The excitation of muscle stretch receptors by suxamethonium

1. Changes in the response of cat soleus muscle spindle primary, secondary and "intermediate" sensory endings to ramp stretchin during intravenous and intra-arterial infusions of suxamethonium (SCh) in the intact anaesthetised animal have been studied. The effects of SCh on the spindle sensory endings have been related to the known action of SCh and ACh on the two nuclear bag intrafusal muscle fibres of isolated cat muscle spindles in vitio (Gladden 1976) 2. The application of repetitive short-duration ramp-and-hold test stretches to the soleus muscle during continuous slow infusion of SCh enabled the gradual activation of the spindle sensory endings to be studied in detail. Typical muscle spindle primary sensory endings were activated in three consecutive stages during infusion of SCh, which will be termed Phases I, II and III of excitation. In Phase I of excitation, a gradual facilitatory effect of SCh on tha discharge of the Ia sensory endings accompanied by a progressive reduction in the sensitivity of the endings to liuscle shortening was observed. This occurred without a potentiation of, and usually a decrease in, the dynamic and position sensitivities of the Ia endings to muscle stretch. The Phase I facilitatory effects of SCh on la endings appear not to be caused by the contraction of intrafusal muscle fibres, and are probably the result of a direct or indirect depolarising action of SCh on the sensory nerve terminals of the la afferent axon. In Phase II of excitation, the sensitivity of the Ia endings to the dynamic phase of the ramp stretch increased dramatically, and the response of the endings to stretch was very similar to that of la endings during strong dynamic fusimotor activation. This increase in the dynamic sensitivity is very likely the result of the recruitment of the dynamic nuclear bag (DNB) fibre, which has the lower threshold of SCh of the two nuclear bag fibres (Gladden 1976), and which is made to contract by dynamic fusi- motor axons (Boyd, Gladden, McWilliam&Ward 1977), In Phase III of excitation, a marked increase in the position sensitivity of the Ia endings was superimposed on the already enhanced dynamic sensitivity, and the response of the la endings to stretch became very similar to that during combined stimulation of powerful dynamic and static fusi-motor axons. The increase in the position sensitivity of the Ia endings in Phase III of activation no doubt reflects the contraction of the static nuclear bag (SNB) fibre, which has the higher threshold to SCh of the two nuclear bag fibres (Gladden 1976). The characteristic response to stretch of la endings in Phase III of excitation is thus the result of the combined activation of the dynamic and static nuclear bag fibres by SCh. 3. Unlike soleus muscle spindle primary sensory endings, secondary endings showed only a gradual facilitation of their discharge during infusion of SCh, and behaved in a similar way to Ia endings in Phase I of excitation. while Ia endings subsequently experienced large increases in their dynamic and position sensitivities following the recruitment of the two nuclear bag fibres, secondary sensory endings did not show similar effects. The activation of typical secondary sensory endings by SCh therefore appears to be entirely the result of a direct or indirect depolarising action of SCh on the afferent nerve terminals, without any apparent contribution from the contraction of the two nuclear bag fibres