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Comment by jdc | Hacker News Reader

Is anyone familiar with the literature able to ELI5?

Essentially they’re talking about using electrical stimulation to restore function in people with complete paralysis of the legs due to injury of the spinal cord in the lower back (lumbar region). In particular they’re focused on injury to the junction of the lumbar spine and sacrum. The result has been a restoration of function such that people who previously had no control over their lower body could (when lying on their side) make motions as though walking, and walk while suspended from a support rig.

More exciting is that after 43 weeks of this stimulation and comprehensive training, they were able to walk on a treadmill unassisted bearing their own weight! It sounds like balance issues and strength remain potentially problematic, but it’s still amazing. The last line of the introduction really sums it up:

To our knowledge, this is the first report of independent stepping enabled by task-specific training in the presence of EES by a human with complete loss of lower extremity sensorimotor function due to SCI.

SCI = Spinal Cord Injury

Something to add to this that most non-neuroscientists don't realize is that the spinal cord is not just a bundle of wires that connect the brain to muscles. The spinal cord is a complex circuit that contains "central pattern generators" (CPG) that can produce rhythmic movement. When activated, these networks can be pushed into a dynamic state where they repeat the same movement repeatedly, e.g. walking. For more, look up the work by Sten Grillner on stingray CPGs.

One approximate way of looking at this is that the brain's motor cortex sends a "go" signal to the spinal cords CPGs and these start generating the signals to the muscles to walk. You can also think of motor control as hierarchical where as you go up the hierarchy. The bottom of the hierarchy are the muscles/actuators, then the neurons that stimulate the muscles, then the circuits in the spinal cord that have these CPGs and other "primitives", then primary motor cortex and further up the cortical hierarchy. At the higher levels of the cortical hierarchy are representations of whole movements, like moving your hand to mouth. As you descend the hierarchy, the neural signals control gradually more details components of the movement. For more, look up the work by Michael Graziano.

Are CPG the source of "muscle memory", ie reflexive actions? So much of physical training is to achieve the vaunted muscle memory as it allows unconscious (ie faster) reaction times.

I would argue that muscle memory has less to do with speed or strength and more to do with 'remembering' motion. But of course this is just based off personal experience and intuition so I have nothing to back this up.

Connecting this to the OP is that it looks like the stimulation is initially activating these CPGs -- all of the movement is happening based on control from the spinal cord alone. It's not clear how much communication is taking place between the motor cortex and the spinal cord in this patient.

So is it like a pacemaker for the legs?

So ballet/dance/sports and other complex precision choreographed movements training is in major part about fine tuning these CPG?

the decerebrate cat experiments illustrate this perfectly (although perhaps people dont like to talk about it because of the ethics of the experiments)

https://www.youtube.com/watch?v=wPiLLplofYw&gl=SG&hl=en-GB

This article is very informative:

https://www.washingtonpost.com/health/2018/09/24/paralyzed-p...

I was also going to recommend the WP article. It covers the results fairly well.

I still dont get it. Is this sending signals into the severed spinal cord from an artificial device? If so, how is training going to help? Is it reading from one end of the spinal chord and transfering to the other?

The spinal cord in this case is injured, but not severed.

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