The role of the spinal cord is often simplified to that of a simple relay station, carrying messages between the brain and the body. However, the spinal cord can actually learn and remember movements on its own.

A team of researchers details how two different neuronal populations enable the spinal cord to adapt and recall learned behavior in a way that is completely independent of the brain. These remarkable findings, published in Science, shed new light on how spinal circuits might contribute to mastering and automating movement. The insights could prove relevant in the rehabilitation of people with spinal injuries.  

The spinal cord modulates and finetunes our actions and movements by integrating different sources of sensory information, and it can do so without input from the brain. What’s more, nerve cells in the spinal cord can learn to adjust various tasks autonomously, given sufficient repetitive practice. How the spinal cord achieves this remarkable plasticity, however, has puzzled neuroscientists for decades.  

The researchers carried out studies to determine how the spinal cord recovers from injuries by exploring how the nerve connections are wired, and how they function and change when we learn new movements. 

Part of the problem is the difficulty in directly measuring the activity of individual neurons in the spinal cord in animals that are not sedated but awake and moving. The team took advantage of a model in which animals train specific movements within minutes. In doing so, the team uncovered a cell type-specific mechanism of spinal cord learning. 

To check how the spinal cord learns, the lab built an experimental setup to measure changes in movement in mice, inspired by methods used in insect studies. “We evaluated the contribution of six different neuronal populations and identified two groups of neurons, one dorsal and one ventral, that mediate motor learning.” 

"These two sets of neurons take turns," explains the author. "The dorsal neurons help the spinal cord learn a new movement, while the ventral neurons help it remember and perform the movement later."  

"You can compare it to a relay race within the spinal cord. The dorsal neurons act like the first runner, passing on the critical sensory information for learning. Then, the ventral cells take the baton, ensuring the learned movement is remembered and executed smoothly." 

The detailed results, published in the journal Science, illustrate that neuronal activity in the spinal cord resembles various classical types of learning and memory. Further unravelling these learning mechanisms will be crucial, as they likely contribute to different ways in which we learn and automate movement, and may also be relevant in the context of rehabilitation, says the senior author. 

https://www.science.org/doi/10.1126/science.adf6801