Previously unrecognized part of our masticatory muscle discovered – the masseter muscle in the human cheek consists of three instead of two layers –

Anatomical surprise: In one of our masseter muscles, researchers have identified a previously overlooked part. According to this, the so-called masseter muscle does not consist of two, but of three anatomically and functionally delimitable layers. The third, deepest layer is essential for the retraction of our lower jaw, as the scientists report. This means that some textbooks may now have to be adapted.

Anyone who thinks that the human anatomy has long been researched to the last is wrong. In recent years in particular, scientists have tracked down several previously overlooked or newly developed structures in our body – from hidden conduction pathways to ligaments to arm arteries.

Massive muscle with controversial anatomy

Szilvia Mezey and her colleagues from the University of Basel have now made another anatomical find – in our masseter muscles. Among the strands of muscles that move our jaw, the masseter muscle is the most prominent. It begins on the zygomatic arch and extends over the cheek to the lower jaw. If you place your fingers on the back of your cheek and press your teeth together, you can feel it tense up.

In the common anatomy textbooks, the masseter muscle is usually described as consisting of two layers. But that was not always the case: In 1995, the standard work “Gray’s Anatomy” presented the mastication muscle in three layers and a study from 2003 saw it that way. In both cases, the superficial layer was subdivided again.

Additional layer in the depth

“In view of these contradicting descriptions, we wanted to thoroughly examine the structure of the masseter muscle again,” explains senior author Jens Christoph Türp. To do this, the team dissected the jaw area of ​​almost 30 human dead that had been preserved with formaldehyde, analyzed tissue samples and computed tomography images of 16 dead. To see the muscle in motion, the researchers also recorded the muscle anatomy of a living test person using magnetic resonance imaging (MRI).

The newly discovered part of the masseter muscle attaches to the coronoid process of the lower jaw (left). Its fibers also run obliquely to those of the remaining layers of the masseter muscle. © Jens C. Türp / UZB

The surprising result: the deeper layer of the masseter muscle turned out to be subdivided again. In addition to the already known part of this masticatory muscle, there is also a delimited deeper layer. “This deep part of the masseter muscle can be clearly distinguished from the other two layers in terms of its course and function,” reports Mezey. Among other things, the muscle fibers in this layer run at an angle to the ones above.

Important for the retraction of the lower jaw

According to the research team, the observed structures are an independent, previously overlooked part of the masseter muscle. “Our find is a bit as if zoologists had discovered a new vertebrate species,” says Türp. The new muscle layer begins at the posterior zygomatic process and ends at the coronoid process of the lower jaw – a protruding bone spur. This is why the researchers have named this layer the masseter muscle pars coronidea – the coronoid part of the masseter.

The analyzes also revealed the function of the newly discovered muscle part: the location of its attachment points suggests that this layer is involved in stabilizing the lower jaw. More importantly, however: “Of all parts of the masseter muscle, the coronoid part is the only one that can retract the lower jaw,” report Mezey and her colleagues. Without it, we would not be able to move our lower jaw back and forth. (Annals of Anatomy, 2021; doi: 10.1016 / j.aanat.2021.151879)

Source: University of Basel

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