Effects of muscle thixotropy on afferent activity in the hindlimb of the rat

Abstract

The increase with time of the stiffness of resting muscle and its effect on afferent nerve activity arising within the muscle were investigated in the leg of the anaesthetized rat. Nerve activity was recorded from the branch of the tibial nerve innervating the gastrocnemius muscle. A sinusoidal torque from a motor coaxial with the ankle produced small (less than 0.25 deg) oscillations of the resting foot that were interrupted by several cycles of a much larger amplitude displacement. After the perturbation, the same small torque caused larger (greater than 0.40 deg) oscillations, demonstrating a reduction in the resting or short-range stiffness of muscles acting at the joint. Turning off the torque for 30 s allowed the stiffness to return to its enhanced level. Afferent nerve activity in response to the small torque was greater following the perturbation, reflecting the larger oscillations. We conclude that thixotropic stiffening of muscles at rest reduces postural displacements produced by small torques as well as the quantity of afferent signals converging on the central nervous system from these events.