But for the single-neuron approach to be useful to a paralyzed patient, it will need to successfully scale up. Contracting one arm muscle offers little practical reward; movements like reaching and grasping require many muscles to work in concert. The researchers have already taken steps in this direction. First, they showed that a single cell could work two different muscles: a high firing rate triggered the wrist to flex, while a low firing rate caused it to extend. Next, they hooked up two rerouted connections at the same time, with one neuron wired to the wrist-extending muscle and another to the wrist-flexing muscle.
But Andrew Schwartz, a professor of neurobiology at the University of Pittsburgh, is skeptical. A moving arm, says Schwartz, is “a very complicated mechanical system.” Any given sophisticated arm movement requires not only a large number of precisely coordinated muscles acting across several complex joints, but also the propagation of forces along the limb. “If your intention is to generate a movement, you have to somehow calculate the effect of all these forces across the arm,” says Schwartz. “It’s not just, ‘Activate a muscle and the arm goes where you want.’ There’s a lot of math involved.”
According to the University of Washington group, it may be possible to circumvent the question of how to generate intricate movements by wiring a single brain cell directly to a specific region of the spinal cord. “Stimulating a single location in the spinal cord will often activate 10 to 15 different muscles in a precise balance,” says Moritz.
Beyond any theoretical shortcomings of the single-neuron strategy, there are a number of technological hurdles to overcome before it could be used in patients. Electrode readings from an individual brain cell can degrade over time, potentially destroying the rerouted connection. As a result, says Moritz, any long-term setup would need some degree of redundancy.
Also, he adds, the system would ideally be fully implantable. Whenever wires protrude through the skin, as they did in the monkey experiments, they introduce risks of infection and disruption. The group plans to tackle this problem with miniaturized components and wireless technology.
Because their approach requires relatively little computing power, says Moritz, “we think we may be one step closer to low-power, fully implantable systems.”