Mechanisms Underlying the Excitation of Muscle Afferents by Suxamethonium
Topical application or systemic administration of suxamethonium (succinylcholine, SCh) has proved to be a useful technique for the relatively rapid, reversible excitation of muscle spindles, independently of their gamma fusimotor innervation. Current evidence indicates that the powerful excitation of primary Ia afferents is due to the contracture of the bag1 and bag2 intrafusal muscle fibres in the presence of SCh. The effects of SCh on long chain fibres are presently unknown. Secondary endings are excited much less powerfully than primary endings, and there is debate about the mechanisms responsible. Secondaries may be excited as a result of the hyperkalaemic “side-effects” of systemic SCh administration or by depolarisation of collateral sensory terminals they may have on the bag intrafusal fibres. It is likely that all spindle afferents (as well as other non-spindle afferents) are affected by the increase in serum potassium levels, while only those spindle afferents with substantial collateral terminals on bag fibres are affected by their contraction. The application of SCh as a means of assessing the contribution of individual intrafusal fibres to the afferent discharge of different spindle endings is briefly reviewed.
KeywordsAcetylcholine Succinylcholine Carbachol Suxamethonium Hexamethonium
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