With roughly 650 muscles in our body in need of instruction, the human nervous system has a mammoth task in sending out the correct commands to each and every one of them. The intricate web of nerve cells that carry these commands can easily be damaged.
For Hollywood actor Christopher Reeve, this took place in one short moment when he was thrown from a horse, leaving him tetraplegic – unable to move all four of his limbs. Repairing this damage is often impossible. It is from this seemingly insurmountable roadblock that neuroprosthetics springs; a field dedicated to the development of devices that substitute for the loss of nerve cell function.
The damage to Reeve’s nerve cells that led to his tetraplegia was almost instantaneous. For 52-year-old Jan Sherman, however, this same fate was 13 years in the making. Spinocerebellar degeneration was the culprit – a genetic disorder that causes nerve cells to degrade, gradually chipping away at the body’s ability to control its muscles until finally paralysis takes hold. The degenerating nerve cells are specifically found within the spine and a region of the brain called the cerebellum, which coordinates commands from the rest of the brain telling our muscles to move. Sherman’s cerebellum was luckily unaffected by her condition, but without fully functional nerves running down her spine, she was still left paralysed.
Unlike most patients who receive this diagnosis, Sherman was given the opportunity to test the latest in brain-machine interfaces. Two electrodes were connected to the region of her brain that controls movement, called the motor cortex. These electrodes were plugged into a recording device that captured the signals being fired off from her nerve cells. A computer then converted these signals into a language a prosthetic arm could understand, using a ‘neural decoder’. Connecting the computer to the motorised arm completed the information highway and a 13-week training regime began.
The prosthetic arm was designed to move with ‘seven degrees of freedom’ – about the amount of flexibility it takes to reach out for a glass of water and bring it to your mouth. A human arm is capable of mastering 25 degrees of freedom. As the weeks rolled by Sherman grappled with a variety of tasks usually used to assess stroke patients. These trials focused on gripping and grasping movements that would be of use during daily living, including picking up different sized blocks and balls and stacking cones.
Without the aid of the prosthetic arm, Sherman was unable to complete any of these tasks, scoring 0 out of a possible 27 points. Week on week her brain learnt to tweak its activity patterns to better communicate with the prosthetic arm. By the end of the training regime her score had risen to between 15 and 17. Any increase greater than 5.7 points is said to be a clinically significant improvement in function, with the potential to improve a patient’s quality of life.
Work is already underway to further develop the range of movements the prosthetics can achieve and also remove the need for cables by using wireless technology. Stepping farther into the future, it is hoped the prostheses will one day be able to send impulses back to the brain to convey the sensation of touch, ultimately providing those suffering from paralysis a greater ability to engage with their surroundings.