Arm and hand movements have been restored in paralyzed monkeys by zapping their spinal cords with electricity.
The breakthrough offers hope of stricken accident or stroke victims regaining use of their upper limbs. Patients are now being recruited for a clinical trial.
Electrical stimulation of surviving nerves in a severely damaged backbone can improve motor control.
Senior author Dr. Marco Capogrosso, of Pittsburgh University in the US, said: "To perform even the simplest arm movement, our nervous system has to coordinate hundreds of muscles, and replacing this intricate neural control with direct electrical muscle activation would be very difficult outside a laboratory.
"Instead of stimulating muscles, we simplified the technology by designing a system that uses surviving neurons to restore the connection between the brain and the arm via specific stimulation pulses to the spinal cord, potentially enabling a paralyzed person to perform tasks of daily living."
Losing use of the arms and hands - ranging from struggling to bend the wrist to complete paralysis - are among the most life-altering complications of stroke and severe spinal cord injury.
Even mild immobility significantly limits quality of life and independence - making it an important focus in the field of neuro-rehabilitation.
Currently, there are no therapies or medical technologies that provide motions and dexterity - skills that set primates and humans apart from other mammals.
They include rotating the arm in the shoulder, bending it at the elbow, flexing and extending the wrist and altering the grip by changing positions of individual fingers.
It allows for extremely complex control of the way we hold objects and otherwise interact with the world.
That amazing ability is also what makes getting arm and hand movement back extraordinarily difficult.
The technology developed by the US team activates the remaining healthy nerves connecting the brain and the spinal cord to control the muscles of the arm.
It uses seamless external stimuli that requires little to no training - allowing individuals to continue familiar motor tasks the way they did before.
In experiments, macaques with partial arm paralysis learned to reach, grasp and pull a lever to receive a favorite food treat.
They were fitted with brain implants that detected electrical activity from regions controlling voluntary movement.
When they detected the animal's intention to move its arm a small array of electrodes connected to a stimulator the size of a pencil rubber were switched on.
It was placed over the nerve roots sprouting from the spinal cord toward the muscles of the arm and hand.
Co-first author Dr. Sara Conti, of Harvard Medical School, Boston, said: "Our protocol consists of simple stimulation patterns that are initiated by detection of the animal's intention to move.
"We don't need to know where the animal wants to move. We only need to know they want to move - and extracting that information is relatively simple.
"Our technology could be implemented in clinics in many different ways, potentially without requiring brain implants."
The system was extensively verified using a combination of computational algorithms and medical imaging.
It ensured each monkey's unique anatomy was compatible with the device, said Dr. Conti.
Stimulation significantly boosted precision, force and range of movement - allowing each animal to move its arm more efficiently.
Importantly, they continued to improve over time as they adapted to the groundbreaking technique.
Co-first author Dr. Beatrice Barra, now at New York University, added: "Taking a step back and tackling a very complex clinical problem from a different and simpler perspective compared to anything that was done before opens more clinical possibilities for people with arm and hand paralysis.
"By building a technology around the nervous system that mimics what it is naturally designed to do, we get better results."
The electrical spinal cord stimulation, described in the journal Nature Neuroscience, will be tested on paralyzed stroke patients in the US later this year.
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