Discharge of human muscle spindle afferents innervating ankle dorsiflexors during target isometric contractions

Abstract

There are discrepancies in the literature about the reproducibility of forces at which human muscle spindle afferents accelerate their discharge during isometric voluntary contractions. The aim of this study was to determine for single muscle spindle afferents both the reproducibility of the ‘acceleration threshold’ and the factors contributing to variability of ‘acceleration threshold’. Microneurographic recordings were made from muscle spindle afferents innervating tibialis anterior while subjects performed isometric ankle dorsiflexions. Subjects matched the force of their contractions with a visually displayed ‘ramp‐and‐hol? template. Template parameters were determined by the force of maximal isometric ankle dorsiflexion (MVC), and expressed as per cent MVC. The required ‘ramp’ rate and ‘hold’ force was adjusted between trials (range, 0.5‐5% MVC s− and 0.5‐20% MVC, respectively). The duration of the hold phase was 4 s and, following each contraction, stretch was applied transversely to the tendon to minimize the influence of any ‘after‐effects’ on spindle afferent responses in subsequent contractions. For each contraction, the force at which the rate of muscle spindle discharge increased was defined as the ‘acceleration threshold’. Of twenty‐six muscle spindle afferents innervating tibialis anterior, all but two increased their discharge in the test contractions. In 90% of contractions, acceleration thresholds were less than 3.2% MVC (range, 0.01–11.9% MVC). Individual muscle spindle afferents increased their discharge at similar but not identical forces in repeated contractions. There was a positive correlation between the rate of contraction and the acceleration threshold (P < 0.001), but the strength of the target contraction had no effect on the threshold, and there was no trend for thresholds to change over time. The results suggest, first, that most muscle spindle endings in the human pretibial muscles receive a significant increase in fusimotor drive during relatively weak isometric efforts and secondly, that when fusimotor after‐effects are controlled, much of the residual variability in ‘acceleration threshold’ for any one spindle in repeated contractions is due to extrafusal factors, particularly variability in contraction rate.

Changes in Human Muscle Spindle Sensitivity during a Proprioceptive Attention Task

“…It has always been a common and intriguing observation that muscle spindle firing rates are much lower in humans than in animals (Prochazka and Hulliger, 1983), and the effect of the fusimotor system on muscle spindle sensitivity may also be weaker than in animals. This notion is supported, for example, by the fusimotorinduced increase in muscle afferent discharge during isometric contractions, which is 5-15 times lower in humans than in awake cats (Prochazka et al, 1977;Wilson et al, 1997).…”

Muscle history, fusimotor activity and the human stretch reflex

“…During voluntary isometric contractions there is good evidence for co-activation of skeletomotor and fusimotor neurones (Vallbo, 1971;Burke et al 1979;Wilson et al 1997). Our interpretation of the increase in stretch reflex size produced by the voluntary contraction is that slack in muscle fibres, both extrafusal and intrafusal, produced by pre-conditioning, is taken up by the voluntary contraction and that fusimotor activation is necessary to remove the slack in spindles.…”

“…A 25% contraction produces no greater fusimotor activation than 10% or, more likely, leaves intrafusal fibres in a mechanical state no different from that after a 10% contraction. In this context, the recent findings by Wilson et al (1997) showed that, in human tibialis anterior, contraction threshold for spindle acceleration, presumably representing fusimotor threshold, was less than 3·2% MVC. Although we have not systematically studied this point, observations in animal experiments suggest that take-up of slack in intrafusal fibres depends on both the duration and rate of fusimotor stimulation (J. E. Gregory, D. L. Morgan & U. Proske, unpublished observations).…”

Effects of muscle action type on corticospinal excitability and triceps surae muscle-tendon mechanics

“…These efferents provide the equivalent of alpha-gamma coactivation; when skeletofusimotor neurons are activated, unloading of the spindle by activation of extrafusal fibers is at least partially compensated by loading due to intrafusal contraction. (Gregory et al 1998;Kandel et al 2000, 725;Vallbo 1971;Wilson et al 1997) Stretch reflex. Mechanical deformation of the intrafusal muscle fibers generates APs by activating mechanically gated ion channels in the afferent axons coiled around the spindle.…”