M. Mileusnic

Force estimation from ensembles of Golgi tendon organs.

Golgi tendon organs (GTOs) located in the skeletal muscles provide the central nervous system with information about muscle tension. The ensemble firing of all GTO receptors in the muscle has been hypothesized to represent a reliable measure of the whole muscle force but the precision and accuracy of that information are largely unknown because it is impossible to record activity simultaneously from all GTOs in a muscle. In this study, we combined a new mathematical model of force sampling and transduction in individual GTOs with various models of motor unit (MU) organization and recruitment simulating various normal, pathological and neural prosthetic conditions. Our study suggests that in the intact muscle the ensemble GTO activity accurately encodes force information according to a nonlinear, monotonic relationship that has its steepest slope for low force levels and tends to saturate at the highest force levels. The relationship between the aggregate GTO activity and whole muscle tension under some pathological conditions is similar to one seen in the intact muscle during rapidly modulated, phasic excitation of the motor pool (typical for many natural movements) but quite different when the muscle is activated slowly or held at a given force level. Substantial deviations were also observed during simulated functional electrical stimulation.

Development of a muscle spindle model

We are constructing a physiologically realistic model of the muscle spindle to assist in the analysis of natural sensorimotor control and to design biomimetic systems for restoration of reach and grasp tasks of paralyzed arm and hand muscles by functional electrical stimulation (FES). The model is composed of mathematical elements that are closely related to the anatomical components of spindles. The current spindle model is reasonably accurate in predicting spindle afferent activity recorded during a variety of ramp, triangular and sinusoidal stretches applied under various fusimotor conditions.

Modeling Spinal Sensorimotor Control for Reach Task

The spinal sensorimotor control system executes movement instructions from the central controller in the brain that plans the task in terms of global requirements. Spinal circuits serve as a local regulator that tunes the neuromuscular apparatus to an optimal state for task execution. We hypothesize that reach tasks are controlled by a set of feedforward and feedback descending commands for trajectory and final posture, respectively. This paper presents the use of physiologically realistic models of the spinal sensorimotor system to demonstrate the feasibility of such dual control for reaching movements

An integrated package of neuromusculoskeletal modeling tools in Simulink/spl trade/

An integrated neuromusculoskeletal (NMS) modeling tool has been developed to facilitate the study of the control of movement in humans and animals. Blocks representing the skeletal linkage, sensors, muscles, and neural controllers are developed using separate software tools and integrated In the powerful simulation environment of Simulink (Mathworks Inc., USA). Musculoskeletal Modeling In Simulink (MMS) converts anatomically accurate musculoskeletal models generated by SIMM (Musculographics Inc., USA) into Simulink blocks. It also removes runtime constraints in SIMM, and allows the development of complex musculoskeletal models without writing a line of code. Virtual Muscle builds realistic Simulink models of muscle force production under physiologic and pathologic conditions. A generic muscle spindle model has also been developed to simulate the sensory output of the primary and secondary afferents. Neural control models developed by various Matlab (Mathworks Inc., USA) toolboxes can be integrated easily with these model components to build complete NMS models in an integrated environment.

Proprioceptors and Models of Transduction

Even when deprived of exteroceptive sensory information such as vision and touch, we are aware of the posture and motion of our bodies (kinaesthesia) and the amount of effort being exerted by our muscles.

Strategic development of sensorimotor prosthetic technology

Reanimation of paralyzed limbs to restore functional movement requires a lot more than electrical activation of muscles. It is a true test of the validity and completeness of our theories of sensorimotor control. These theories can be used to inform the specifications of the neuromuscular interfaces that must be built that include sensors of voluntary command signals and multimodal feedback from the limb as well as neuromuscular stimulators. The theories can also be used to design and test control systems both before and after the interfaces are deployed in the patient. We are working on marrying a family of modular hardware interfaces with a computer modeling environment where clinical conditions and proposed therapeutic interventions can be developed and tested.