Effect of load disturbances during centrally initiated movements

TLDR

These movements, subserved by muscles well endowed with muscle spindles, are part of the coordinated eye‑head response to the appearance of a stimulus in the animal's visual field. The study used a chronically vestibule‑ectomized monkey trained to make a visual discrimination, and evaluated neck torque under unexpected load disturbances by surgically interrupting the reflex afferent loop and by simulating a mathematical head‑neck model. The compensatory torque of reflex origin was less than 10–30% of that needed for perfect compensation, with the majority of compensation arising from the mechanical properties of the neck musculature, yet reflex and mechanical processes together never fully compensated for the disturbance. Footnotes 1–3 are included.

Abstract

1. We have investigated the relative contributions of mechanical and reflex mechanisms in generating the forces produced by the neck muscles when loads were unexpectedly applied during centrally programmed head movements in monkeys. These movements, subserved by muscles well endowed with muscle spindles, are part of the coordinated eye-head response to the appearance of a stimulus in the animal's visual field. Our preparation was a chronically vestibulectomized monkey trained to make a visual discrimination. 2. Two procedures were used to evaluate the torque generated by the neck musculature when an unexpected load disturbance was applied: first, by surgically interrupting the afferent loop subserving the reflex action (section of cervical dorsal roots) and second, by building a mathematical model of the head-neck system and carrying out a process of simulation. 3. Our results indicated that the compensatory torque of reflex origin stimulated by the application of an opposing force was less than 10--30% of that required for perfect compensation, and the larger fraction of the observed compensation was due to the mechanical properties (inertial, viscous, and elastic) of the neck musculature. The combined action of reflex and mechanical processes never completely compensated for the disturbance.