Birds and humans share an ability that’s not too common among our tetrapod vertebrate cousins–walking on two feet. While some animals can manage on their hindlimbs for a limited time (think prairie dogs, gorillas, and running frilled lizards), true bipedalism is only found among humans, birds, and a few other animals like kangaroos. It’s a handy bodily feature, as using just two feet to walk frees up the forelimbs up for other tasks. In our case, it frees our hands to build tools; for birds it makes their forelimbs available for flight. But how birds and humans manage the complex task of bipedalism appears to be quite different.
Birds, unlike us, have a structure near the bottom of the spinal column called the lumbosacral organ. This bulbous structure, first described in the late 1800s by anatomists, has recently been the subject of more inquiry. What is its purpose? Does it act like a second balancing organ, as its anatomy and behavioral experiments suggest, where lesions in the area cause balance issues? If so, why didn’t we evolve something similar?
2024 Grass Fellow Hannah Martin found this oddity intriguing from her perspective as a cellular and molecular neurophysiologist and wanted to be among the first to investigate the lumbosacral organ’s neurons in detail.
“I’m interested in all the senses that provide us the ability to move around the world,” said Martin. “Previously I worked on the balance organ in the inner ear, and became really interested in this very alien organ in the bird spinal cord because it really had an unknown function.”
Martin first wanted to know if the neurons in the lumbosacral organ are proprioceptive–if they’re capable of perceiving the bird’s position and motion in space. To do that, the neurons would need to have mechanically sensitive ion channels. And if there is evidence of neurons with ion channels that respond to mechanical stimulus, are they the most common and well-known type, the Piezos (specifically either Piezo1 or Piezo2)?
To answer these questions during her fellowship in the Grass Lab at the Marine Biological Laboratory in Woods Hole, Mass, Martin had to first see exactly where the neurons were located in the lumbosacral organ. After isolating the tissue from chick embryos, she bathed them in a dye that stains neurons, finding them clustered at the center of the lobe. Furthermore, the dye she used is most readily absorbed through the large pores of mechanically sensitive channels, so the strong uptake of these neurons is evidence of their function for proprioception.
Next, she set up an experiment where she could provide a mechanical stimulus and measure the electrical response from one neuron at a time. Poke, measure, repeat. Her preliminary results showed that half of the neurons responded to mechanical stimuli. She repeated this experiment in a small number of neurons in the spinal grey matter, finding none were mechanically sensitive.
Lastly, Martin looked for the expression of Piezo genes that code for Piezo ion channels, which convert a mechanical stimulus such as touch into the electrochemical language of neurons. She used a method called fluorescent in situ hybridization chain reaction (HCR-FISH) to see if Piezo genes were active in the neurons in the lumbosacral organ, but interestingly, didn’t find a detectable signal. As a control, she used the same method with the dorsal root ganglion–well known for its sensitivity for mechanical stimulation–and found a strong signal. If the avian lumbosacral organ doesn’t rely on Piezo ion channels, what does it use?
An example of science at its best, Martin answered some of her questions and highlighted new areas to explore. We now know more about the neurons in this mysterious organ, but there are ion channels yet to be discovered and further questions about biological purpose. Martin is equally interested in the details of the lumbosacral organ’s function as she is about its evolutionary significance and possible future applications. “From the perspective of evolution, how do two different body plans get to the same walking strategy? And what are good ways to design sensory systems? For this kind of work, one application you might think about is biomimetic approaches such as bipedal robots.”
Her summer as a Grass Fellow allowed her the freedom to delve deep into the mysteries of neuroscience and evolution while developing the skills she’ll need as a future principal investigator. “This totally, 100%, reaffirmed my desire to be a PI, an academic group lead. It does validate your technical and scientific skills to just be thrown into this environment and undertake a new project,” she said. “I set the stage and laid the seeds for a research direction that I intend to pursue throughout my career.”
Feature image credit: Tightrope-Walking Chicken “Graceful Gertie,” IQ Zoo, Chrome Litho Postcard, circa 1955. Courtesy of Steve Shook, Moscow, Idaho, USA.