Jason H. Mateika

Co-activation of tongue protrudor and retractor muscles during chemoreceptor stimulation in the rat

1 Our primary purpose was to test the hypothesis that the tongue protrudor (genioglossus, GG) and retractor (styloglossus, SG and hyoglossus, HG) muscles are co‐activated when respiratory drive increases, and that co‐activation will cause retraction of the tongue. This was addressed by performing two series of experiments using a supine, anaesthetized, tracheotomized rat in which tongue muscle force and the neural drive to the protrudor and retractor muscles could be measured during spontaneous breathing. In the first series of experiments, respiratory drive was increased progressively by occluding the tracheal cannula for thirty respiratory cycles; in the second series of experiments, the animals were subjected to hyperoxic hypercapnia and poikilocapnic hypoxia. 2 Airway occlusion for thirty breaths caused progressive, quantitatively similar increases in efferent motor nerve activity to protrudor and retractor tongue muscles. Net tongue muscle force was always consistent with tongue retraction during occlusion, and peak force rose in parallel with the neural activites. When airway occlusion was repeated following section of the lateral XIIth nerve branch (denervation of retractor muscles) the tongue either protruded (15/21 animals; 10 ± 2 mN at the 30th occluded breath) or retracted weakly (6/21 animals; 6 ± 2 mN at 30th occluded breath). 3 To ensure that our findings were not the result of damage to the muscle nerves, occlusion experiments were also done in eight animals in which GG EMG activity was recorded instead of nerve activities. Changes in peak integrated GG electryomyogram (EMG) activity and peak retraction force during occlusion were highly correlated (r2= 0.86, slope = 1.05). 4 In separate experiments in fourteen rats, we found that hyperoxic hypercapnia and poikilocapnic hypoxia also result in parallel increases in the respiratory‐related EMG activity of the GG and HG muscles. Also, as in the occlusion experiments, augmentations of protrudor and retractor muscle EMG activities were associated with parallel changes in tongue retraction force. 5 These studies in anaesthetized rats demonstrate that tracheal occlusion and independent stimulation of central or peripheral chemoreceptors results in inspiratory‐related co‐activation of the protrudor and retractor muscles, and proportional changes in tongue retraction force. These observations also demonstrate that recording GG EMG activity in isolation could lead to erroneous conclusions about respiratory‐related movements of the tongue.

Respiratory-related activation of human abdominal muscles during exercise

We tested the hypothesis that abdominal muscles are active during the expiratory phase of the respiratory cycle during exercise. Electromyographic (EMG) activities of external oblique and rectus abdominis muscles were recorded during incremental exercise to exhaustion and during 30 min of constant work rate exercise at an intensity of 85 % of the peak oxygen consumption rate (V̇O2). High amplitude intramuscular EMG activities of both abdominal muscles could be evoked with postural manoeuvres in all subjects. During cycling, respiratory‐related activity of the external obliques was evoked in four of seven subjects, whereas rectus abdominis activity was observed in six of the seven subjects. We measured only the activity that was confined exclusively to the expiratory phase of the respiratory cycle. Expiratory activity of both muscles increased with exercise intensity, although peak values averaged only 10‐20 or 20‐40 % of the peak activity (obtained during maximal, voluntary expiratory efforts) for the external oblique and rectus abdominis muscles, respectively. To estimate how much of the recorded abdominal muscle activity was supporting leg movements during exercise, we compared the activity at the very end of incremental exercise to that recorded during the first five respiratory cycles after the abrupt cessation of exercise, when ventilation was still very high. Although external oblique activity was reduced after exercise stopped, clear expiratory activity remained. Rectus abdominis activity remained high after exercise cessation, showing a gradual decline that approximated the decline in ventilation. During constant work rate exercise, EMG activities increased to 40‐50 and 5‐10 % of peak in rectus and external oblique muscles, respectively, and then plateaued for the remainder of the bout in spite of a continual upward drift in V̇O2 and pulmonary ventilation. Linear regression analysis showed that the rise in respiratory‐related expiratory muscle activity during progressive intensity exercise was significantly correlated with ventilation, although weakly. In constant work rate exercise, expiratory EMG activities increased, but the changes were highly variable and did not change as a function of exercise time, even though ventilation drifted significantly with time. These experiments suggest that abdominal muscles play a role in regulating the ventilatory response to progressive intensity bicycle exercise, although some of the observed activity may support postural adjustments or limb movements. The contribution of abdominal muscles to ventilation during constant work rate exercise is variable, and expiratory activity does not ‘drift’ significantly with time.