Risorius muscle: what it is, functions and characteristics

You may be familiar with the 55-38-7 rule, known professionally as the “Mehrabian rule”. According to the postulation made by the emeritus professor of psychology Albert Mehrabian, 55% of the communication in the human being falls on the body language, 38% in the use of the voice and only 7% of the communicative potential is due to the content of the words that we articulate.

This proposition (if true) shows that, no matter how "advanced" we are evolutionarily, great part of our behavior continues to be conditioned by instincts inherited from our ancestors. Without a doubt, a picture communicates more than a thousand words.

Based on this very interesting premise, today we will tell you everything about a very small muscular structure, but with impressive functionality from a communicative point of view in humans. Keep reading, because here you will find all relevant information on the risorius muscle.

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What is the risorius muscle?

The risorius muscle is a skeletal (striated) muscular structure found on both sides of the face, small in size and triangular in shape. Before continuing naming the physiological characteristics of this muscle, we must lay certain foundations as far as the muscular system is concerned.

The human body has more than 600 muscles. These perform every function imaginable, from pumping blood through the heart to allowing us to lift a heavy item at work. In any case, if we refer to the muscular system, we are only encompassing those muscles that can be contracted voluntarily by the body, that is, the skeletal or striated.

Outside this definition would be the smooth muscle (which lines the digestive tract, for example) and the heart, since its contractions and distensions are involuntary and do not occur with a purpose aware. As you can imagine, the risorius muscle It is within the category of striated musculature, since we can contract it at will in order to gesticulate.

Origin of the risorius muscle

The risorius muscle, together with the arterial conglomerate that feeds it, derives from the second pharyngeal arch of the mesoderm (layer between the endoderm and ectoderm), which forms approximately at week 4 of gestation of the fetus.

For his part, the innervation of this musculature derives from the cells of the neural crest. These cells (CCN) are defined as a totipotent cell population originating from the dorsal portion of the tube. neural, from where they migrate through the developing organism to give rise to a wide variety of structures.

In addition, it should be noted that it has been postulated that the risorio is an exclusive muscle of hominins, that is, a subfamily of primates included in the family Hominidae. Thus, it is believed that only gorillas, chimpanzees, and humans have the ability to smile mediated by this muscular structure. Undoubtedly, this data further evidences the phylogenetic closeness to our primate relatives.

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structure and function

As we have said, the risorius muscle is located on both sides of the face, it is small in size and has a triangular shape. It is located in the superficial plane of the face, on each side of the cheeks, close to the upper and lower jaw bones. It is inserted into the skin at the corner of the mouth, so its simultaneous contraction allows us to smile, among other expressions of joy.

In any case, it is striking to know that, by itself, the risorius muscle generates a "false smile", since it does not involve the skin around the eyes. Compared to a real smile, this raises the lip corners, along with the zygomaticus major and minor, among other muscular structures. The smile is a truly complex gesture reserved for a few living beings, since no more and no less than 12 muscles participate in it.

Vascularization and innervation

The risorius muscle receives the blood necessary to nourish its cells from the facial artery, which originates from the external carotid and communicates with the structures of the superficial part of the face, such as the muscle that we see here regards The transversal artery of the face also participates in its blood supply.. On the other hand, the facial and transverse veins drain the deoxygenated blood from the risorium, which ends up flowing into the jugulars.

Moving to the nervous system, and like all the muscles involved in facial expression, the risorium is innervated by cranial nerve VII, also known as the facial nerve. This is the somatic motor nerve of the cutaneous muscles of the face and neck, which allows us to perform multiple movements and gestures to relate to each other and to the three-dimensional environment that surrounds us. surrounds.

Do we all have laughing muscle?

Although the answer may surprise you, the risorius muscle presents an important physiological variation between individuals. Without going any further, an investigation studied the shape and location of the risorio in 18 Caucasian cadavers and the results were surprising.

7 of them did not have a risorium directly, 2 showed inconclusive results and only 4 presented a completely symmetrical structure.. Thus, it is considered that the risorio presents variety in terms of bilateral symmetry, asymmetry and presence or absence.

The importance of laughter in society

In general, we use about 12 muscles to smile, although this number can depend on various factors and the emotion that is trying to communicate. Some of those involved in almost all cases are the following: the levator flexure muscle mouth, the upper lip, the orbicularis oculi, the risorium, the zygomatic major and the zygomatic minor.

Laughter and smile have historically been considered as signs of joy, well-being and pleasure at both individual and social levels.. These forms of communication help us to quickly express an optimal state of mind, but, nevertheless, their physical relationship does not always have to do with positive emotional issues. There are 3 main types of smile:

• Duchenne smile: it is the most studied, the one that involves the major and minor zygomatic muscles. It is the only one that is considered genuine and also modifies the orbicularis musculature, near the eyes.
• Professional smile: one that is used to show cordiality, but that does not have to be preceded by a real positive feeling.
• Sardonic smile: a medical condition caused by tetanus, where the muscles of the face contract involuntarily due to the toxins tetanolysin and tetanospasmin.

Besides, in the rest of hominins the smile does not always imply complicity. In other primates, showing the closed mouth with its teeth stuck together is primarily a gesture of submission, so it is speculated that the "goodness" that we have attributed to smiling in modern society derives, in part, from this primal interaction of our ancestors.

On the other hand, if a primate curves its lips and presents space between its teeth, that "smile" indicates that an attack is soon to take place. The animal is preparing its oral structure to bite its possible threat, and thus get rid of danger. All these gestures are found in the genetic gear of primates, since they are not learned over time based on the observation of adult individuals.

For example, you may be surprised to learn that a blind child who has never seen a smile will do so under the same circumstances and in the same way as a person without any visual impairment. The smile and the rest of the muscles involved in this gesture act in a primal and unlearned way: smiling is in our genetic code.

Summary

Who was going to tell us that such a small and apparently anecdotal muscle was going to give us so much to talk about? Although the laugh by itself does not cause the appearance of a smile, it is an important element in this gestural mechanism that is so essential in today's society.

Without going any further, it is estimated that adult human beings smile an average of 25 times a day, depending on the number of social interactions that occur to the individual. This gesture can be sincere, cordial, ironic or fake, but the most important thing is that it expresses much more than a string of words in a few seconds.

Bibliographic references:

• Bae, J. H, Lee, J. H., Youn, K. H., Hur, M. S., Hu, K. S., Tansatit, T., & Kim, H. J. (2014). Surgical consideration of the anatomical origin of the risorius in relation to facial plans. Aesthetic surgery journal, 34(7), NP43-NP49.
• Deny, J., Sudharsan, R. R., & Kumaran, E. m. (2021). An orbicularis oris, buccinator, zygomaticus, and risorius muscle contraction classification for lip-reading during speech using sEMG signals on multi-channels. International Journal of Speech Technology, 1-8.
• Diogo, R., Wood, B. A., Aziz, M. A., & Burrows, A. (2009). On the origin, homologies and evolution of primate facial muscles, with a particular focus on hominoids and a suggested unifying nomenclature for the facial muscles of the Mammalia. Journal of Anatomy, 215(3), 300-319.
• Germann, A. M., & Al Khalili, Y. (2019). Anatomy, Head and Neck, Risorius Muscle. StatPearls [Internet].
• Kim, H. S., Pae, C., Bae, J. H., Hu, K. S., Chang, B. M., Tansatit, T., & Kim, H. J. (2015). An anatomical study of the risorius in Asians and its insertion at the modiolus. Surgical and Radiological Anatomy, 37(2), 147-151.