Activation of Muscle Spindles by Succinylcholine and Decamethonium.: The Effects of Curare.

Abstract

Summary. The effect of the neuromuscular blocking agent succinylcholine‐iodide (Sch) was studied, by intraarterial injection, on single muscle spindles in cats. Spindle afferents were isolated in the dorsal roots. The injection was made into the ipsilateral arteria profunda femoris or at the aortic bifurcation from a contralateral cannula introduced through the femoral artery. In all, some 80 muscle spindles were studied under myographic control (strain gauge). Of these 21 belonged to m. soleus , 16 to m. tibialis anticus , 33 to m. gastrocnemius , and the rest to the joint gastrocnemius‐soleus muscle. In addition 20 Golgi tendon or B‐organs were studied. Sch in subparalytic doses caused a strong transient regular discharge of the muscle spindles, reaching lower maximum frequencies in soleus than in the other muscles. The same stimulating effect was obtained with larger doses after full paralysis of alpha motor end plates. The Sch‐discharge was analyzed with respect to threshold, time course, initial muscle tension and efferent (small fibre or) gamma excitation of the muscle spindles. Though spindle excitation by Sch was favoured by an increase in initial tension or stretch, it could always be obtained in wholly slack muscle by slightly increasing the dose of Sch. After full paralysis by d‐tubocurarine of the excitatory effect of both the ordinary alpha motor fibres as well as the gamma fibres sufficiently large doses of Sch still elicited in most spindles the typical transient increase of discharge rate. Sch ultimately paralyzed the gamma end plates. Before d‐tubocurarine or Sch paralyzed the gamma end plates there often appeared a transient phase of facilitation. Decamethonium, tried in a few cases, also increased the discharge rate of the muscle spindles but the effect, apart from lasting for at least an hour (longer times not tried) as against some 3–10 minutes with Sch (depending upon dosage and preparation), consisted in irregular bursts of discharges which could be blocked by curare in large doses. It thus differed greatly from the highly regular and transient outburst of spikes obtained by Sch which rapidly reached its maximum and then slowly decayed. Golgi tendon or B‐organs were not fired by Sch. In the discussion it is pointed out that several of the results are best understood on the basis of the assumption that Sch, whatever it may do to the gamma fibre end plates, also affects the sensory Spindle organs directly.