This is a report on the resistance to block of the motor terminals on intrafusal fibres of cat soleus muscle spindles using the drug gallamine triethiodide (Flaxedil). To minimize diffusion barriers and to permit accurate measurements of time courses, rather slow rates of gallamine infusion were used (0.15 mg min-1). The main finding made was that after gallamine infusion, when extrafusal tension had dropped to half, all dynamic fusimotor effects and eight of twenty static effects had fallen to 40% or less of their control value. The remaining static effects persisted at 60-80% of their control value. Where fusimotor fibres were stimulated together with one or two skeletomotor fibres, the influence of the skeletomotor axons was significant only after spindle biasing had fallen to low levels. When gallamine infusion was stopped extrafusal tension returned to control levels within 20-75 min, depending on the length of the block, while fusimotor responses did not fully recover within the recording period of up to 150 min. The combination for some fusimotor responses of an early fall and a late recovery when compared with extrafusal tension, suggested a greater sensitivity of these endings to the drug. A comparison of spindle responses to the drug succinyl choline (SCh) and to fusimotor stimulation in the presence of gallamine showed that SCh responses were rapidly reduced by gallamine and had a long recovery time course, as were some fusimotor responses. From this it is argued that fusimotor effects with a high sensitivity to gallamine blockade were associated with nuclear bag fibre contractions and the more resistant effects with nuclear chain fibre contraction. It is generally believed that intrafusal neuromuscular junctions are more resistant to neuromuscular blockers than extrafusal junctions. The present experiments provide evidence to the contrary for some intrafusal junctions. Since muscle relaxants are often used in general anaesthesia it is interesting to speculate about the recovery of function of proprioceptive reflexes and of kinaesthesia during the immediate post-anaesthetic period, in view of the large difference in recovery time for transmission at intrafusal and extrafusal junctions.

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