Interaction of the movement-dependent, extrafusal and fusimotor after-effects in the firing of the primary spindle endings.

The after-effects of the firing of the primary spindle endings were studied in ankle extensor muscles of cats under Nembutal anesthesia. The activities of 27 primary endings of the muscle spindles from mm. soleus, plantaris, and gastrocnemius have been analysed in various combinations of fusimotor and extrafusal stimulation and application of the mechanostimulation to the spindle bearing muscle. Short-term simulation of static gamma-axons evoked a post-stimulation increase in the spindle ending firing, which can be recorded under both isometric and isotonic conditions on applying a weak extrafusal stimulation or without it. The movement-dependent after-effects were tested with a double-trapezoid pattern of muscle length (or load) changes. The after-effects consisted of the difference of firing rates at the same values of muscle length (or load) with opposite direction of movement to the steady states; these uncertainties were also present during constant stimulation of static gamma-axons. The rate difference showed a tendency to a certain decrease with stimulation rate increment. For diapason of the stimulation rates up to 125 impulses/s a small negative correlation (r = -0.61) between the firing rate differences and the gamma-stimulation rate has been registered in the population of primary endings tested under length servo-control conditions. Using a frequency-modulated intrafusal stimulation, a clockwise hysteresis dependence of the spindle firing rate upon stimulation rate was demonstrated. The pronounced after-effects were shown to exist for steady rates of stimulation: the discharge rates were always higher after stimulation rate increase and lower after its decrease. Fusimotor after-effects were effectively destroyed by both the extrafusal stimulation and the cyclic length (load) changes evoked lengthening-shortening movements of the muscle. The results obtained can be considered as evidence for a hypothesis that history-dependent behavior of muscle spindles is mainly connected with hysteresis of the intrafusal muscle fibers and the whole spindle bearing muscle.