Control of position and movement is simplified by combined muscle spindle and Golgi tendon organ feedback

Whereas muscle spindles play a prominent role in current theories of human motor control, Golgi tendon organs (GTO) and their associated tendons are often neglected. This is surprising since there is ample evidence that both tendons and GTOs contribute importantly to neuromusculoskeletal dynamics. Using detailed musculoskeletal models, we provide evidence that simple feedback using muscle spindles alone results in very poor control of joint position and movement since muscle spindles cannot sense changes in tendon length that occur with changes in muscle force. We propose that a combination of spindle and GTO afferents can provide an estimate of muscle-tendon complex length, which can be effectively used for low-level feedback during both postural and movement tasks. The feasibility of the proposed scheme was tested using detailed musculoskeletal models of the human arm. Responses to transient and static perturbations were simulated using a 1-degree-of-freedom (DOF) model of the arm and showed that the combined feedback enabled the system to respond faster, reach steady state faster, and achieve smaller static position errors. Finally, we incorporated the proposed scheme in an optimally controlled 2-DOF model of the arm for fast point-to-point shoulder and elbow movements. Simulations showed that the proposed feedback could be easily incorporated in the optimal control framework without complicating the computation of the optimal control solution, yet greatly enhancing the system's response to perturbations. The theoretical analyses in this study might furthermore provide insight about the strong physiological couplings found between muscle spindle and GTO afferents in the human nervous system. © 2013 the American Physiological Society

Supraspinal Control of Automatic Postural Responses Which Pathway Does What?

Rapid corrective actions, termed automatic postural responses, are essential to counter the destabilizing effect of mechanical perturbations during natural behaviors. Previous research has demonstrated that automatic postural responses of the limbs and body share a number of capabilities in adapting to the prevailing circumstances and these abilities reflect contributions from multiple supraspinal pathways, including brainstem nuclei, basal ganglia, and primary motor cortex. However, we do not know the context-dependent contribution from specific generators, whether different neural pathways have a common role across different effectors, and how sensory and central deficits in one pathway are accommodated by those remaining. Bridging these gaps is essential to integrate the diverse set of studies, develop general theories of motor control, and explicate how the nervous system addresses the partially distinct behavioral demands of co-evolved effector system. The considerable flexibility and multiple interacting pathways of automatic postural responses also make it ideal for understanding how powerful formal theories, like optimal feedback control, are achieved by a distributed hierarchical neural network

Long-latency reflexes account for limb biomechanics through several supraspinal pathways

Accurate control of body posture is enforced by a multitude of corrective actions operating over a range of time scales. The earliest correction is the short-latency reflex which occurs between 20-45 ms following a sudden displacement of the limb and is generated entirely by spinal circuits. In contrast, voluntary reactions are generated by a highly distributed network but at a significantly longer delay after stimulus onset (greater than 100 ms). Between these two epochs is the long-latency reflex (around 50-100 ms) which but acts more rapidly than of voluntary reactions but shares some supraspinal pathways and functional capabilities. In particular, the long-latency reflex accounts for the arm’s biomechanical properties rather than only responding to local muscle stretch like the short-latency reflex. This paper will review how the long-latency reflex accounts for the arm’s biomechanical properties and the supraspinal pathways supporting this ability. Relevant experimental paradigms include clinical studies, non-invasive brain stimulation, neural recordings in monkeys, and human behavioral studies. The sum of this effort indicates that primary motor cortex and reticular formation contribute to the the long-latency reflex either by generating or scaling its structured response appropriate for the arm’s biomechanics whereas the cerebellum scales the magnitude of the feedback response. Additional putative pathways are discussed as well as potential research lines

Long-latency reflexes for inter-effector coordination reflect a continuous state-feedback controller

Successful performance in many everyday tasks requires compensating unexpected mechanical disturbance to our limbs and body. The long-latency reflex plays an important role in this process as it is the fastest response to integrate sensory information across several effectors, like when linking the elbow and shoulder, or the arm and body. Despite the dozens of studies on inter-effector long-latency reflexes there has not been a comprehensive treatment of how these reveal the basic control organization, which set constraints on any candidate model of neural feedback control such as optimal feedback control. We considered three contrasting ways that controllers can be organized: multiple independent controllers versus multiple-input multiple-output (MIMO) controller, continuous feedback controller versus intermittent feedback controller, and direct MIMO controller versus state-feedback controller. Following a primer on control theory and review of the relevant evidence, we conclude that continuous state-feedback control best describes the organization of inter-effector coordination by the long-latency reflex