Participation of Reflex Mechanisms and : Reaction-Time Processes in the Compensatory Adjustments to Mechanical Disturbances

The purpose of this chapter is to review the actions and functions of different neural processes in controlling the compensatory adjustments to mechanical disturbances. Although I would prefer to discuss neural systems with strictly defined anatomical pathways. disagreement always arises concerning the segmental and suprasegmental origins of response components. Thus, throughout most of the chapter, I will make distinctions between different neural processes on functional grounds and will confine discussion concerning the probable anatomical pathways to a later section. Ordinarily. adjustments to mechanical disturbances are ~ntegrated with voluntary motor behavior, suggesting that they be considered within this general perspective. Movements and body postures are the consequence of an ongoing interaction between the forces produced by the body muscula-ture and the opposing forces that result from the mechanical loads that are imposed upon the muscles. A fundamental function of neQI"omuscular control systems is to adjust these muscular forces in such a manner as to insure the mechanical stability of the body. Note that the only essential requirements for mechanical stability are that muscular fQrces counterbalance the mechanical loads and that thcrc be damping-the positions of the joints need not be rigidly controlled. While there is no fundamental requirement for a control of joint position. movements are usually well compensated for differences in mechanical ,l?ad tbat would otherwise disturb them. This and other considerations 'led MER roN [1953] to propose tbat the most basic of the motor regulatory mechanisms, the stretch reflex, functions as a servo system that controls muscle length. and hence joint position. Thus, servo control of muscle length is supposed to be the origin of compensation for disturbances in load.