'Driving' a new pair of arms: Neurology, recovery and rehabilitation

Brendan Mar­rocco, an Iraq War vet­eran who lost all four limbs in a road­side bomb attack, was recently released from a Bal­ti­more hos­pital after receiving a double-​​arm trans­plant. Northeastern University news office asked Christo­pher Hasson, a sen­so­ri­motor con­trol expert and a newly appointed assis­tant pro­fessor in the Depart­ment of Phys­ical Therapy, to explain the brain's role in the long recovery and reha­bil­i­ta­tion process.

Marrocco received the double-arm transplant six weeks ago and has already reported movement in the elbow of his left arm. What is the brain's role in learning how to control a novel object, which in this case is a new pair of arms?

A human arm is mar­velously complex—and presents a for­mi­dable con­trol problem for the brain. The scale of this problem is best illus­trated by com­par­ison with dri­ving a car. With an auto­matic trans­mis­sion you have three things to con­trol: The steering wheel turns the car left or right, the gas pedal speeds up the car, and the brake slows it down. Healthy adults learn the basics of dri­ving rel­a­tively quickly, but fine-​​tuning takes much longer and can only be achieved through many hours of prac­tice. During this fine-​​tuning process the brain refines its knowl­edge of how the car responds to con­trol actions. In Marrocco's case, he must learn to "drive" his new arms; how­ever, the con­trol problem explodes in com­plexity. For each arm he must learn to con­trol motions at three joints with 12 mus­cles; if you include the hand that adds at least 14 more joints and more than 20 more mus­cles. Imagine trying to learn to drive a car with more than 30 dif­ferent con­trols! For­tu­nately, Mar­rocco has a head start, as he has had prior expe­ri­ence con­trol­ling arms. This may explain why he learned to per­form basic move­ments rel­a­tively quickly. How­ever, fine-​​tuning his con­trol will take much longer.

You are the principal investigator of Northeastern's Neuromotor Systems Laboratory, in which you study how movement control in older adults is affected by age-related changes in the neuromuscular system. How will Marrocco's relative youth—he is only 26 years old—contribute to the recovery and rehabilitation process?

Although it's nat­ural to think that a rel­a­tively young adult such as Mar­rocco would have a clear advan­tage over someone who is older, age may not be a crit­ical factor in terms of the motor learning aspects of recovery. It was once widely thought that after you reach adult­hood the struc­ture of your brain sta­bi­lizes and becomes fixed, and there­fore older adults would have dif­fi­culty learning new skills as their brains are more resis­tant to change.

How­ever, recent studies have shown that the brain remains mal­leable into old age, and that older adults can often learn new tasks just as well as younger indi­vid­uals, although they may move more slowly. Of course, there are still advan­tages to being young. For example, as you age, the volume and quality of brain tissue declines, which may have detri­mental effects on move­ment con­trol. In addi­tion, older adults tend to use more of their brain's resources to per­form motor tasks. This increased cog­ni­tive effort may make an older adult fatigue more easily than a young adult like Mar­rocco, giving him an advan­tage in the reha­bil­i­ta­tion process. There are many open ques­tions about how aging affects motor learning and con­trol. Adults 65 and older who wish to par­tic­i­pate in my lab's research on aging can con­tact neuromotorsystemslab@​neu.​edu for more information.

Marrocco says he looks forward to driving, swimming, and competing in a marathon using a handcycle. Given that the nerves in his arms will regenerate at a rate of only one inch per month, how do you foresee his recovery process unfolding? Will he ever regain enough motor control of his arms to be independent?

The time it takes to regen­erate the nerves within the arm will limit the pace of recovery. Mar­rocco won't be able to per­form some move­ments until his ner­vous system can prop­erly "talk" to all of his mus­cles. There will also be a mul­ti­tude of changes in the brain, which is con­stantly reor­ga­nizing its struc­ture in response to changes in behavior, the envi­ron­ment, and in Marrocco's case, to injury. This mal­leability is called neu­ro­plas­ticity. After Mar­rocco lost his arms, the regions in the brain that are respon­sible for arm sen­sa­tion and con­trol prob­ably shrunk as a result of disuse, but these regions should expand as he regains con­trol of his arms. It is crit­ical that Mar­rocco keeps trying to use his new arms, as this will drive neu­ro­plas­ticity. Every time Mar­rocco per­forms an action the asso­ci­ated neural con­nec­tions will strengthen. Ide­ally, Mar­rocco will enter a cycle of pos­i­tive feed­back: neu­ro­plas­ticity will enable him to do more with his arms, which should moti­vate him to per­form more actions, which will fur­ther rein­force con­nec­tions in his brain.