There are few biological functions that we take for granted more than gait, the intricate symphony of motion that happens almost automatically when we walk or run. Gait is programmed deep into the nervous system of animals, an activity so robust that it is maintained even when large segments of brain are removed. Those crude, early experiments suggested that the machinery for gait was somehow packed into the spinal cord. But scientists are still mapping the networks of neurons – called central pattern generators – that give fish, mice, and men the ability to propel themselves forward at slow or fast speed.
For the past 5 years, the laboratory of Kamal Sharma, associate professor of neurobiology at the University of Chicago, has been looking at one key component of this gait machinery: the V2a interneuron. Interneurons are the go-betweens of the spinal cord, coordinating the message between the motor and sensory systems. In animals with a gait that alternates left and right feet as they walk or run, interneurons are a likely candidate for coordinating this action – suppressing the left limb as the right moves forward, and vice versa. So after a paper identified the developmental background of the V2a interneurons, Sharma and postdoctoral researcher Steven Crone began studying whether these cells were in fact an important component of the gait system.
The quickest way to determine a cell type’s importance is to remove it, and that’s just what Crone did in a 2009 paper – genetically inserting a toxin that kills V2a interneurons. Other than appearing smaller than their wild-type peers, the mice without the neurons seem to be fairly normal. When placed on treadmills running at a slow speed, the modified mice alternate left and right paws just as the normal mice did. But when the treadmill was sped up, a difference was revealed – where normal mice continued alternating left and right at a faster pace, mice lacking V2a interneurons broke out into a “gallop,” moving left and right limbs together.
That result suggested that the system controlling gait at slow speeds may differ from the gait system when an animal is traveling at faster speed. That dynamic has been seen in other animals, such as the zebrafish, where different spinal cord interneurons are active when swimming at high speed compared to low speed.
“The question is when you are walking slow versus when you are walking fast, do you use the same neurons that are firing at different speeds, or are you actually recruiting more neurons?,” Sharma said.
But zebrafish are translucent, and it’s thus easier to track the activity of neurons live while they are moving. Mice don’t have the benefit of translucency (yet), necessitating a quite different approach to monitor V2a interneuron activity. Researchers removed the spinal cord from mice and artificially stimulated “fictive locomotion” – the electrical activity that normally accompanies limb movement, even when there are no longer limbs to move. They could then monitor the activity of V2A interneurons in the spinal cord during this simulated locomotion, and see whether a larger number were involved when a faster pace was simulated.
The study, published Tuesday in Nature Communications, found that only some of the V2a population are involved in locomotion – their activity aligns with the rhythm of the stimulation. But when the frequency of stimulation is increased, to replicate a faster pace of locomotion, more V2a neurons join the rhythm, supporting that similar recruitment principles are at work in both zebrafish and mice. However, unlike the zebrafish, a large subset of V2a interneurons in mice never synced up with fictive locomotion, suggesting they could be dedicated to other motor programs.
“If you look at this from an evolutionary sense, then it makes some kind of logic, because fish wouldn’t need these for, say, lung breathing,” Sharma said. “Maybe these neurons have always existed, and they could provide some kind of an excitatory drive to a central pattern generator, to regulate how fast or slow that circuit works.”
In fact, Sharma’s laboratory is exploring the role of V2a interneurons in breathing as well as many other “automatic” processes, such as chewing and swallowing. The theory is that these neurons are the light switch for central pattern generators, providing the excitatory input that starts them up when you want to walk or breath or chew. But when those light switches break down – as may occur in degenerative diseases such as Parkinson’s or amyotrophic lateral sclerosis (Lou Gehrig’s disease) – a person may lose the ability to maintain their gait at faster speeds, Sharma theorizes.
“As we get older, we don’t have trouble walking slow, we have trouble walking fast,” Sharma said. “Is that just weakness, or is a part of our circuit that was dedicated to walking fast somehow not working any more, or not working as it is supposed to?”
Zhong, G., Sharma, K., & Harris-Warrick, R. (2011). Frequency-dependent recruitment of V2a interneurons during fictive locomotion in the mouse spinal cord Nature Communications, 2 DOI: 10.1038/ncomms1276
Crone SA, Zhong G, Harris-Warrick R, & Sharma K (2009). In mice lacking V2a interneurons, gait depends on speed of locomotion. The Journal of neuroscience : the official journal of the Society for Neuroscience, 29 (21), 7098-109 PMID: 19474336
McLean DL, Fan J, Higashijima S, Hale ME, & Fetcho JR (2007). A topographic map of recruitment in spinal cord. Nature, 446 (7131), 71-5 PMID: 17330042