DIFFERENCES IN THE TUBOCURARINE ANTAGONISM OF THE ACTIVATION OF MUSCLE SPINDLE AFFERENTS BY SUCCINYLCHOLINE, ACETYLCHOLINE AND NICOTINE

¹ Department of Pharmacology, University of Illinois College of Medicine, Chicago, Illinois

Oscillographic recordings of the activity of group Ia and II afferent fibers from deefferented muscle spindles in the triceps surae muscle group were carried out in acutely spinalized cats. Smaller doses of i.v. administered tubocurarine (TC) were required to abolish extrafusal neuromuscular transmission than to abolish transmission from fusimotor nerve fibers to intrafusal muscle fibers. After the administration of sufficient TC to abolish fusimotor neuromuscular transmission the stimulation of the group Ia or II afferent discharge induced by succinylcholine (SCh) or nicotine was reduced but not abolished, whereas the stimulation by acetylcholine (ACh) was abolished. It is concluded that SCh and nicotine act, at least in part, postsynaptically on the intrafusal muscle fibers whereas the action of ACh could be largely confined to the presynaptic endings on intrafusal fibers. However, some of these differences between the actions of ACh and of the other agents could be interpreted as due to variations in degree of direct antagonism of TC.