Numerous muscles are involved and use coordination just to perform simple actions like picking up a pen. For instance, the eyes and head need to turn towards the object, the hand needs to reach towards the pen and the fingers need to pick it up.
This complex coordination of muscles is managed as a series of shortcuts in the brain’s motor cortex to make these actions more manageable. Rather than controlling each individual muscle separately, researchers believe that the cortex activates these muscles in groups, called “muscle synergies,” and that various combinations of these synergies exist to allow a wide range of movements.
According to a new MIT study in collaboration with Harvard Medical School and the San Camillo Hospital in Venice, which was published in the latest issue of Proceedings of the National Academy of Sciences, these muscle synergies are activated in different ways after a stroke, and that these disruptions follow particular patterns depending on stroke’s severity and the amount of elapsed time after the stroke. Senior author of the study, Emilio Bizzi, an Institute Professor at MIT says that the findings could lead to improved rehabilitation for stroke patients and also offer better insight into how the motor cortex coordinates movements.
Bizzi, who is a member of the McGovern Institute for Brain Research at MIT, says:
“The cortex is responsible for motor learning and for controlling movement, so we want to understand what’s going on there. How does the cortex translate an idea to move into a series of commands to accomplish a task?”
A good way to investigate motor cortical functions is by studying how these motor patterns are disrupted in stroke patients with damaged motor areas. Bizzi and his team first discovered muscle synergies in 2009 in the arms of mild stroke victims when they measured electrical activity in each muscle as the patients moved. The team used a specially designed factorization algorithm to identify characteristic muscle synergies in the stroke victims’ affected and unaffected arms.
“To control, precisely, each muscle needed for the task would be very hard. What we have proven is that the central nervous system, when it programs the movement, makes use of these modules. Instead of activating simultaneously 50 muscles for a single action, you will combine a few synergies to achieve that goal.”
In both studies, the 2009 and the new one, the team demonstrated that synergies in the affected arms of mild stroke patients’ in the cortex are very similar to those in the unaffected arms despite different patterns in muscle activation. This proves that muscle synergies are structured within the spinal cord, and that cortical stroke changes the brain’s ability to activate these synergies in the appropriate combinations.
The new study also discovered a very different pattern in those who suffered more severe strokes. They found that the synergies in the affected arm merged to form a smaller number of larger synergies, whilst in a third group of stroke victims whose stroke occurred many years earlier, the muscle synergies of the affected arm were divided into fragments of the synergies seen in the unaffected arm, a phenomenon known as fractionation.
Fractionation does not restore the synergies to what they would have looked like prior to the stroke, and leading author Vincent Cheung, a research scientist at the McGovern Institute, remarks: “These fractionations appear to be something totally new. The conjecture would be that these fragments could be a way that the nervous system tries to adapt to the injury, but we have to do further studies to confirm that.”
The team hypothesizes that these patterns of synergies that are determined by both the severity of the deficit and the elapsed time after the stroke could be used as markers to achieve a more detailed description of an individual patients’ impaired status. Cheung says: “In some of the patients, we see a mixture of these patterns. So you can have severe but chronic patients, for instance, who show both merging and fractionation.” The discovery could also help to design more targeted rehabilitation programs. Based on these markers, the MIT team is currently collaborating with several hospitals in order to develop new therapeutic protocols.
In the U.S., around 700,000 people fall victim to strokes every year, and even though there are many different rehabilitation programs available, and according to Bizzi, choosing the best method for an individual is more of an art than medical science.
“There is a great deal of need to sharpen current procedures for rehabilitation by turning to principles derived from the most advanced brain research. It is very likely that different strategies of rehabilitation will have to be used in patients who have one type of marker versus another.”