The regularity of muscle spindle discharge in man.

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Abstract
The variability of discharge of 39 muscle spindle afferents from the pretibial muscles of normal human subjects was determined for spike train sequences recorded with the ankle joint fixed in 25.degree. plantar flexion, during further stretch and during graded voluntary contractions of the receptor-bearing muscle. In non-contracting muscles with the ankle joint in 25.degree. plantar flexion, a sustained discharge was maintained by 24 of the 39 endings. The mean discharge frequency for the active endings was 11.1 Hz (range 4.8-22.1 Hz), the mean coefficient of variation 0.073 (range 0.021-0.183). With further stretch, the discharge of endings maintaining frequencies below 10-12 Hz became more regular. For endings maintaining higher frequencies, changes in the coefficient of variation were small and occurred in either direction. All secondary endings maintained a highly regular discharge, but, at these frequencies, there was no statistically significant difference in the variability of primary and secondary endings. There appears to be no functionally effective background fusimotor drive to non-contracting muscles of normal human subjects. A voluntary contraction sufficient to accelerate a spindle ending invariably decreased the regularity of its afferent discharge. During voluntary contractions, coefficients of variation up to 0.345 were recorded. Coefficients as low as 0.1 were not uncommon, and thus the absence of fusimotor drive cannot necessarily be inferred from a regular afferent discharge pattern. With contractions of different strength the increase in the coefficient of variation did not parallel the increase in discharge frequency. Not all fusimotor influences acting on a spindle ending are translated into variability, and measurements of the variability of discharge do not accurately reflect the level of fusimotor drive. The discharge frequency of some spindle endings decreased slightly in some contractions and this was accompanied by an increase in the variability of discharge. Contracting extrafusal muscle fibers can modulate the discharge pattern of spindle endings and contribute to the variability of discharge during a voluntary contraction. In contracting muscles the irregular fusimotor-driven spindle discharge contained a ''hidden'' periodicity, but this was not as extensive as has been reported for the cat. No such periodicity could be demonstrated for spindle endings in noncontracting human muscles.