Next Chet Moritz presented on Developing Neuroprosthetic Devices to Promote Plasticity and Rehabilitation after Brain and Spinal Cord Injury.
Chet stated that the goal of neuroprosthetic devices is two-fold: 1) to promote plasticity and regeneration after incomplete injury and 2) to re-animate limb function with complete injury through the use of a brain-controlled spinal interface after SCI or stroke. Chet’s group is using intraspinal stimulation—a stim device implanted within the motor laminae of the spinal cord. Where as others which have been previously discussed in the conference are using epidural stim—lying on top of the spinal cord. Chet states that mostly the epidural stim only gets movement when the device is turned on and connected to the stim machine outside the body. Chet’s group is hypothesizing that by directly stimming the motor laminae, there will be neuroplastic changes that will show improved motor control and function even when the person isn’t hooked up to the machine and have it turned on. Some other folks have also looked at residual EMG activity hooked into a spinal stimulator to make for activity dependent spinal stim—you move a little to tell the stimulator to kick on, it kicks on and completes the movement. They are also working on what I would call an injury by-pass. A recording device is implanted over the M1 motor cortex to record intended movement. The signal is decoded and travels to either a spinal stimulator which causes the movement or to surface stim electrodes that then stim the muscles to perform the movement.
So there are 3 different kinds of brain-controlled spinal interfaces (BCSI)s at play here. I think it’s fairly obvious how they are meeting goal 2; however, how these meet goal 1 is a little trickier. Let’s say we connect Area 1 to a signal recorder then that recorder to a stimulator on a completely unrelated area of the brain: Area 2. So you’ve now connected Area 1 to Area 2, so that each time Area 1 fires, Area 2 gets stimulated too. After a while, you take all that external hardware away and the 2 areas are now physiologically linked via neuroplastic changes. Chet is hypothesizing if we do the same thing, but make Area 2 in the spinal cord, we’ll get some long term potentiation in incomplete people and re-wire their system basically. However, the timing of these signals is really important. Right now they’ve got it down to where the stimulator turns the desired muscles on approximately 10-15 milliseconds after a natural thought process would have done it. Which is good, but not quite fast enough to drive the desired plasticity… yet. They have discovered that much slower than that actually depresses the second site, and we definitely don’t need that!
Besides neuroplastic changes, spinal stim is just overall better than indirect surface stim. The order of motor unit recruitment is in the natural order (unlike surface stim), which makes the force smoother (probably doesn’t hurt so much this way, which is why I personally can’t stand surface stim) and leads to more fatigue-resistance and more repetitions which we all know is a bed rock principle of neuro rehab—lots of reps! Spinal stim is easier to elicit functional muscle synergies (like gait) with as stimming where the CPGs live with just 1 electrode, so there are fewer wirers hanging everywhere to trip up everyone in the range of 4 wires vs. 32-48 wires. Right now they are focusing more on using this tech to elicit the 5 grips necessary for ADLs in the hand as return of hand and arm movement is the main goal of tetraplegics. That is followed by bladder/bowel function, sexual function and reducing autonomic dysreflexia, which also applies to paraplegics. They are hoping to be able to hijack the spinal cord stimulators that neurosurgeons are already implanting in spinal cord injury patients for neuropathic pain control and adapt them for motor use.
Chet wanted us to take this away if nothing else though: that since intraspinal e-stim is enouch to create function, supraspinal control of movement is not necessary for function.
That concludes the mind bending portion of the lectures. (Or at least mind bending to me.)