Changes in respiratory muscle activity in conscious cats during experimental dives at 101 ATA

Le rôle joué par les fibres afférentes métabosensibles dans les mécanismes adaptatifs neuromusculaires

Les adaptations de l'organisme à l'exercice sont permises par l'ajustement de l'activité des neurones centraux qui est en partie régulée par l'activité des afférences I et II (mécanosensibles), par la mise en jeu des afférences III et IV (métabosensibles) et par les modifications du métabolisme musculaire au cours de l'exercice. Le rôle des afférences métabosensibles apparaît comme fondamental dans les mécanismes adaptatifs à l'exercice et dans la tolérance à la fatigue. Néanmoins, de nombreuses interrogations demeurent. Cette revue fait le bilan des connaissances concernant l'implication de ces afférences dans les boucles de rétrocontrôle sensori-motrices et les mécanismes d'adaptation neuromusculaire. Il semble désormais établi que l'activation des afférences métabosensibles soit à l'origine de l'adaptation cardiovasculaire et respiratoire à l'exercice. De plus, ces afférences seraient à l'origine d'un mécanisme de protection du muscle contre la fatigue en modulant la commande motrice centrale au niveau spinal et supraspinal.

Métabosensibilité musculaire et adaptations physiologiques au cours de l’exercice

The first role played by group III (thin myelinated) and group IV (unmyelinated) afferent fibres from skeletal muscle is to transmit nociceptive information from muscle to the central nervous system. The second role of these free endings located in the interstitium of the muscle is to induce cardiovascular and respiratory adjustments during muscular exercise. These respiratory and circulatory responses during muscular exercise may be reflexly induced via muscular afferents. Indeed, static contraction of hindlimb muscles in anaesthetised mammals has been shown to reflexly increase the ventilatory function, the myocardial contractility and heart rate. The mechanical muscle deformation and the accumulation of metabolites in its intersitium are the cause of raised activity in small nerve fibres which in turn induces the physiological responses. It is also admitted that the central locomotor areas on the medullary and spinal neuronal pools control ventilatory and cardiovascular function during exercise. This mechanism is called "central command". Furthermore, adjustments of the locomotor activity during exercise is mediated by the thinly myelinated and unmyelinated fibres with endings in the working muscle. These fibres, also called "metaboreceptor" may be responsible of the coupling between the ventilation and the locomotion. Thickly myelinated muscle afferents (i.e. group I and II) appear to play little role in causing the reflex autonomic responses to contraction.

EMG power spectrum of respiratory and skeletal muscles during static contraction in healthy man

Changes in EMG power spectrum during isometric voluntary contraction maintained until exhaustion in the range of 20-80% MVC were studied in three skeletal muscles (adductor pollicis or AP, vastus lateralis, and medialis) and two respiratory muscles (diaphragm and rectus abdominis). Quantitative EMG analysis consisted of computation of the median frequency (MF) of power spectra and also the continuous measurement of EMG power in two bands of high (EH) and low (EL) frequencies using bandpass filters. This allowed the calculation of the H/L ratio and its time constant of decay rate (TC delta H/L) throughout the sustained static contraction. The main results were: (1) highly significant, positive correlations between TC delta H/L and the maximal MF changes and also the endurance time to fatigue; (2) EMG changes were determined early, within the first 10-20 s of contraction; and (3) EL always increased throughout the fatiguing isometric contraction, but EH changes markedly varied within the five muscle groups studied. These observations are discussed in terms of the differences in muscle fiber composition and also the variations in motor unit recruitment.