Skeletal Muscles are Hyper-Reinnervated According to the Axonal Capacity of the Surgically Rewired Nerves
Vlad Tereshenko, M.D., Ph.D.1,2, Dominik Dotzauer, Medical Student3, Martin Schmoll, PhD3, Lisa Gfrerer, MD, PhD4, Kyle R. Eberlin, MD5, William G., Jr. Austen, MD6, Roland Blumer, PhD7, Dario Farina, PhD8 and Oskar C. Aszmann, MD3, 1Massachusetts General Hospital, Boston, MA, 2Medical University of Vienna, Vienna, MA, 3Medical University of Vienna, Vienna, Austria, 4Weill Cornell Medicine, New York City, NY, 5Massachusetts General Hospital/Harvard Medical School, Boston, MA, 6Harvard Medical School, Boston, MA, 7Systemic Anatomy, Medical University of Vienna, Vienna, Austria, 8Imperial College London, London, United Kingdom
Advances in robotics have outpaced the capabilities of man-machine interfaces to decipher and transfer neural information to and from prosthetic devices. Muscles can serve as an interface for amplifying the neural output from the spinal cord, following selective nerve transfers. In this study, we emulated clinical scenarios where high- or low-neural capacity donor nerves (i.e., nerves with a large or small number of motor axons) were surgically rewired to a skeletal muscle controlled by a very small number of motor axons (sternomastoid muscle) (Figure 1). We found that the sternomastoid muscle successfully underwent functional reinnervation after nerve transfer using either facial (high motor axonal count) or ulnar (low axon count) nerves as the donor of motor axons. Using retrograde neural tracing and electrophysiological tests, the reinnervated muscle showed almost a 15-fold hyper-reinnervation after the high-capacity nerve transfer (facial nerve), indicating its capability of incorporating a multifold of neural signals (Figure 2). Additionally, the surgically redirected neuronal sources redefined the physiological properties of the reinnervated muscle by altering the expression of myosin heavy chain types according to the donor nerve. These findings suggest that skeletal muscles can serve as a biological amplifier of neural information from the spinal cord for controlling bionic prostheses, with the potential of expressing high-dimensional nerve function for high-information transfer interfaces.
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