Comparison of the Effects of Stimulating Groups of Static γ Axons With Different Conduction Velocity Ranges on Cat Spindles

Abstract

In cat peroneus tertius muscles, static γ axons were prepared in groups of three to four according to the conduction velocity of their axons (fast, intermediate, or slow). Effects of stimulating these groups (at 20, 30, and 50 Hz) on spindle ensemble discharges during sinusoidal stretch (peak-to-peak amplitude, 0.5 mm; frequency linearly increasing from 0.5 to 8 Hz in 10 s) were compared. Ensemble discharges were obtained by digital treatment of the discharges in afferent fibers from all the spindles in peroneus tertius as recorded from the muscle nerve. Stimulation of each group prevented ensemble discharges from falling to very low levels during shortening phases. However, this effect was clearly larger when the group of fast-conducting axons was stimulated. In view of the known effects of the activation of bag2 and chain fibers (either separately or together) on single primary ending discharges during comparable sinusoidal stretches, this stronger effect supports the view that static γ axons with faster conduction velocities are more likely to supply more bag2 fibers than slower ones. Possibly the proportions of bag2 and chain fibers activated during motor activity are determined by a recruitment of static γ motoneurons related to their size.