THE RESPONSES OF SECONDARY ENDINGS OF CAT SOLEUS MUSCLE-SPINDLES TO SUCCINYL CHOLINE

This report describes the effects of succinylcholine (SCh) on the seco ndary endings of cat soleus muscle spindles and attempts to explain th em in terms of the action of the drug on intrafusal fibres. All but 2 of 41 secondary endings studied in detail showed a significant respons e to a single intravenous injection of 200 mu g kg(-1) SCh. This consi sted of a rise in the resting rate or development of a resting dischar ge if the spindle had previously been silent and an increase in the re sponse to stretch. The increases in the responses to stretch were weak er than those observed for primary endings of spindles, but were much larger than those of tendon organs, which showed very little effect wi th this concentration of drug. The response to SCh showed two features consistent with its action being mediated via an intrafusal muscle fi bre contraction rather than a direct depolarising action on the affere nt nerve ending. In the presence of SCh, secondary endings were able t o 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 depende nce similar to that for fusimotor stimulation. Further support for the action of SCh being principally via an intrafusal fibre contraction w as provided by the observation that its effects were abolished by the neuromuscular blocker gallamine triethiodide. The time course of recov ery of SCh responses, following their blockade by gallamine, was much slower than recovery of extrafusal tension and closely paralleled that for the recovery of fusimotor responses. In three separate experiment s on the medial gastrocnemius muscle the possibility that SCh may exer t an excitatory action on spindle sensory endings through the liberati on of potassium ions from the muscle was tested by tetanic stimulation of the muscle. This had no detectable excitatory effect. Several obse rvations were made on the effect of SCh on responses of cutaneous rece ptors. SCh did not change levels of spontaneous activity or responses to mechanical stimulation of either slowly or rapidly adapting mechano receptors. It was argued for both tendon organs and cutaneous receptor s that if SCh had a direct action on the nerve ending at the concentra tions used here, some responses of these receptors to the drug might h ave been expected. All of the above supports the view that secondary e ndings of spindles are able to respond to SCh by the development of an intrafusal fibre contracture. The question of the intrafusal fibre ty pes involved is discussed.