AUTHOR=Li Si , Zhuang Cheng , Hao Manzhao , He Xin , Marquez Ruiz Juan Carlos , Niu Chuanxin M., Lan Ning TITLE=Coordinated Alpha and Gamma Control of Muscles and Spindles in Movement and Posture JOURNAL=Frontiers in Computational Neuroscience VOLUME=9 YEAR=2015 URL=https://www.frontiersin.org/journals/computational-neuroscience/articles/10.3389/fncom.2015.00122 DOI=10.3389/fncom.2015.00122 ISSN=1662-5188 ABSTRACT=

Mounting evidence suggests that both α and γ motoneurons are active during movement and posture, but how does the central motor system coordinate the α-γ controls in these tasks remains sketchy due to lack of in vivo data. Here a computational model of α-γ control of muscles and spindles was used to investigate α-γ integration and coordination for movement and posture. The model comprised physiologically realistic spinal circuitry, muscles, proprioceptors, and skeletal biomechanics. In the model, we divided the cortical descending commands into static and dynamic sets, where static commands (αs and γs) were for posture maintenance and dynamic commands (αd and γd) were responsible for movement. We matched our model to human reaching movement data by straightforward adjustments of descending commands derived from either minimal-jerk trajectories or human EMGs. The matched movement showed smooth reach-to-hold trajectories qualitatively close to human behaviors, and the reproduced EMGs showed the classic tri-phasic patterns. In particular, the function of γd was to gate the αd command at the propriospinal neurons (PN) such that antagonistic muscles can accelerate or decelerate the limb with proper timing. Independent control of joint position and stiffness could be achieved by adjusting static commands. Deefferentation in the model indicated that accurate static commands of αs and γs are essential to achieve stable terminal posture precisely, and that the γd command is as important as the αd command in controlling antagonistic muscles for desired movements. Deafferentation in the model showed that losing proprioceptive afferents mainly affected the terminal position of movement, similar to the abnormal behaviors observed in human and animals. Our results illustrated that tuning the simple forms of α-γ commands can reproduce a range of human reach-to-hold movements, and it is necessary to coordinate the set of α-γ descending commands for accurate and stable control of movement and posture.