Quantitative Aspects of the Use of Succinylcholine in the Classification of Muscle Spindle Afferents

Summary

Testing with succinylcholine (SCh) has been shown to give valuable information about the nature of the contacts of muscle spindle afferents on the three different intrafusal muscle fibre types. The important features of the methods to be used in order to allow quantitative interpretation of the test are reviewed. From large populations of gastrocnemius and jaw muscle afferents in the cat it was concluded that only two measurements of the ramp and hold stretch responses were needed to detect significant endings on bag₁ and bag₂ fibres. These were the increments in dynamic stretch response and in initial frequency caused by the SCh. Evidence is reviewed to support the view that essentially all the effects of SCh, under the dosage conditions used, are due to contraction of the bag fibres. The resulting predictions regarding patterns of afferent terminations on intrafusal fibres are compared with the conclusions from independent histological observations. From studies of jaw muscle afferents it is concluded that the SCh classification can give new insights into factors which influence central connectivity patterns.