Functional role and structure of the scalene: an accessory inspiratory muscle in hamster

Fournier, Mario, and Michael I. Lewis. Functional roleand structure of the scalene: an accessory inspiratory muscle inhamster. J. Appl. Physiol. 81(6):2436-2444, 1996.Although the scalene muscle (Sca) is a primaryinspiratory muscle in humans, its respiratory function in other speciesis less clear. The electromyographic (EMG) activity of the Sca wasstudied during resting ventilation (eupnea) in both the awake andanesthetized hamster and after a variety of respiratory challenges inthe anesthetized animal. The EMG activities of the medial Sca and thecostal diaphragm were compared. The medial Sca, the major component ofthe Sca, originates from cervical transverse processes 2 to 5 andinserts primarily onto rib 4, with a small segment onto rib 3. In both the anesthetized and awake animal, the Sca was always silent during quiet breathing. WithCO₂-stimulated hyperpnea, the Scawas always recruited during inspiration in phase with the diaphragm.Active recruitment of the Sca was also observed after resistive loading and total airway occlusion. After ipsilateral phrenicotomy, the Sca waspersistently recruited during eupnea. The specificity of the EMGsignals was tested both by excluding cross contamination from other ribcage muscles and by selective denervation studies. Muscle spindles wereidentified in the medial Sca histochemically, suggesting that therespiratory activity of the Sca can also be modulated by changes inmuscle length and/or load. These results indicate that the Scafunctions as an accessory inspiratory muscle in the hamster and mayplay an important role in conditions of chronic load.