Discovery of a new muscle layer on the jaw


The newly discovered muscle layer extends from the back of the cheekbone to the anterior muscle process of the lower jaw. (S = surface layer, D = deep layer, C = coronoid layer). Credit: Jens. C. Türp, University of Basel / UZB

Human anatomy still has a few surprises in store for us: Researchers at the University of Basel have discovered a hitherto neglected part of our jaw muscles and have described this layer in detail for the first time.

The masseter muscle is the largest of the muscles in the jaw. If you place your fingers on the back of your cheeks and clench your teeth together, you will feel the muscle tighten. Anatomy textbooks generally describe the masseter as composed of a superficial part and a deep part.

Today, researchers led by Dr Szilvia Mezey from the Department of Biomedicine at the University of Basel and Professor Jens Christoph Türp from the University Center for Dentistry Basel (UZB) described the structure of the masseter muscle as consisting of a additional third, even deeper layer. In the scientific journal Annals of anatomy, they propose to give this layer the name Musculus masseter pars coronidea — in other words, the coronoid section of the masseter — because the newly described muscle layer is attached to the muscular (or “coronoid”) process of the jaw. lower.

The anatomical study was based on a detailed examination of the formalin-fixed jaw musculature, computed tomography scans, and analysis of stained tissue sections from deceased people who had donated their bodies to science. This was in addition to the MRI data of a living person.

As if a new animal species had been discovered

“This deep section of the masseter muscle is clearly distinguished from the other two layers in terms of course and function,” explains Mezey. The arrangement of muscle fibers, she says, suggests that this layer is involved in stabilizing the lower jaw. It also appears to be the only part of the masseter that can pull the lower jaw back, i.e. towards the ear.

A review of historical anatomy studies and textbooks reveals that the structure of the masseter muscle has raised questions in the past. In a previous edition of Grey’s Anatomy, from the year 1995, the editors also describe the masseter muscle as having three layers, although the studies cited are based on the jaw musculature of other species and contradict each other in part.

Other individual studies from the early 2000s also reported three layers, but they split the superficial section of the masseter into two layers and agreed with the standard work in their description of the deeper section.

“In view of these contradictory descriptions, we wanted to take another in-depth look at the structure of the masseter muscle,” explains Türp. “While it is generally accepted that anatomical research over the past 100 years has left nothing to chance, our discovery is a bit like the discovery of a new species of vertebrate by zoologists.”

Risk of frailty associated with changes in oral function

More information:
Szilvia E. Mezey et al, The human masseter muscle revisited: first description of its coronoid part, Annals of anatomy – Anatomischer Anzeiger (2021). DOI: 10.1016 / j.aanat.2021.151879

Provided by the University of Basel

Quote: New muscle layer discovered on the jaw (2021, December 20) retrieved December 20, 2021 from

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