SIMULATION OF DYNAMIC FUSIMOTOR EFFECTS IN THE DISCHARGE FREQUENCY OFIA EFFERENTS BY PRESTRETCHING THE MUSCLE-SPINDLE

The discharge patterns of primary muscle spindle afferents from the ti bial anterior muscle of the cat were recorded under a ramp-and-hold st retch of constant amplitude (7 mm) and stretch rates varying between 1 and 50 mm/s. With seven Ia fibers, the discharge patterns were record ed under various dynamic gamma stimulation frequencies of between 10 a nd 120 stimuli per second. With 26 passive spindle fibers of the type known as bag, Ia fibers, the discharge patterns were obtained under pr ogressively increasing prestretch of the muscle. From each discharge p attern the following discharge frequencies were read: the initial acti vity (the discharge frequency before the start of ramp stretching), th e peak dynamic discharge (the discharge frequency at the end of the dy namic phase of stretching), the maximum static value (MSt; the dischar ge frequency at the beginning of the static phase of stretching), and the final static value (the discharge frequency at the end of the 3rd s of the plateau phase). These four discharge frequency values were pl otted against MSt, in separate diagrams for the Ia fibers under dynami c gamma stimulation and for the bag, Ia fibers. The relationship betwe en the four discharge frequency values and the MSt turned out to be th e same or much the same - for both groups of Ia fibers. This means tha t the two groups of Ia fibers produced (more or less) identical discha rge patterns in response to the ramp-and-hold stretch. In addition, wh ere Ia fibers of the two groups had the same MSt, their dynamic and st atic responses were determined. Under these circumstances no differenc e was found in respect to their stretch properties between Ia fibers o f dynamically gamma-activated spindles and bag, Ia fibers of passive s pindles. In the Discussion, the high degree of similarity in the behav ior of the two groups of Ia fibers is explained in terms of the mechan ical properties of intrafusal bag, fibers, which render it likely that in passive intrafusal ba,ol fibers stretch activation will evoke the same mechanical behavior as dynamic gamma activation.