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

Although the scalene muscle (Sca) is a primary inspiratory muscle in humans, its respiratory function in other species is less clear. The electromyographic (EMG) activity of the Sea was studied during resting ventilation (eupnea) in both the awake and anesthetized hamster and after a variety of respiratory challenges in the anesthetized animal. The EMG activities of the medial Sea and the costal diaphragm were compared. The medial Sea, the major component of the Sea, originates from cervical transverse processes 2 to 5 and inserts primarily onto rib 4, with a small segment onto rib 3. In both the anesthetized and awake animal, the Sea was always silent during quiet breathing. With COP-stimulated hyperpnea, the Sea was always recruited during inspiration in phase with the diaphragm. Active recruitment of the Sea was also observed after resistive loading and total airway occlusion. After ipsilateral phrenicotomy, the Sea was persistently recruited during eupnea, The specificity of the EMG signals was tested both by excluding cross contamination from other rib cage muscles and by selective denervation studies. Muscle spindles were identified in the medial Sca histochemically, suggesting that the respiratory activity of the Sea can also be modulated by changes in muscle length and/or load. These results indicate that the Sea functions as an accessory inspiratory muscle in the hamster and may play an important role in conditions of chronic load.