Activation of cat muscle spindle primary, secondary and intermediate sensory endings by suxamethonium.

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Abstract
Acetylcholine (ACh) when applied to the isolated cat muscle spindle caused contraction of the dynamic and the static nuclear bag fibers, but not of the nuclear chain fibers, and the dynamic nuclear bag fiber had a lower threshold to ACh than the static nuclear bag fiber. Suxamethonium (SCh) has similar effects on the intrafusal muscle fibers. The response of cat soleus muscle spindle primary, secondary and intermediate sensory endings to repetitive ramp stretches during continuous slow infusions of SCh were studied. The changes observed are interpreted on the basis of the known action of SCh on the intrafusal muscle fibers of the isolated spindle. Primary sensory endings, with afferent axon conduction velocities > 80 m/s, were activated during SCh infusion in 3 stages. In phase I of excitation, a gradual facilitation of the discharge of the Ia endings was seen, without potentiation of the dynamic or length sensitivity to stretch. In Phase II of excitation, the dynamic sensitivity of the Ia endings increased very markedly. In phase III of excitation, an increase in length sensitivity was superimposed on the already enhanced dynamic sensitivity. Secondary sensory endings, with afferent axon conduction velocities < 60 m/s, only experienced a gradual facilitation of their discharge during SCh infusion, similar to the Phase I effects of SCh on primary endings. The majority of intermediate sensory endings, with afferent axon conduction velocities between 60 and 80 m/s, were activated by SCh in the same way as primary endings, or in the same way as secondary endings. A significant number of these sensory endings behaved in a truly intermediate manner during SCh infusion, and may represent an intermediate form of spindle sensory ending. The afferent axon conduction velocities of the truly intermediate sensory endings were restricted to the range 69-77 m/s. All the intermediate endings which were activated in a primary-like manner had afferent axons conducting at velocities > 74 m/s. Intermediate endings which were activated in a secondary-like manner had afferent axons conduction velocities < 72 m/s. The probable contribution of the static and dynamic nuclear bag fibers to the discharge of each type of spindle sensory ending is discussed.

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