Take a good look at the following link that takes you to an image of the temporal muscle in the Wikipedia:Wikipedia Temporal Muscle

The motor units of the many muscles that move about the mandible are pointing out to every possible direction. I would like to point out that they can’t be active, all of them, simultaneously. Agonist and antagonist muscles don’t work at the same time.  Like the rest of our body, muscular activity in one direction switches off the activity of other muscles that would have an antagonistic direction of force.

Then, open another browser window from the following Wikipedia link:Wikipedia Medial Pterygoid Muscle

Put these browser windows side by side on your computer screen and watch. It is easily understood that the medial pterygoid and the temporal muscles are antagonistic for each other, for the most part of their respective motor unit fibers. Both these muscles are the elevators of the mandible and they should be the main agonists in the process of gum chewing. However, simultaneous operation of these two muscles is not possible. The most part of temporal muscle fibers would be drawing the mandible up and back. This movement conflicts with the most part of fibers of the medial pterygoid muscle, that are almost directly opposite, but at least perpendicularly oriented to those of the temporal muscle. There must be a mechanism that prevents a force-directional conflict between these two muscles.

There is a simple solution – timing. The point of interest is the timing of the burst of activity in each of these two muscles.

The temporal muscle and the medial pterygoid can never act simultaneously. The temporal muscle moves the jaw up, the medial pterygoid moves the jaw medially, but whenever the other one is active, the other one can’t be active. One after the other: first the jaw moves up by the activity of the temporal muscle, then the jaw moves medial by the activity of the medial pterygoid muscle. We normally like to see the gum-chewing to happen in a smooth lateral swing of the mandible from lateral excursion into the maximum intercuspal position:

Looking at the jaw-closing phase of the chewing process in detail, however, there can be just two main directional muscular force vectors happening, up and medial. These two events don’t happen simultaneously. A slow-motion presentation would show a stepwise locomotion of the mandible as follows:

I am a dentist, but that’s not the only reason I like to watch people chewing. The ability to chew is, in fact, a social skill. Amicable social cohesion in a group is best created at a dinner table. The jaw movements of the table-mates pace the discussions and the social atmosphere of the dinner group. Not sharing the dinner, not showing one’s jaws moving, would cast an individual outside the dinner-table group society. Take yourself a break and watch the following YouTube video in a separate browser:Beldent Casi Identicos chewing gum ad

It is a commercial advertisement by Beldent chewing gum company. You could see identical twins chewing gum. Take another careful look, and see how individual people deal differently with their jaw muscles while chewing gum. Others have broad lateral excursions of their mandible, whereas others seem to be careful just to move their mandible carefully up and down. The police sergeant in this video seems to me to present the sort of confident, assertive smoothness of his jaw movements.  He has got a beautiful lateral swing of his mandible closing in. First, the temporal muscle seems to squeeze completely the chewing gum bolus, after which the medial pterygoid draws the jaw back to the middle.

In contrast, the young man in white pullover seems to me to be unconfident in his jaw movements.

There are those less fortunate individuals who seem to be more cautious in their jaw movements. First, they do the initial crushing of a part of the bolus, then they move their mandible a little bit to the middle direction, then a bit of crushing-down again, then again the jaw movement proceeds a little into the middle. To me it looks like the white pullover guy would feel uncomfortable, as if he would have difficulties to decide whether to chew his gum or not. In a dinner table, it could be interpreted as if this young man would have difficulties to accept the same food that the rest of the table-mates are enjoying. A social stigma.

Yet I believe, the inability to chew smoothly for this white pullover guy and his co-sufferers, is not so much about the dynamics of social group behavior, it can be better explained by the difference in the anatomy of the dental arches of these individuals. The difference in chewing patterns between the police officer and the young man in white pullover is the result of how smoothly their opposed jaw-elevator muscle-force-direction-vectors are timed to co-ordinate.

In my previous articles in the www.blogbites.net I have already explained how the sensory information between the back teeth and the front teeth differs. The sensory information from the back teeth stimulates the temporal muscle, whereas sensory information from the front teeth switches off the activity of the temporal muscle and causes a burst of activity in the medial pterygoid muscle instead. The anatomical set-up of teeth on dental arches determines and guides the activity of the agonist and antagonist muscles of mastication. I have also discussed these matters in my previous articles, such as “Filibustering in the House of Parliament of Occlusion” and “Anterior guidance illustrated”. Sign up to follow my blog. Forthcoming articles discussing the details of jaw-closing business are underway.