SUMMARY1. The effects of the muscle-depolarizing drug succinylcholine (SCh) on the stretch responses of jaw-closer muscle spindle afferents were studied in the anaesthetized cat. Using ramp and hold stretches repeated every 6 s the basic measurements made were: initial frequency (IF), peak frequency (PF) and static index (SI), the frequency 0-5 s after the end of the ramp of stretch. Derived from these were: dynamic difference (DD) = PF -IF, dynamic index (DI) = PF -SI and static difference (SD) = SI -IF. Increases in these measures caused by a single i.v. dose of SCh (200 gtg kg-') are symbolized by the prefix A.2. In a population of 234 units, ADD and AIF were each distributed bimodally, but were uncorrelated, thus defining four subgroups.3. ADD was argued to be an index of the effect of bag1 intrafusal fibre contraction and AIF to be an index of the effect of bag2 fibre contraction. On this basis it is proposed that units can be divided into four groups according to the predominant influences of the bag1, bag2 and chain fibres as blc (6-8%), b1b2c (22-2%), b2c (54-3%) or c (16&7%).

SUMMARY1. A population of 269 gastrocnemius muscle spindle afferents have been studied in anaesthetized cats for the effects of succinylcholine (SCh) on their response to ramp and hold stretches repeated every 6 s. The effectiveness and reliability of the SCh test was improved by prior stimulation of the muscle at 10 Hz for 30 s to increase the blood flow.2. Responses have been assessed from averaged cycle histograms before and after a single iv. dose of SCh of 200 jag kg-'. As for previous studies of jaw muscle spindles the basic measurements were initial frequency (IF), peak frequency (PF) and static index (SI), the frequency 05 s after the end of the ramp of stretch. Dynamic difference (DD = PF-IF), dynamic index (DI = PF-SI) and static difference (SD = SI -IF) were derived from these and increases caused by SCh indicated by the prefix A. 3. ADD and AIF were each distributed bimodally and since they were uncorrelated formed the basis for a four-way classification. Since ADD can be attributed to activation of bag1 (b1) intrafusal fibres and AIF to bag2 (b2) fibres, while all afferents receive input from chain (c) fibres it is proposed as with the jaw spindles that the classes correspond to predominant influence from b1 c, b1 b2 c, b2 c and c intrafusal fibres.4. The proportion of units in the different groups were similar to those in the jaw muscles except for there being very few b1 c type in gastrocnemius. 5. Conduction velocity was bimodally distributed with the best dividing line at 63 2 m s51. The b1 b2c units were all, save one, in the fast group, while the b2 c units were equally divided between fast and slow.6. Mean control values for DD did not differ between the b1b2c and the b2c groups, which is taken to indicate that the b1 fibre does not contribute significantly to the dynamic stretch response of spindles with no intrafusal contraction.7. The results emphasize the importance of recognizing that some apparently primary afferents lack b, fibre influence, while many secondaries have marked b2fibre influence. 8. The importance of the SCh classification is discussed in relation to the identification of fusimotor effects on spindle discharge and in relation to studies of central connectivity.MS 9904

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SUMMARY1. A method of classification of muscle spindle afferents using succinylcholine (SCh) and ramp stretches has recently been described, which appears to estimate separately the strength of influence of bag, (b1) and of bag2 (b2) intrafusal fibres.Increase in dynamic difference (ADD) indicates b1 influence whilst increase in initial frequency (AIF) indicates b2 influence. The significance of this classification has now been examined by correlation with the strength of synaptic projection of jaw muscle spindle afferents to the fifth motor nucleus (MotV) and the supratrigeminal region (STR) in anaesthetized cats.2. Projection strength was estimated by computing the extracellular focal synaptic potential (FSP) from spike-triggered averages of 1024 sweeps at 100 ,pm

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