When the central nervous system (CNS) generates voluntary movement, many muscles, each comprising thousands of motor units, are simultaneously activated and coordinated. Computationally, this is a daunting task, and investigators have strived to understand whether and how the CNS’s burden is reduced to a much smaller set of variables. In the last few years, my collaborators and I have searched for physiological evidence of simplifying strategies by exploring whether the motor system makes use of motor modules, to construct a large set of movements. The core argument for the neural origin of motor modules rests on studies of the spinal cord in several vertebral species, conducted using a variety of techniques. With these approaches, we were able to provide the experimental basis for a modular organization of the spinal cord circuitry in vertebrates. A spinal module is a functional unit of spinal interneurons that generates a specific motor output by imposing a specific pattern of muscle activation (muscle synergy). Muscle synergies are neural coordinative structures that function to alleviate the computational burden associated with the control of movement and posture. In this mini review I will address two critical questions: 1) Are the muscle synergies explicitly encoded in the nervous system? and, 2) How do muscle synergies simplify movement production? I will argue that shared and task-specific muscle synergies are neurophysiological entities whose combination, orchestrated by the motor cortical areas and the afferent systems, facilitates motor control and motor learning.
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