A larger number of studies have been dedicated to elucidate the morphological, biochemical, and neurophysiological properties of the spindle ( Matthews, 1962 Granit, 1970 Boyd and Smith, 1984). A large amount of cortical outputs is directed to γ motoneurons that supply fusimotor control of spindles ( Boyd and Smith, 1984). Muscle spindle is a unique sensory organ that has dual efferent and afferent innervations ( Boyd, 1980 Matthews, 1981 Hulliger, 1984). The results suggest that the information of joint angles may be communicated between the CNS and muscles via the descending γ s efferent and Ia afferent signals. With the quadratic strategy of spindle sensitivity control, γ s commands may serve as the CNS outputs that inform the periphery of central coding of joint angles. However, if γ s commands were quadratically modulated with joint angles, a robust linear relation between Ia afferents and joint angles could be obtained in both mono-articular and bi-articular muscles. Among the three hypotheses, the constant and linear strategies did not yield Ia responses that matched the experimental data, and therefore, were rejected as plausible strategies of spindle sensitivity control. Simulation results revealed a nonlinear landscape of Ia afferent with respect to both γ s activation and joint angle. In simulation experiments within the operational range of joint movements, the gamma static commands (γ s) to the spindles of both mono-articular and bi-articular muscles were hypothesized (1) to remain constant, (2) to be modulated with joint angles linearly, and (3) to be modulated with joint angles nonlinearly. In this study, we addressed the issue of communication between the central and peripheral sensorimotor systems using a computational approach based on the virtual arm (VA) model. Furthermore, what information is carried in the fusimotor signal from the motor cortex to the muscle spindle is largely unknown. How fusimotor control of spindle sensitivity affects proprioceptive coding of joint position is not clear. The sensitivity of muscle spindle is nonlinearly dependent on the activation of gamma (γ) motoneurons in the spinal cord that receives inputs from the motor cortex. Proprioceptive afferents from muscle spindles encode information about peripheral joint movements for the central nervous system (CNS).
2School of Dentistry, Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, CA, USA.1School of Biomedical Engineering, Med-X Research Institute, Shanghai Jiao Tong University, Shanghai, China.
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