This is a report of observations on the responses of the primary and secondary endings of soleus muscle spindles of the anesthetized cat to the combined effects of the depolarizing neuromuscular blocker succinyl choline (SCh), given intravenously, and fusimotor stimulation. The findings were interpreted in terms of a dual pacemaker model for activity generated in the bag1 intrafusal fiber interacting with activity coming from bag2 and chain fibers. 2. In preliminary experiments it was found, using whole ventral root stimulation at fusimotor strength, that spindle responses to fusimotor stimulation were not blocked by SCh, whereas extrafusal junctions blocked rapidly. In the presence of SCh, fusimotor responses of spindle secondary endings were, on average, slightly larger than their control values before SCh was given, whereas fusimotor responses of primary endings were slightly smaller. 3. A study of the responses of spindle primary endings to stimulation of single dynamic (gamma D) and static (gamma S) axons in the presence of SCh revealed a fundamental difference in behavior. None of the responses to stimulation of gamma D axons (9 gamma D axons with 8 primary endings) showed significant summation with the responses to SCh. By contrast, the 20 gamma S axons studied showed varying degrees of summation with the responses to SCh. The responses of secondary endings to gamma S stimulation in the presence of SCh resembled those of primary endings and gamma S stimulation. 4. To explain these differences it is proposed that the primary ending has two separate sites of impulse initiation, one close to terminals on the bag1 intrafusal fiber (innervated by gamma D axons) and a second close to terminals on the bag2 and chain fibers (innervated by gamma S axons). It is proposed that the maintained increase in spindle firing observed during SCh infusion is the result of a bag2 contracture. The response to gamma S stimulation, contracting bag2 and chain fibers, adds to the SCh response. The degree of summation varies depending on whether the gamma S activates bag2 fibers, chain fibers, or both. The bag1 contracture, together with the effects of gamma D stimulation, acts through a separate pacemaker and therefore does not sum with the steady increase in spindle firing in the presence of SCh. There may be pacemaker switching between the bag1 generator and the bag2 and chain generator. 5. If the model is representative of most spindles containing the three kinds of intrafusal fibers, and the contractions of bag2 and chain fibers generate activity through a common impulse generator, then this bears on the question of the functional independence of the bag2 and chain fiber systems.
