Risorius muscle: what it is, functions and characteristics.

Let's see what is the functioning of the risorius muscle and its psychological implications.

You may be familiar with the 55-38-7 rule, known in the professional world as the "Mehrabian rule". According to the postulation made by the emeritus professor of psychology Albert Mehrabian, 55% of human communication lies in body language, 38% in the use of the voice and only 7% of the communicative potential is due to the content of the very words we articulate.

This proposition (if true) shows that, however "advanced" we may be evolutionarily, much of our behavior is still conditioned by instincts inherited from our ancestors. Undoubtedly, a picture communicates more than a thousand words.

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

What is the risorius muscle?

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

The human body has more than 600 muscles. They perform every imaginable function, from pumping Blood in 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 including those muscles that can be contracted voluntarily by the organism, i.e., the skeletal or striated muscles.

Outside this definition would be the smooth musculature (which lines the digestive tract, for example) and the cardiac musculature, since their contractions and distensions are involuntary and are not produced with a conscious purpose. As you can imagine, the risorius muscle falls into the category of the striated musculature, as we can contract it at will in order to gesticulate..

Origin of the risorius muscle

The risorius muscle, together with the arterial conglomerate that nourishes it, derives from the second pharyngeal arch of the mesoderm (layer located between the endoderm and ectoderm), which is formed approximately in the 4th week of gestation of the fetus.

For its part, the innervation of this musculature derives from the cells of the neural crest.. These cells (NCC) are defined as a totipotent cell population originating from the dorsal portion of the neural tube, 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 risorium is a muscle exclusive to 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 fact further evidences the phylogenetic closeness with our primate relatives.

Structure and function.

As we have said, the risorius muscle is located on both sides of the face, is small in size and presents 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 in the skin of 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" because it does not involve the skin around the eyes.. Compared to a real smile, it elevates the labial commissures, along with the zygomaticus major and minor, among other muscular structures. The smile is a truly complex gesture reserved for only a few living beings, as 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 in the external carotid artery and communicates with the structures of the superficial part of the face, such as the muscle that concerns us here. The transverse artery of the face also participates in its irrigation.. On the other hand, the facial and transverse veins drain the deoxygenated blood from the risorius, which end up flowing into the jugular veins.

Moving to the nervous system, and like all the musculature involved in facial expression, the risorius 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 around us.

Do we all have a facial muscle?

Although you may be surprised by the answer, the risorius muscle presents an important physiological variety among individuals. Without going any further, an investigation studied the shape and location of the risorius in 18 Caucasian cadavers and the results were surprising.

Seven of them had no risorius directly, two showed inconclusive results and only four presented a completely symmetrical structure.. Thus, the risorium is considered to present variety in terms of bilateral symmetry, asymmetry and presence or absence.

The importance of the risorius in society

In general, we use about 12 muscles to smile, although this number may depend on various factors and the emotion we are trying to communicate. Some of those involved in almost all cases are the following: the levator anguli oris, upper lip, orbicularis oris, risorius, zygomaticus major and zygomaticus minor muscles.

Laughter and smiling have historically been considered signs of joy, well-being and pleasure at both individual and social levels.. These forms of communication serve to quickly manifest an optimal mood, but their physical relationship is not always related to positive emotional issues. There are 3 main types of smile:

• Duchenne smile: this is the most studied, the one that involves the zygomaticus major and minor muscles. It is the only one considered genuine and it also modifies the orbicularis oculi muscles, near the eyes.
• Professional smile: the one used to show cordiality, but not necessarily 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.

In addition, in the rest of hominins, smiling does not always imply complicity.. In other primates, showing a closed mouth with teeth stuck together is primarily a gesture of submission, so it is speculated that the "goodness" we have attributed in modern society to the smile derives, in part, from this primordial 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 about to occur. The animal is preparing its oral structure to take a bite out of its potential threat, and thus free itself from danger. All these gestures are in the genetic makeup of primates, as they are not learned over time based on 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 laugher and the rest of the muscles involved in this gesture act in a primitive and unlearned way: smiling is in our genetic code.

Summary

Who would have thought that such a small and apparently anecdotal muscle would give us so much to talk about? Although the chuckle alone 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 presented to the individual. This gesture can be sincere, cordial, ironic or feigned, but the most important thing is that it expresses much more than a string of words in a few seconds.

Bibliographical 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 anatomic origin of the risorius in relation to facial planes. 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 Radiologic Anatomy, 37(2), 147-151.