Abstract
Control of adaptive walking requires the integration of sensory signals of muscle force and load. We have studied how mechanoreceptors (tibial campaniform sensilla) encode "naturalistic" stimuli derived from joint torques of stick insects walking on a horizontal substrate. Previous studies showed that forces applied to the legs using the mean torque profiles of a proximal joint were highly effective in eliciting motor activities. However, substantial variations in torque direction and magnitude occurred at the more distal femorotibial joint, which can generate braking or propulsive forces and provide lateral stability. To determine how these forces are encoded, we used torque waveforms of individual steps that had maximum values in stance in the directions of flexion or extension. Analysis of kinematic data showed that the torques in different directions tended to occur in different ranges of joint angles. Variations within stance were not accompanied by comparable changes in joint angle but often reflected vertical ground reaction forces and leg support of body load. Application of torque waveforms elicited sensory discharges with variations in firing frequency similar to those seen in freely walking insects. All sensilla directionally encoded the dynamics of force increases and showed hysteresis to transient force decreases. Smaller receptors exhibited more tonic firing. Our findings suggest that dynamic sensitivity in force feedback can modulate ongoing muscle activities to stabilize distal joints when large forces are generated at proximal joints. Furthermore, use of "naturalistic" stimuli can reproduce characteristics seen in freely moving animals that are absent in conventional restrained preparations.NEW & NOTEWORTHY Sensory encoding of forces during walking by campaniform sensilla was characterized in stick insects using waveforms of joint torques calculated by inverse dynamics as mechanical stimuli. Tests using the mean joint torque and torques of individual steps showed the system is highly sensitive to force dynamics (dF/dt). Use of "naturalistic" stimuli can reproduce characteristics of sensory discharges seen in freely walking insects, such as load transfer among legs.
Highlights
Regulation of force is considered an integral component in the control of adaptive leg movements in walking [1,2,3,4]
Torques at the femorotibial joint are quite variable and can show changes in sign. These variations in torques could potentially be associated with fluctuations in the femorotibial joint angle, which can occur in free walking stick insects [27, 28]
Changes in joint torque were not associated with variations in joint angle, even when the changes in torque were >50% of the maximum value
Summary
Regulation of force is considered an integral component in the control of adaptive leg movements in walking [1,2,3,4]. Muscle forces and loads are monitored mainly by sense organs in the legs and feet [5, 6]. For example, early experiments established that the discharges of Golgi tendon organs, which monitor forces through strains at muscle insertions, could encode force magnitude, providing information necessary in the detection and regulation of body load [7,8,9]. Tendon organs have strong dynamic sensitivities that could compromise linear encoding of force magnitude in walking: receptor discharges were, instead, considered to function mainly to provide signals on variations in forces, and the detection of “static muscle force might require the combined processing of discharges from. Subsequent studies have proposed that information about force magnitude is derived from sensory receptors as a population that converges in the central nervous system, the specific mechanisms have not been elucidated
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