The effects of suxamethonium and acetylcholine on the behaviour of cat muscle spindles during dynamic stretching, and during fusimotor stimulation

1. The responses of primary and secondary afferent fibres from muscle spindles in cat soleus were studied during constant velocity stretching. 2. Intravenous Suxamethonium (SCh) caused a large increase in the response of the primary afferents to the dynamic phase of stretching, and a smaller increase in their response to static extension. The effects of SCh were similar to the effects of dynamic fusimotor stimulation. 3. Increasing doses of SCh increased the response of primary afferents to dynamic stretching up to a point, but a peak discharge frequency was encountered beyond which the afferent fibre could not be induced to discharge. 4. Suxamethonium increased the response of the secondary afferent fibres by a smaller amount than the primaries, and in particular caused a smaller increase in the response to dynamic extension. 5. Acetylcholine given by close arterial injection had an effect similar to the effect of SCh. 6. The effects of static and dynamic fusimotor stimulation on the response of primary afferents summed with the effects of small doses of SCh. When large amounts of SCh were used fusimotor stimulation sometimes had no further effect on the afferent discharge. It was not possible to say whether the fusimotor activity was then inhibited, or submerged in the SCh activity. 7. The actions of SCh and of acetylcholine emphasized the differences in response of primary and secondary afferent endings to dynamic stretching. The use of these drugs enabled us to classify fibres of intermediate conduction velocity. 8. Suxamethonium is known to activate slow muscle fibre systems with distributed nerve endings. The similarity between dynamic fusimotor activity and the effect of SCh suggests that the dyanimic fusimotor fibres act on slow intrafusal muscle fibres through multiple distributed endings.