The responses of secondary endings of cat soleus muscle spindles to succinyl choline.

This report describes the effects of succinylcholine (SCh) on the secondary endings of cat soleus muscle spindles and attempts to explain them in terms of the action of the drug on intrafusal fibres. All but 2 of 41 secondary endings studied in detail showed a significant response to a single intravenous injection of 200 micrograms kg-1 SCh. This consisted of a rise in the resting rate or development of a resting discharge if the spindle had previously been silent and an increase in the response to stretch. The increases in the responses to stretch were weaker than those observed for primary endings of spindles, but were much larger than those of tendon organs, which showed very little effect with this concentration of drug. The response to SCh showed two features consistent with its action being mediated via an intrafusal muscle fibre contraction rather than a direct depolarising action on the afferent nerve ending. In the presence of SCh, secondary endings were able to maintain a discharge during muscle shortening at rates, on average, more than 5 times greater than under control conditions. Secondly, the increase in spindle discharge produced by SCh showed a length dependence similar to that for fusimotor stimulation. Further support for the action of SCh being principally via an intrafusal fibre contraction was provided by the observation that its effects were abolished by the neuromuscular blocker gallamine triethiodide.(ABSTRACT TRUNCATED AT 250 WORDS)