- Animal bodies bend and move when locomotion happens.
- Whole-body motion changes the movement of cerebrospinal fluid deep within the spinal cord and brain.
- Spinal cord sensory receptors detect fluid motion associated with body movement and tune motor circuits.
- Physical activity is a critical part of maintaining brain and body health.
We have all heard the old adage that humans are mostly made of water. Right away, lots of folks would think about blood as a fluid that's mostly water. But how many think about the brain and spinal cord floating with and surrounded by fluid? That cerebrospinal fluid has been something often ignored but serves hugely important functions.
A big part of being an animal is moving around to get food or avoid being food. All animals have a way to "locomote," from cilia on single cells to fins on fish to arms and legs on you and me. That locomotor movement carried out by our limbs moves the bulk of our bodies around from place to place quickly, slowly, and all speeds in between.
Walking With a Spinal Cord in Motion
Safely moving around means knowing what's in the environment within which that movement is happening. That's where our sensory system plays a critical role. Vision is hugely important but can't often be useful in critical times like when we slip, trip, or stumble. We need to rely on our somatosensory system and the vast array of receptors in the skin, muscles, and tendons to tell us about mechanical changes in what we are intending to do.
Research has investigated mechanosensation in the spinal cord but this has mainly been in animals like zebrafish and lamprey eels moving in an aquatic environment. The relevance and function of such a sensorimotor pathway in terrestrial animals like humans and mice has been less studied.
Everything is Connected by Water
Which brings us to Yuka Nakamura and colleagues at Niigata University in Japan. Over a number of studies, but culminating in a recent publication, these researchers carefully mapped connections within the mouse spinal cord. They wanted to know where the signals from mechanical sensing neurons deep within the spinal cord where cerebrospinal fluid moves actually project to and what they might do to spinal cord circuitry that controls limb movement.
What's super cool is that these neurons, despite the literally accurate but clunky name of "cerebrospinal fluid contacting interneurons", provide a great way for sensing the effects of whole body movement and control of posture in animals just like you, me, and our pets. These Japanese scientists show clearly that intact signalling from within the spinal cord due to the movement of fluid arising from locomotion actually tunes and supports the movement itself. This work by Nakamura and colleagues adds to the ever-accumulating evidence that the structural and functional connectivity between spinal fluid regulation and movement control are evolutionarily conserved within vertebrates.
Be Like Water, My Friend
On a practical level, this immediately resonated with me as a martial artist. In martial arts training (and, of course, many other practices such as yoga, pilates, Qigong, etc.) we constantly focus on postural stability and alignment, dynamic limb movement, and whole-body motion. While it may not look like walking necessarily, I often consider and teach martial arts as a kind of highly specialized locomotion. The takeaway from the work of Nakamura and friends is that there is mechanistic evidence for the integrating role and holistic effects that this approach contains.
This is another excellent example of the importance of physical activity for all animals—including humans—for overall brain and body health. So, the next time you think about forgetting about form while out for a run, bike ride, martial arts training, or whatever, keep in mind you are being monitored. Your slouching, but also your good postural control, is constantly being "watched over" by the neurons deep within your spinal cord.
(c) E. Paul Zehr (2023)
Nakamura Y, Kurabe M, Matsumoto M, Sato T, Miyashita S, Hoshina K, Kamiya Y, Tainaka K, Matsuzawa H, Ohno N, Ueno M. Cerebrospinal fluid-contacting neuron tracing reveals structural and functional connectivity for locomotion in the mouse spinal cord. Elife. 2023 Feb 21;12:e83108. doi: 10.7554/eLife.83108. PMID: 36805807; PMCID: PMC9943067.