Effects of torque disturbances on elbow joint movements evoked in unanesthetized cats by microstimulation of the motor cortex

Summary

Flexion and extension movements were evoked in the elbow joint of unanesthetized cats by intracortical microstimulation (ICMS) applied to deep layers of the motor cortex (areas 4 and 6). Pulse trains with duration up to 3–4 s, current intensities of 15–50 μA and rates of approximately 100/s were used. Cortically evoked movements (CEMs) were tested mechanically by applying servo-controlled torque disturbances to the joint. The disturbances consisted of two reciprocating sinusoidal pulses of torque with fixed frequencies (1.2 or 3.2 Hz). A pronounced torque-angle hysteresis with long-lasting after-effects was revealed in the presence of the torque disturbances that opposed the CEMs and/or assisted them. Two parameters were introduced to describe the mechanical testing of the CEMs quantitatively: (1) the resulting stiffness (RS) denned during the forward and reverse phases of the disturbed movement as a ratio between the amplitudes of torque wave and the overal change of angle at these phases; (2) uncertainty index (UI) defined as the subtraction of forward and reverse angle changes, which was normalized by the first of these two values. RS was shown to be dependent on the immediate past movement history of the joint, it increased with changes in the direction of movement, and its magnitude during such changes could be several times higher than when the disturbance was in the same direction as the movement. When the directions of the preceding movement and the initial phase of the disturbed one coincided, a steady divergence between joint angles before and after application of the torque disturbance occurred, and the mean values of UI obtained in the experiment with various combinations of torque disturbances ranged from 0.52 to 0.8. In the same experiment when torque disturbances opposed the preceding movement, the reactions were significantly more unstable and the mean values of UI varied from +0.07 to -0.62. The possible mechanisms of the observed hysteresis and its role in motor control are discussed. Thixotropy of the muscle is considered to be the main factor of these effects. The muscle hysteresis seems to be significantly increased by a corresponding behaviour of the muscle spindles, this leads to a pronounced asymmetry of stretch and unloading reflexes acting on spinal and supraspinal levels. Based on the data obtained, it could be concluded that muscle hysteresis and its influence on the overal motor performance should not be ignored. Application of the so-called spring model of muscle and the equilibrium point hypothesis can lead to a mistaken treatment of experimental results in various motor control problems.