Sympathetic modulation by cold pressor test alters the spike-triggered average torque and discharge rate of low-threshold motor units

Roatta, Silvestro and Arendt-Nielsen, Lars and Cescon, Corrado and Farina, Dario (2007) Sympathetic modulation by cold pressor test alters the spike-triggered average torque and discharge rate of low-threshold motor units. In: Proceedings of the 37th annual meeting of the Society for Neuroscience, 3-7 Nov 2007, San Diego, CA, USA.

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The sympathetic nervous system mediates functions that support motor activity and may affect the contractility of skeletal muscle as indicated by animal and in vitro studies (1,2). Evidence of this modulatory action in humans is still missing. This study aimed at investigating changes in motor unit twitch torque and discharge rate during sympathetic activation induced by the cold pressor test (CPT) in healthy male subjects. Two groups of subjects were studied according to two experimental protocols. In both studies subjects in sitting position were asked to perform ankle dorsi-flexions before, during, 1min after and 9 min after CPT (left hand immersed in water at 4°C for 5 min) (Fig. 1). The following signals were recorded: intramuscular EMG with wire electrodes from the tibialis anterior muscle, torque produced at the ankle joint, heart rate (HR), arterial blood pressure (ABP), and subjective pain rating. STUDY I: constant discharge rate. 12 subjects (age, mean ± SD, 28 ± 4.6 yr) were asked to maintain the discharge rate of a given motor unit (target motor unit) as low as possible (~8 pps), being provided with visual and auditory feedback of the intramuscular EMG signal. The twitch torque of the target motor unit was extracted from the torque signal by spike-triggered averaging. STUDY II: constant force. 11 subjects (age, 26.5 ± 4.5 yr) were asked to maintain an isometric contraction at 10% of the maximum voluntary contraction with force feedback. Individual motor units activated during the contractions were off-line identified and their discharge rate estimated. Statistics: Kruskal-Wallis test, Wilcoxon matched Paired test. Histograms: mean+SE 1) During CPT the motor unit twitch torque is shortened (study I); the resulting decreased degree of fusion of the twitches explains the observed increase in the estimated twitch amplitude. These effects are likely mediated by a direct action of adrenaline on skeletal muscle fibers. 2) The force produced by low-threshold motor units at a fixed discharge rate presumably decreases during CPT due to twitch shortening. 3) Motor unit discharge rate increases during CPT (study II), probably as a compensatory mechanism to maintain a constant force level. The study shows for the first time in humans that sympathetic activation substantially affects the contractility of skeletal muscle fibers and the discharge rate of motor units

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