The motor system plays the violin: a musical metaphor inferred from the oscillatory activity of the α-motoneuron pools during locomotion

Abstract

Chiovetto E. The motor system plays the violin: a musical metaphor inferred from the oscillatory activity of the ␣-motoneuron pools during locomotion. J Neurophysiol 105: 1429 -1431, 2011. First published January 27, 2011 doi:10.1152/jn.01119.2010.-Despite substantial advances in the field, particularly resulting from physiological studies in animals, the neural mechanisms underlying the generation of many motor behaviors in humans remain unclear. A recent study (Cappellini G et al. J Neurophysiol 104: 3064 -3073, 2010) sheds more light on this topic. Like the string of a violin, the ␣-motoneuron pools in the spinal cord during locomotion show continuous and oscillatory patterns of activation. In this report, the implications and relevance of this finding are discussed in a general framework that includes neurophysiology, optimal control theory, and robotics. motoneuron activity; spinal cord; electromyographic activity; locomotion COMPLEX BEHAVIORS INVOLVE the coordination of many muscles and hence motor neuron activity. Previous studies have provided evidence that the patterns of coactivation of multiple muscles can be broken down into more basic units, usually referred to as muscle synergies (Chiovetto et al. 2010;Danion and Latash 2010;d'Avella et al. 2003). This work has motivated the so-called "piano model" of movement generation: according to this model, synergies can be thought of as the keys of a piano, on which a musician has to act to play a song. A severe limitation in this metaphor is, however, that it provides limited insight on how the keys of the piano might be related to particular spinal structures.A recent study by Cappellini and colleagues (2010) has provided some new remarkable findings, in the framework of locomotion, which might shed more light on this question. Their work, however, suggests a metaphor that relates more to the violin than the piano. The authors studied the relationship between the spinal motor output and the mechanics associated with the whole body center of mass (CoM) during walking and running. They recorded the electromyographic activity (EMG) of 30 bilateral leg muscles during different kinds of gait, and mapped such EMG patterns onto the spinal cord in approximate rostrocaudal locations of the motor neuron pools. In the human spinal cord, ␣-motoneurons (MNs) present a very specific topography: each muscle is innervated by several spinal segments, and each segment supplies several muscles. By using specific myotomal charts reporting the proportion of total muscle activation attributable to each spinal segment, authors could reconstruct the spatiotemporal patterns of the MNs and calculate the global center of activity (CoA) of the corresponding MNs in the lumbosacral enlargement.The CoA computation method, which represents the most crucial point of the study, is based on strong assumptions. First, it assumes that rectified EMG provides an indirect measure of the net MNs firing of that muscle. Second, it assumes that the population of subjects has the same spinal topog...