The effect of succinylcholine on cat gastrocnemius muscle spindle afferents of different types

1. 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 I.V. dose of SCh of 200 micrograms kg-1. As for previous studies of jaw muscle spindles the basic measurements were initial frequency (IF), peak frequency (PF) and static index (SI), the frequency 0.5 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 delta. 3. delta DD and delta IF were each distributed bimodally and since they were uncorrelated formed the basis for a four-way classification. Since delta DD can be attributed to activation of bag1 (b1) intrafusal fibres and delta IF 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 b1c, b1b2c, b2c 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 b1c type in gastrocnemius. 5. Conduction velocity was bimodally distributed with the best dividing line at 63.2 m s-1. The b1b2c units were all, save one, in the fast group, while the b2c 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 b1 fibre influence, while many secondaries have marked b2 fibre 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.