IEEE Spectrum November, 2013 - 49

The neurons in the spinal cord don't only
receive signals from the brain; they also process sensory feedback from the body as the
muscles move and balance shifts. The importance of that sensory feedback gradually
emerged with some animal experiments
Edgerton reported in Nature Neuroscience
in 2009. The study suggested that sensory
input could actually control the motor commands produced by the spinal cord.
Harkema, a former student of Edgerton's,
ran with that concept. In her experiments
with Summers, she stimulated his spinal
cord just enough to wake it up and then
let the sensory input do its thing. "It's like
putting a hearing aid on the spinal cord,"
says Edgerton. "We've changed the physiological properties of the neural network
so that now it can 'hear' the sensory information much better and can learn what to
do with it."
Harkema's group use s an off-the shelf neurostimulation system-made by
Minneapolis-based Medtronic-that's FDA
approved for pain management. The system's array of 16 electrodes is surgically
implanted in the epidural space next to
the outermost protective layer of the spinal
cord. The array is then connected to a pulse
generator (which resembles a pacemaker)
that's implanted nearby. Finally, the pulse
generator receives a wireless signal from
a programming device outside the body.
The array spans approximately six spinalcord segments, the ones generally responPAtiEnt no. 3: Drew Meas practices standing independently during an electrical stimulation session
sible for movement in the lower half of the
at the Frazier Rehab Institute, in Louisville, Ky. Neuroscientist Susan Harkema [standing at right]
body. By placing the electrodes over them,
varies the electrical signals sent to Meas's spinal cord to control different muscle groups.
the researchers can generate a response
in the corresponding muscle groups. Electrode 5, for example, is located near a segment of the spinal cord that controls hip
researchers have demonstrated the concept definitively in paramuscles. Electrode 10 is located at the bottom of the array, over
the segment that controls the lower leg.
lyzed cats, rats, and a few humans. But in most of these demonEach of the array's 16 electrodes can be set to act as a cathode
strations, researchers were blasting a large amount of electrical
or an anode or be completely shut off. Stimulation intensities can
current into the body to force the muscles to move. "Everyone,
range from 0 to 10.5 volts with pulses sent at frequencies ranging
including us, was hung up on the idea that you have to stimulate
from 2 to 100 hertz, although the researchers usually don't go
at this high level to induce the movement," says Edgerton. What
beyond 45 Hz. Picking the right combination of electrodes and
they missed was that the stimulation was essentially overwhelmstimulation parameters to generate a simple response in a single
ing the neurons in the lower spinal cord and was actually intermuscle is relatively straightforward. But generating a complex
fering with their ability to process sensory information that can
behavior like standing, which involves many muscle groups and
help the body move on its own.

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Table of Contents for the Digital Edition of IEEE Spectrum November, 2013