cavity, in the buccolabial group of muscles |
| Form | Variable, ranging from a fleshy slender fascicle to a broad thin superficial fan |
| Origin points | Zygomatic arch, parotid fascia, masseteric fascia, fascia over the mastoid part of the temporal bone, etc. |
| Insertion points | Modiolus and overlying skin of angle of the mouth |
| Action | Pulls the angle of the mouth laterally and slightly superiorly, producing a smile |
| Innervation | Buccal branches of facial nerve (CN VII) |
| Arterial supply | Superior labial artery |
| Absence | Absent or asymmetrical in some people, and absent in most people of Asian descent |
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What You'll Learn
The risorius muscle is a muscle of facial expression
The risorius muscle is a slender, narrow, and thin muscle of facial expression. It is deemed the "smiling muscle" as its bilateral contraction pulls the angles of the mouth laterally and slightly superiorly, producing a smile. The risorius muscle is highly variable and inconsistent, believed to be unique to African apes and humans. If present, it is found in the superficial layer of the facial muscles on either side of the lips, overlying the buccinator muscle.
The risorius muscle originates from the masseteric fascia, specifically the parotid fascia of the parotid gland and the masseter muscle. It then converges medially, taking a horizontal course towards the angle of the mouth. The muscle fibres interlace with other muscles forming the modiolus, a dense fibromuscular mass. The risorius muscle inserts into the skin at the angle of the mouth, with potential muscular bundle variations. This insertion allows the risorius to pull the angle of the mouth sidewise, creating expressions of pleasure and laughter.
The risorius muscle demonstrates high variability, with different arrangements of muscle fibres observed in cadaver studies. This variability extends to its origin points, which can include the zygomatic arch, parotid fascia, fascia over the masseter, platysma, and fascia over the mastoid. The risorius muscle receives motor innervation from the buccal branch of the facial nerve (CN VII) and arterial supply primarily from the superior labial artery.
The risorius is part of the buccolabial group of muscles and is absent in most people, further highlighting its unique nature among muscles. Its presence contributes to the formation of facial expressions, particularly those associated with pleasure and happiness. The risorius muscle's ability to pull back the angle of the mouth, or Orbicularis Oris, is integral to the act of smiling, though additional muscles are involved in completing the expression.
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It is highly variable and inconsistent
The risorius muscle is a highly variable and inconsistent muscle of facial expression. It is believed to be unique to African apes and humans, but it is absent in most people. When present, it is found in the superficial layer of the facial muscles on either side of the lips, overlying the buccinator muscle. It originates from the parotid fascia of the parotid gland and the masseter muscle, and inserts into the apical portion of the modiolus of angulus oris. However, it has numerous and very variable origins, and its peripheral attachments may include the parotid fascia, masseteric fascia, fascia overlying the mastoid part of the temporal bone, or the zygomatic arch. Its form is also highly variable, ranging from a fleshy slender fascicle to a broad thin superficial fan.
The risorius muscle is often referred to as the "smiling muscle" because it pulls the angle of the mouth sidewise during smiling or laughing. This action creates an incomplete smile, as wrinkling of the eyes or raising of the lips requires the involvement of the zygomaticus muscle. The risorius muscle receives motor innervation from the buccal branch of the facial nerve (CN VII) and arterial supply from the superior labial artery.
The high variability of the risorius muscle makes it challenging to distinguish its fibers, and detailed knowledge of their different morphological arrangements is essential when planning and performing facial procedures. This variability is further highlighted by a study that dissected 80 hemifaces of Asian cadavers and reported three different types of risorius muscle based on insertion patterns: the zygomaticus risorius, the platysma risorius, and the triangularis risorius. These different types of risorius muscles may permit a range of functions, including retracting the labial commissures, being part of the lip elevators, or even depressing the mouth angle downwards to form an expression of sorrow.
The risorius muscle is a fascinating example of the complexity and diversity of human anatomy. Its variable nature and involvement in facial expressions highlight the importance of understanding its morphology when considering facial procedures or studying the mechanics of emotion expression.
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It is deemed the 'smiling muscle'
The risorius muscle is a very thin muscle of facial expression that enables us to smile. It is unique to humans and African apes. It is also highly variable and inconsistent, and may be absent in some people. When contracted, the risorius pulls the angle of the mouth laterally and slightly superiorly, producing a smile.
The risorius is a slender and narrow muscle that belongs to the buccolabial group of muscles. It is found in the superficial layer of the facial muscles on either side of the lips, overlying the buccinator muscle. The muscle fibres of the risorius converge medially, taking an almost horizontal course towards the angle of the mouth. It interlaces with other muscles that converge towards the mouth, forming a dense, mobile, fibromuscular mass called the modiolus.
The risorius is deemed the 'smiling muscle' due to its function of pulling the mouth from the sides when smiling. Its origin of attachment is typically the parotid fascia, but it may also involve the fascia over the masseter muscle. The risorius receives motor innervation from the buccal branch of the facial nerve (CN VII) and arterial supply from the superior labial artery.
The risorius muscle presents a high degree of variability, and its specific functions may differ among individuals. For example, it may act to retract the labial commissures, elevate the lips, or even depress the mouth angle downwards, forming an expression of sorrow. The risorius muscle is an interesting aspect of human anatomy that contributes to our ability to express a range of emotions through facial expressions.
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It pulls the angle of the mouth sideways
The risorius muscle is a very thin muscle of facial expression that enables us to smile. It is also known as the "smiling muscle". It is located laterally to the opening of the oral cavity. The risorius muscle is highly variable and inconsistent, and it is believed to be unique to African apes and humans. If present, it is found in the superficial layer of the facial muscles on either side of the lips, overlying the buccinator muscle.
The risorius muscle pulls the angle of the mouth sideways. It originates from the masseteric fascia and inserts into the skin at the angle of the mouth. Upon activation, the risorius pulls the angle of the mouth laterally, producing facial expressions of pleasure and laughter. This action is used when laughing, grinning, and smiling. Its bilateral contraction pulls the angles of the mouth laterally and slightly superiorly, resulting in a smile.
The risorius muscle has a very variable form, ranging from a fleshy slender fascicle to a broad thin superficial fan. It has numerous and variable origin points, which are sometimes inconsistent. These origin points include the zygomatic arch, parotid fascia, fascia over the masseter anterior to the parotid gland, and fascia enclosing the platysma. From these origin points, the muscle fibres of the risorius converge medially, taking an almost horizontal course towards the angle of the mouth.
The risorius muscle is innervated by the buccal branch of the facial nerve (CN VII) and receives its arterial supply primarily from the superior labial artery. It is important to note that the risorius muscle may be absent or asymmetrical in some individuals.
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It is innervated by the buccal branch of the facial nerve
The risorius muscle is a very thin muscle of facial expression that enables us to smile. It is highly variable and inconsistent, and it is believed to be unique to African apes and humans. The risorius is a slender and narrow muscle that belongs to the buccolabial group of muscles. It is found in the superficial layer of the facial muscles on either side of the lips, overlying the buccinator muscle.
The risorius muscle has a very variable form, ranging from a fleshy slender fascicle to a broad thin superficial fan. It has numerous origin points, including the zygomatic arch, parotid fascia, and fascia over the masseter anterior to the parotid gland. From these origin points, the muscle fibres of the risorius converge medially, taking a horizontal course towards the angle of the mouth.
The risorius muscle is innervated by the buccal branch of the facial nerve (CN VII). It receives motor innervation from this branch, which controls the movement of the risorius muscle. The facial nerve is the seventh cranial nerve and is responsible for supplying motor function to the muscles of facial expression. It has several branches, including the buccal branch, which specifically innervates the risorius muscle.
The risorius muscle is responsible for pulling the angle of the mouth sidewise during smiling or laughing. Its bilateral contraction pulls the corners of the mouth laterally and slightly superiorly, producing a smile. This muscle is unique in that it has no bony origin, and its fibres can be arranged in different patterns, which may permit different functions. For example, it can act as a retractor of the labial commissures, a lip elevator, or it may help depress the mouth angle downwards, forming an expression of sorrow.
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Frequently asked questions
The risorius is a very thin muscle of facial expression located laterally to the opening of the oral cavity. It is also known as the 'smiling muscle' as its bilateral contraction pulls the angles of the mouth laterally and slightly superiorly, producing a smile.
The risorius muscle pulls the angle of the mouth sidewise, such as during smiling or laughing. It is innervated by the buccal branch of the facial nerve (CN VII) and receives its arterial supply mainly from the superior labial artery.
The risorius muscle is believed to be unique to African apes and humans. However, it is absent or asymmetrical in some people and is reported to be absent in most people.
