How does your right arm know to be as long as your left? What tells your body how tall you are? Why does a giraffe’s neck grow tall, as its body stays the same size? As much as we have learned in biology, we still don’t know how organisms are aware of their size–at least not on a cellular level. New research from a team of physicists suggests that subtle chemical frequencies tell organisms how large they are.
A frequency is a pattern, and nature, whether we notice it or not, is full of patterns. We see them in plants and animals, in chemicals, in rocks, and even in outer space. These patterns count for much more than just aesthetics; we can use them to answer big questions about how our world works.
Sizing up the problem
Physics grad student at the Universität Saarland, Frederic Folz, didn’t plan on doing biology research when he started his Bachelor’s thesis in 2016, but after talking with physics professors Giovanna Morigi and Karsten Kruse, they came up with an interesting idea, one that integrated electrodynamics with biology.
A few years ago, Nobel Prize-winning quantum physicist Robert B. Laughlin, wrote a unique paper on a topic he called “the length problem”. He wrote, “On the matter of length determination, per se, very little progress has been made beyond Thompson’s 1917 treatise on biological form.” Laughlin goes on to suggest living things can essentially “measure themselves” through a process commonly seen in electrodynamic systems.
See, for an organism to regulate their growth, their body must first somehow be aware of their own size–they must have some biological way of measuring themselves. Then, they must have some way to respond to this information, alerting their appendages to grow, shrink, or maintain the status quo. Perhaps, thought Laughlin, biological organisms are measuring their size using the frequency of oscillating chemical signals in their cells.
“For example” explained Folz, “if you have a bacteria, you can consider it as a cavity where chemicals are traveling. When certain resonances occur, the cell could stop its growth”
Folz and his collaborators sought to test Laughlin’s idea using axons, the long fibers connected to neurons, as a model. These spindly filaments are perfect to study, because unlike other parts of the body, they have a relatively simple geometry. When an axon grows, it’s really only growing lengthwise, unlike most body parts with complex and irregular growth cycles.
Your axons and most other cells are part of a highway system of chemicals racing back and forth, delivering messages throughout your organs. In the nervous system, motor proteins called kinesin and dynein run back and forth from the root to the end of the axon in a loop.
Inside the axon, as chemicals travel to the growth cone (the end of the axon) they jumpstart the motion of other chemicals to the soma (the root of the axon). If the axon lengthens, then this cycle would slow, and we’d see what’s called a “negative feedback loop”. Researchers have studied this process in the lab, finding that if the concentrations of these motor proteins are changed, then the length of the axon itself will change.
Beyond nerves
Together with fellow physicist Lukas Wettmann, the researchers recreated the axon’s chemical system in their model, breaking it down into a mechanics problem. Simply put, If chemical “I” moves back and forth at a constant velocity “X”, completing “N” oscillations over a certain time, then how far did it travel?
Because it will take the chemical more time to travel through a longer axon, a lower frequency means a longer axon. Just as you can measure the length of an axon, their equations can also be used to measure the frequency and shape of the oscillations themselves.
An artists rendering of a neuron. The axion is the long spindly filament at the end of the cell.In the future, the team is looking to expand its model beyond axons to other, more complex organ systems. They think this model could be applied not only to the length problem but to a myriad of biologic processes, including how tiny cells can measure their internal pressure.
“We can also cast our equations in the physical form as an electronic circuit. We can build a circuit that emulates the behavior we see here [in the axon]…You can use this mechanism to regulate different physical quantities that, at first glance, have nothing to do with one another” said Folz
Even though circuits and brain cells don’t seem like they have much in common, the nature of their signals allows researchers to extract information about how physical elements move inside them. While it started as a biology project, this research could grow into much more than nerves.