The influence of increased muscle spindle sensitivity on Achilles tendon jerk and H-reflex in relaxed human subjects

Abstract

Whether the fusimotor system contributes to reflex gain changes during reinforcement maneuvers is re-examined in the light of new data. Recently, from direct recordings of spindle afferent activity originating from ankle flexor muscles, we showed that mental computation increased the muscle spindle mechanical sensitivity in completely relaxed human subjects without concomitant alpha-motoneuron activation, providing evidence for selective fusimotor drive activation. In the present study, the effects of mental computation were investigated on monosynaptic reflexes elicited in non-contracting soleus muscle either by direct nerve stimulation (Hoffmann reflex, H) or by tendon tap (Tendinous reflex, T). The aim was to relate the time course of the changes in reflex size to the increase in spindle sensitivity during mental task in order to explore whether fusimotor activation can influence the size of the monosynaptic reflex. The results show changes in reflex amplitude that parallel the increase in muscle spindle sensitivity. When T-reflex is consistently facilitated during mental effort, the H-reflex is either depressed or facilitated, depending on the subjects. These findings suggest that the increased activity in muscle spindle primary endings may account for mental computation-induced changes in both tendon jerk and H-reflex. The facilitation of T-reflex is attributed to the enhanced spindle mechanical sensitivity and the inhibition of H-reflex is attributed to post-activation depression following the increased Ia ongoing discharge. This study supports the view that the fusimotor sensitization of muscle spindles is responsible for changes in both the mechanically and electrically elicited reflexes. It is concluded that the fusimotor drive contributed to adjustment of the size of tendon jerk and H-reflex during mental effort. The possibility that a mental computation task may also operate by reducing the level of presynaptic inhibition is discussed on the basis of H-reflex facilitation.

Sympathetic outflow enhances the stretch reflex response in the relaxed soleus muscle in humans

“…An increased firing rate from primary muscle spindle afferents and increased stretch sensitivity during mental computation have also been demonstrated (26). In a study of a mental computation task, Rossi-Durand and colleagues (28) observed that the increases in stretch reflex amplitude occurred without H-reflex changes, indicating changes in muscle spindle sensitivity rather than changes in ␣-motoneuron excitability or presynaptic inhibition on Ia afferents. They ascribed the increased stretch sensitivity during mental computation to a fusimotor sensitization of muscle spindles and suggested that, in relaxed subjects performing mental computation, the presetting of the fusimotor activation in priming spindle sensitivity could prepare the muscle spindles to better play their role in proprioception and motor control.…”

“…A mental computation task (28) and unpleasant picture viewing (3) both clearly facilitated the stretch reflex in the soleus muscle. An increased firing rate from primary muscle spindle afferents and increased stretch sensitivity during mental computation have also been demonstrated (26).…”

“…Macefield et al (17) failed to observe any changes in muscle spindle firing during a strong and sustained increase in muscle sympathetic nerve activity (MSNA) in the relaxed human leg. In contrast Rossi-Durand and coworkers (26,28) found that mental arithmetic, known to increase MSNA in leg muscles, facilitated the stretch reflex in the soleus muscle. Furthermore, a recent study suggested that proprioceptive acuity involving the muscle spindles was unchanged or, in one condition, improved during sympathetic activation (20).…”

Respiratory response to passive limb movement is suppressed by a cognitive task

“…A cognitive task has been shown to affect the H reflex in humans (38). It is therefore possible that neural activity via a cognitive task may modulate other neural signals at the level of the spinal cord in a manner similar to the way in which central motor command that has been shown to interact with afferent feedback (14).…”

Effects of hypoxia on muscle response to tendon vibration in humans

“…The H reflex reflects spinal excitability through the integration of afferent pathways and reflex activation of ␣ motoneurons. 29 Furthermore, a recent human study 33 showed that the changes in activity of muscle spindle primary endings are generally positively correlated with those of both the mechanically (tendon jerk) and electrically (Hoffman) elicited reflexes. However, the TVR does not correspond solely to a monosynaptic reflex between spindle afferents and spinal motor neurons.…”