Increased muscle spindle sensitivity to movement during reinforcement manoeuvres in relaxed human subjects

Abstract

The effects of reinforcement manoeuvres, such as mental computation and the Jendrassik manoeuvre, on muscle spindle sensitivity to passively imposed sinusoidal stretching (1.5 deg, 2 Hz) in relaxed subjects were analysed. The unitary activity of 26 muscle spindle afferents (23 Ia, 3 II) originating from ankle muscles was recorded using the microneurographic method. Particular care was paid to the subjects’ state of physical and mental relaxation. The results showed that the activity of 54 % of the Ia afferents was modified during mental computation. The modifications took the form of either an increase in the number of spikes (mean, 26 % among 11 Ia fibres) or a shortening in the latency of the response to sinusoidal stretching (mean, 13 ms among 3 Ia fibres), or both. They were sometimes accompanied by an enhanced variability in the instantaneous discharge frequency. The three secondary endings tested exhibited no change in their sensitivity to stretch during mental computation. The increased sensitivity to passive movements sometimes began as soon as the instructions were given to the subjects and sometimes increased during mental computation. In addition, the increased sensitivity either stopped after the subjects gave the right answer or continued for several minutes. During the performance of a Jendrassik manoeuvre, the Ia units underwent changes similar to those described above for mental computation. It was concluded that muscle spindle sensitivity to movement can be modified in relaxed human subjects. The results reinforce the idea that the fusimotor system plays a role in arousal and expectancy, and contribute to narrowing the gap between human and behaving animal data.

Influence of wearing an unstable shoe construction on compensatory control of posture

“…The decrease of agonist onset latency and antagonist offset latency when WUS indicate a higher performance of the postural control system in this condition. Considering the increase of GM activity (Sousa, et al, 2012) and of postural sway while standing with unstable shoes (Landry, et al, 2010), it can be suggested that the higher performance results from a higher neural drive associated to augmented gamamotoneuron activity that leads to a higher sensitivity of the muscles spindles (Ivanenko et al, 1999;Ribot-Ciscar et al, 2000). Another possibility for the higher postural performance could be an increase of Ia-afferent input onto motoneuron pool of the lower limbs (Loram et al, 2005b) as a result of higher muscle length changes while WUS.…”

On the nature of the electromyographic signals recorded during vibration exercise

“…TVR derives from the stimulation of Ia-afferents when applying a sinusoidal stimulation directly to a muscle or a tendon (Bongiovanni et al 1990;Fromm 1976;Burke et al 1976). However, TVR seems also to be modulated by alterations in spindle sensitivity through γ control (Matthews 1966;Ribot-Ciscar et al 2000). By applying a vibration directly to the tendon, previous studies have suggested TVR to be composed of motor unit activity synchronized and unsynchronized with the vibration cycle (Hori et al 1989;Romaiguère et al 1991).…”

Neural correlates of task-related changes in physiological tremor

“…We found that target-guided contractions produced more 7-9 Hz coherence among MUs than self-guided contractions, which is consistent with the predicted effects of an increase in the magnitude or gain of afferent feedback. Such a change in afferent feedback and/or fusimotor activity might have stemmed from changes in the cognitive aspects of the task (Hospod et al 2007;Ribot et al 1986;Ribot-Ciscar et al 2000, changes in sympathetic drive (Abila et al 1985), or the change in force dynamics associated with error correction (Marsden et al 1970). Our results suggest that the absence/ presence of a target underlies changes in tremor previously associated with the general absence/presence of visual feedback.…”