ADAPTATION IN REGULATION OF MOVEMENT AND POSTURE. A STUDY OF STRETCH RESPONSES IN SPASTIC ANIMALS

Abstract

Stimuli consisting of step and continuous sinusoidal muscle length changes were applied to the ankle extensors of decerebrate and chronic spinal cats. Afferent and efferent spike discharges were recorded but recordings of total tension developed were found to be more significant for the analysis of the stretch response. Step changes of length showed tension responses composed of varying proportions of 2 factors; the 1st component a function of stretch amplitude remained constant, while the 2nd component appeared early and decayed with time. Sinusoidal stretch produced a tension response which increased in intensity when higher sinusoidal stretching frequencies were applied. In individual stretch cycles, the tension response significantly led in time the stretch producing it. It is proposed that these observations indicate a reflex control system with an output determined by both muscle length error and the rate of change of muscle length. The mechanism of the rate of change sensitivity probably involves the adaptation of the muscle spindle receptor system. The compensation for conduction delay and limb inertia provided by such dynamic response would appear essential in the myotatic reflex to provide a stable position and movement control.