A colony of honeybees annoints one of its fortunate females to be their queen, with the mission in life of breeding future generations. Most of the other denizens in the hive are worker bees, consigned to making honey-filled combs. Ants, termites and many other insect varieties have similar social structures.
So does a unique species of mammal.
Naked mole-rats (Heterocephalus glaber) spend their 10-to-30-year life span entirely underground. In the deep darkness of their subterranean tunnel network, H. glaber has little or no use for ears or eyes. The weirdo rodents’ designated breeder queen is waited upon by one to three breeding males, while the 20 to 300 drones in the colony take care of housekeeping business.
The snout end of a mole-rat’s wrinkly, pinkish, hairless hide is equipped with a pair of sharp, curved incisor teeth, about half an inch in length. Full-grown, a mole-rat may measure 1.5 inches in diameter by 6 long.
Neuroscientist Kenneth Catania at Vanderbilt University in Nashville, Tenn., specializes in studying this improbable beast among others. “Mole-rats use their overlapping incisors for a lot of different tasks,” he observed, “for carrying objects, for transporting their young. [They also use] them for digging, which is one of the reasons the mouth doesn’t close over the teeth.
“Their tunnel-system habitat,” he continued, “presents a complex sensory motor challenge. To dig through soil that has hard objects in it,” Catania explained, “exerting a lot of force may overtax this mouth structure that is somewhat delicate. If the animal ran into something hard and broke a tooth, that would be a temporary disaster. They need a lot of feedback from the brain about what they’re digging through and how to regulate the excavation pressure at high speed, while they’re exerting a lot of force.
“And then of course,” he added, “those teeth are used for eating, which is a very complex behavior for all of us mammals. Consuming food is something that really hasn’t been explored much in terms of the brain’s organization.”
Catania, an assistant professor of biological sciences at Vanderbilt, is senior author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), released online April 8, 2002. Its title: “Somatosensory cortex dominated by the representation of teeth in the naked mole-rat brain.”
Evolution Short-Changed Eyes, Ears
“We looked at the organization of the neocortex in this species, the naked mole-rat,” Catania told BioWorld Today, specifically at the sense-of-touch area the somatosensory region that processes information for touch. And we found that this animal, which lives underground primarily, doesn’t have much use for vision or for hearing. A much larger part of its brain is devoted to touch than other mammals that are close to its size and mammals in general. As they went underground millions of years ago,” he went on, “evolution seems to have taken over the brain regions that were devoted to vision and hearing. Mainly, vision is the one that seems to be greatly reduced at the expense of the area that processes touch information. What really surprised us was the huge amount of the cortex taking up a third of the touch areas devoted to the tooth representation.
“The primary somatosensory cortex,” Catania explained, “is a region found in all mammals including humans. It’s the area that receives the major input of touch coming up from lower brain centers, and originally from the sensory receptors on the skin surface of the body. Laboratory rats,” he pointed out, “are model in vivo systems for looking at somatosensation, but they don’t tell us much about the brain’s teeth representations. In fact, we don’t really know what’s going on with how the cortex represents teeth in almost all the other mammals. This PNAS paper opens up a system for looking at how the neurons are processing that kind of touch information from teeth, and also how it might change, depending on how the teeth are used in terms of behavior.
“People don’t know much about the plasticity of the representation of the teeth,” he went on. “We’ve learned a lot about how the hands and arms and touch areas in primates and other mammals change, based on injury or different usages of the sensory surfaces. So stimulating a sensory surface a lot makes it an expansion of the representation in the map on the brain. If you lose your arm in an accident, you may have reorganizations related to that loss of input that may cause a lot of problems such as phantom limb sensations.
“It turns out that the face representation will come to reactivate the area that used to reactivate the hand,” Catania pointed out. “That seems to be one of the underlying mechanisms that causes this phantom limb syndrome, which can be really painful and disturbing.
“We know much about what happens when you lose a limb,” he observed. “Relatively few people lose their arm or hand. But nearly everybody loses teeth, and we don’t know how that brain area reorganizes. It turns out that a phantom tooth sensation occurs in a small percentage of people who have dental work. And there’s a lot we need to understand [about] how you relearn to use sensory surfaces after dental work, when you’re trying to use a dental implant, for example. And how the mechanoreceptors may respond to learning the new surfaces that people are putting into their mouths.”
Knowledge Gap In Tooth Representation
Catania made the point, “There’s a huge gap in our knowledge of how mouth representation is arranged in the cortex. That hiatus is really glaring when you look at the health issues related to oral structures. It’s ubiquitous in terms of what goes on in people’s teeth, especially compared to losing a hand. Dental implants and replacements are very common, and as the population ages, and lives longer than they have historically, total loss of teeth may occur in many elderly. When you lose inputs from the arm, the face takes over for the hand telling the brain the hand is being touched when really the face is being touched. But,” Catania concluded, “the hand cortex presumably signals, Hey! I’m hand!’ And that may help explain phantom limb pain.”