Nerve stimulation research update

I heard about this story on the radio one morning recently. The story reported how nine people who had lost their mobility through spinal cord injury were able to become mobile again, having used epidural electrical stimulation.

I had not heard of epidural electrical stimulation before. A quick search reveals that a small device is implanted over the protective coating of the spinal cord. The device then produces electrical currents to the lower part of the spinal cord which was not previously able to communicate with the upper part. (https://www.medicaldevice-network.com/comment/epidural-stimulation/) What I am not clear about is if this is referring to upper and lower in the context of the site of the issue with the spinal cord or if they are using upper and lower in terms of neurons. The image in the story (link below) shows a spinal cord injury high up along the spine and the device shown implanted in the lumbar spine.

We know that the upper motor neurons degrade over time, which breaks the chain of communication between the brain and the lower motor neurons. My understanding is that in HSP the degradation of the upper motor neurons occurs throughout the nerve rather than at a specific point along the nerve. This suggests that the lower part of the upper motor neurons will have had some degeneration, making external electrical stimulation more challenging. If the epidural electrical stimulation can talk with the lower motor neurons then I hope that there is a potential for use in HSP. I spot a gap here in my understanding of the spine and the upper and lower motor neurons! 

The other sides to the story I heard was that they were using an AI system with stimulating the nerves to be able to help people regain their mobility better. They also reported that general levels of nerve communication were lower when this was being used, which they were speculating was a big reduction in the feedback signals being sent back up to the brain (and not being received due to the spinal cord injury).

This latter aspect gave me thought for reflection - I know that feedback for things like bladder and bowel are affected, giving cause for the often experienced incontinence with HSP, and it also makes sense that some people lose their sensitivity to vibration in their legs. I suspect that I had been focussing most on the communication from the brain above the various forms of information that the brain receives back from the lower body.

The story which was talked about on the radio was this one, from Nature, from a team in Switzerland: https://www.nature.com/articles/s41586-022-05385-7