What kind of feelings would you ascribe to getting a high-five? Do you feel the satisfying sting of palm-on-palm action? Or perhaps the ache in your joints from excessive celebration? What about the heat of your friend's hand or the dull, enduring numbness after the party’s over?
This confluence of sensations is not unique. Touch is a notoriously difficult sense to study precisely because the host of feelings is difficult to separate. Various kinds of stimuli, from pressure to pain to temperature, each undergo unique processes when they’re converted from mechanical inputs to electrical signals sent to the brain, a process called mechanosensation.
To make matters worse (or better, depending on how you look at it), we typically experience all these touch sensations simultaneously. This makes isolating the response to a single stimulus tremendously difficult and has so far impeded our understanding of this fundamental physiological process. But figuring out the intricacies of touch could be crucial to developing treatments when the system breaks down, such as when someone starts experiencing chronic pain.
This week scientists at the Yale University School of Medicine announced that they have found a promising new model for describing touch—light touch specifically—at the cellular level: ducks. The team’s research is being presented this week at a meeting of the Biophysical Society in Baltimore, Maryland.
Ducks that feed by touch, such as mallards, teal and shovelers, use their bills to navigate and find food in murky water. Searching for crustaceans and other small prey, they rely on an intense concentration of neurons in their heads—called trigeminal neurons—that connect to sensory organs inside their bills. The Yale team, led by Sviatoslav "Slav" Bagriantsev in collaboration with Elena Gracheva, discovered that 85 percent of these neurons are low-threshold, meaning they respond to incredibly delicate touch. The concentration is much higher than what's typically seen in other vertebrates. Also, the cells respond to touch much more quickly, more intensely and for longer periods of time.
“When you study [a] physiological phenomenon that’s hard to understand, it’s always great to look at an example where a certain physiological feature has been taken to the extreme,” Bagriantsev explains. “There are so few animals that have taken sense of touch to the extreme!”
The team’s next step is to explore how ducks’ superior sensory abilities work at the molecular level, by injecting certain viruses into fertilized duck eggs. This disrupts specific amino acids, the building blocks of proteins, and could help the researchers identify the proteins responsible for making the nerve cells so sensitive, as well as point to their human counterparts. The ultimate hope is that drug developers could identify targets for new compounds that would help treat mechanosensory disorders such as chronic pain and heightened pain response.
Molecular investigations would “bring this model to a whole new level," Bagriantsev says. "People do mice genetics all the time, and right now bird genetics is actually getting better and better.” Eventually he hopes to study mechanosensation in other animal groups, too. “The key is the ability to rely on touch in the absence of visual or olfactory cues. If an animal can do this—we're interested regardless of what the animal is,” he wrote in an email.
But for the next round, the bird world may offer the best candidates. The order of birds known as Anseriformes, which includes ducks, geese and swans, is a place to start, but the true avian superstars of tactile feeding are shorebirds of the order Charadriiformes. When these sandpipers, curlews and other waders plunge their bills into the sand to find food, they can’t see anything at all.
“Beak-probing shorebirds are the champions,” Bagriantsev says. Unlike mice and rats, though, these animals present significant ethical concerns in scientific research because they do not have lab populations bred specifically for testing. Conducting this kind of research on shorebirds would require recovering carcasses from out in the field, a tedious task. Not that lab-reared ducks are a common occurrence either, but fortunately for the research team, there was a local solution to that problem.
“Our study has no ethical concern because we obtained our duck heads from a farm where they’re just byproducts of poultry production,” Bagriantsev says. “There [are] plenty of farms around Yale which grow ducks and chickens and other types of poultry for human consumption. We just happened to be there during the slaughter and pretty much got what they threw away. We isolate the [nerve tissue] from the heads at the farm, put them in a test tube and drive back to the lab just fast enough so the neurons stay alive.”
High-five for dedication.