Neuromechanical Coupling is Reflected in the Spatial Organization of the Spinal Motoneuron Pools

Recent advances in neuroprosthetics have shown that activating the surviving spinal circuitry below the level of spinal damage can produce complex movements and restore function. This is because the spinal cord circuitry plays an important role in the control of complex movements by integrating descending execution-related signals with feedback sensory signals. To generate efficient movements through muscle contractions, the nervous system embeds the anatomical and dynamical properties of the controlled body parts, a concept termed neuromechanical tuning. The motoneurons innervating the skeletal muscles of the body form complex spatial structures that span multiple segments. The goal of this study is to determine if these structures can support neuromechanical tuning. We performed a comparative analysis of the spatial organization of the motoneuron pools and the functional organization of the muscles they innervate. We developed a three-dimensional model of the spatial organization of motoneuron pools innervating upper limb muscles in the macaque and quantified their relative distances. We then utilized a musculoskeletal model of the macaque upper extremity to characterize the functional relationship between the muscles innervated by the motoneuron pools. We found that the distances between motoneuron pools mimicked the agonistic or antagonistic actions of muscles they innervate. This shows that the spatial organization of the motoneuron pools embeds the functional musculoskeletal anatomy evolved to support the large repertoire of upper limb movements in primates. This spinal anatomy may play an important role in the neuromechanical tuning and simplify the sensorimotor control of the arm.