This week’s post is from Kelsey Stilson, a PhD student in biology at the University of Chicago, specialising in neurobiology and functional anatomy. It’s a post with teeth, in more ways than one! If you would like to write for Anatomy to You, get in touch via Facebook or Twitter.

I study opossums. Specifically, I study the only North American opossum (or marsupial for that matter), Didelphis virginiana. I can relate to the opossum. Like graduate students, they are nocturnal, will eat anything, and live in urban environments while feeling like they are somehow not a part of it.

Unlike graduate students, they also have extremely flexible ankles that allow them to climb down a tree face first, they are immune to many types of snake venom, and have a body temperature that is, on average, two degrees colder than most mammals (leading to very slow metabolisms and fat opossums). And don’t get me started on their reproductive systems!

I could go on, but this isn’t a post about the evolutionary Disneyland of an animal that is D. virginiana. No, this is a post about a specific part we share with them. This is about their (and your) teeth^[1].

You probably don’t think about your teeth all that much unless you are going to the dentist soon or are a devout flosser. Either way, you probably never really THINK about your teeth. You just want your teeth to look nice and clean. If you are in North America, you are likely especially worried (neurotic some would say) about how your teeth look relative to other people’s teeth (Khalid et al. 2015). However, as D. virginiana will tell us, teeth are much more than calcium slabs of social status. They are part of your mouth’s sensory system.

For example, consider this scenario: It’s late night when you finally get to sit down to dinner. The burrito in front of you is a carefully prepared package of beans, cheese, meat, vegetables, and hot sauce. At least you assume so, because you can’t see any of it. All you can see is the tortilla wrapping around this journey of delights. You pick up the burrito and bite in with your lower and upper incisors, easily cutting through the various densities of food to the tortilla below. You rip off the last bit of tortilla with your incisors, canine, and premolars, then you use your tongue and the angle of your jaw to transport the food the middle of your mouth.

The flavors and textures cause a cacophony of chemical sensations, lighting up the reward centers in your brain (Kubo et al. 2015). As you focus on the flavor of the food your tongue pushes the food to one side of your mouth, carefully moving the various beans and cheeses to the tops of your rounded molars. Your closing jaw muscles fire, bringing your top and bottom molars together, mashing the food. Your cheek and tongue reposition the food as you open your mouth and the whole process starts again. In fact, your mouth carefully adjusts the chew cycle with every bite, positioning bits of meat on top of your molars for mastication and mixing them with saliva so they will be safe to swallow (Chen 2009). Unappreciated effort from your teeth goes into sorting bits for swallowing or continued crunching.

Outside, an opossum tips over your garbage and does much the same with your Thai food from the night before (Figure 3). For both of you, teeth are not just a multitool of slicing, dicing, crushing, and smiling^[2], but also an important sensory system for keeping us alive.

The Sensors

Take tooth pulp, for instance. Pain receptors in the pulp let you know when an infection or cavity is active in your tooth, warning you before the infection can spread. Another system is your dentinal tubules (Figure 4), small liquid-filled tubes that run from the pulp, through the dentine, and out at the gum line. Cells attached to nerves (i.e. mechanoreceptors) are pulled or pushed by movement of the liquid in the tube, which will expand or contract based on temperature, acidity, or aridity. This protects your mouth from these extreme conditions.

Teeth also relay sensations of pressure through the tooth roots, telling your brain and spinal column where food is and warning you if you are about to eat a non-organic substance (say, a small rock that made its way into your burrito or whatever it is the opossum is eating in the trash). These tooth pressure sensors are found in the ligaments between the tooth roots and the tooth socket (see figure 5). The roots are connected to the socket (aka alveolar socket) through a meshwork of rubber band-like ligaments called periodontal ligaments (PDL). This ligament system surrounds the tooth root. As a bite of food is applied to the tooth, the tooth moves around in the socket and the ligaments are stretched. Nerves within and between these ligaments send out pressure signals, letting the brain^[3] know how much pressure is being applied to the tooth and what direction the tooth is being loaded. This gives your brain an estimate of how tough the food is. A somewhat mysterious algorithm in your brain then decides whether to keep on chewing or move the food to the back of your throat to initiate the swallow reflex.

Interactive bit: Try this- close your lips and pull your tongue back, so you create an area of negative pressure. You will feel your teeth being pulled by the suction. That is the feeling of your periodontal ligaments tightening, your nerves firing, and your brain responding to the neural signals. Your front teeth will feel like they are being pulled more. Not only are they flatter and so easier to pull, they are also more sensitive, firing at very small amounts of force and quickly maxing out at higher forces. Your molars, which do the majority of the crushing work, are less sensitive to low force levels. We know this from studies done on humans and other animals where teeth were poked with a force transducer as the neural output^[4] was recorded (Cash and Linden 1982, Trulsson et al. 2006). From this we learned that it’s not only size and position that matter in human teeth, they are literally wired differently depending on where they are in the mouth.

Your teeth also initiate protective reflexes. They slow down the jaw during closing, especially if your molars just managed to break through a hard item like a tortilla chip (Türker and Jenkins 2000). This prevents the teeth from slamming against each other. Teeth also protect you from that rock in your burrito by initiating the ‘fast open reflex’ (Anderson et al. 1970) when the rock hits your teeth. Your mouth springs open so you can remove the rock before it does any more damage. It’s not the end of the world today if you chip a tooth, but in the past causing damage to teeth over time or exposing a tooth root could mean death.

          This multimodal sensory tool is especially noteworthy when it is absent. Human infants have a limit on the types of food they can eat until their teeth erupt and when they do they are fascinated with putting items in their mouth. As humans age they often need teeth replaced. A few fake teeth installed in a tooth row won’t be noticed, but a full set of dentures currently provide a substandard mastication experience. Dentures are important, but many patients would rather take their dentures out and just eat with their gums, because it provides a better sensory experience than false teeth (Bergdhal et al. 2007). We currently have no way to replace the periodontal ligament and nerves once the original tooth is pulled out. Once it’s all gone, it’s gone for good (Figure 6).

The Evolutionary Perspective

It’s time to wrap it up by going back to our burrito scenario. You look up at the sound of the opossum going through the trash can and your pet lizard looks back at you from his tank in the adjacent living room, jealous of your masticating ability. And he should be! Your mammalian mouth has a sensory system fine-tuned for breaking down food before it ever gets to the stomach, effectively prepping it for efficient digestion (Chen 2009). It is this system that allows mammals to be incredibly active, to keep our bodies warm (endothermy), and to process lots of information with our large, calorie-loving brains (Kubo et al. 2015). Your lizard can rip food and move it to the back of his mouth for swallowing, but his teeth are constantly being replaced, so precise food processing is beyond his ability.

Other animals have modified the tooth sensory system for tasks beyond our simple human needs. Narwhals’ large spiraling anterior “unicorn” tusk is a highly modified incisor that senses water salinity and possibly other stimuli in the ocean (Nweeia et al. 2012)  through those dentinal tubules mentioned above. One group of deer, the muntjacs or barking deer, have hinged upper canines that spring forward for aggressive displays and fighting (Aitchison 1946). The babirusa pigs of Indonesia have upper canines that rotate during growth and then grow up through the skin of the face, curling backwards in a spiral. These upper canines break relatively easily and are not used during fighting, so the function (if there even is one specific function) of these canines remains unknown (Macdonald et al. 2016). Perhaps these too are sensory systems?

Baleen whales and anteaters are both toothless, but have keratinous baleen and pads (respectively) that are innervated by the same tooth sensory system. It remains to be tested, but this likely gives feedback during feeding and helps to coordinate jaw movement (Ferreira-Cardoso et al. 2019).

Humans and opossums are just one corner of this strange and wonderful sensory system. Due to space limitations and my own verbosity, the precise link between opossums and humans will have to wait for a future post. For now, I hope you appreciate how tooth sensory systems have kept animals (especially mammals) alive through many an exploratory gastronomic experience. Here’s to many more.

References

Aitchison, J. (1946). Hinged teeth in mammals: a study of the tusks of muntjacs (Muntiacus) and Chinese water deer (Hydropotes inermis). In Proceedings of the Zoological Society of London (Vol. 116, No. 2, pp. 329-338). Oxford, UK: Blackwell Publishing Ltd.

Anderson, D. J., Hannam, A. G., & Mathews, B. (1970). Sensory mechanisms in mammalian teeth and their supporting structures. Physiological Reviews, 50(2), 171-195.

Cash, R. M., & Linden, R. W. (1982). The distribution of mechanoreceptors in the periodontal ligament of the mandibular canine tooth of the cat. The Journal of Physiology, 330(1), 439-447.

Chen, J. (2009). Food oral processing—A review. Food Hydrocolloids, 23(1), 1-25.

Crompton, A. W., & Hiiemae, K. (1970). Molar occlusion and mandibular movements during occlusion in the American opossum, Didelphis marsupialis L. Zoological Journal of the Linnean Society, 49(1), 21-47.

Ferreira-Cardoso, S., Delsuc, F., & Hautier, L. (2019). Evolutionary Tinkering of the Mandibular Canal Linked to Convergent Regression of Teeth in Placental Mammals. Current Biology, 29(3), 468-475.

Khalid, A., & Quiñonez, C. (2015). Straight, white teeth as a social prerogative. Sociology of Health & Illness, 37(5), 782-796.

Kubo, K. Y., Chen, H., Zhou, X., Liu, J. H., & Darbin, O. (2015). Chewing, stress-related diseases, and brain function. BioMed Research International, 2015.

Macdonald, A., Leus, K., & Hoare, H. (2016). Maxillary canine tooth growth in babirusa (genus Babyrousa). Journal of Zoo and Aquarium Research, 4(1), 22-29.

Nweeia, M. T., Eichmiller, F. C., Hauschka, P. V., Donahue, G. A., Orr, J. R., Ferguson, S. H., Watt, C. A., Mead, J. G., Potter, C. W., Dietz, R., & Giuseppetti, A. A. (2014). Sensory ability in the narwhal tooth organ system. The Anatomical Record, 297(4), 599-617. 

Piancino, M. G., Isola, G., Cannavale, R., Cutroneo, G., Vermiglio, G., Bracco, P., & Anastasi, G. P. (2017). From periodontal mechanoreceptors to chewing motor control: A systematic review. Archives of Oral Biology, 78, 109-121.

Trulsson, M. (2006). Sensory‐motor function of human periodontal mechanoreceptors. Journal of Oral Rehabilitation, 33(4), 262-273.

Türker, K. S., & Jenkins, M. (2000). Reflex responses induced by tooth unloading. Journal of Neurophysiology, 84(2), 1088-1092.

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^[1] The study of opossum teeth has a long, proud, and weird history, punctuated by the opossum renaissance of the 1970s (i.e. Crompton and Hiiemae 1970).

^[2] Well, not really smiling for the opossum. That would require cheeks, a subject I would like to touch upon in a future post. [Editor: yes please!]

^[3] I’m using the term “brain” very loosly. For a good overview of brain regions and ganglia used in mastication, see Piancino et al. 2017.

^[4] I seriously recommend looking up this paper. The authors measured neural output from the inferior alveolar nerve of live humans by jabbing a needle through the willing participant’s mouth. Not as easy as it sounds.