Role of the soft palate in respiration: an electromyographic study in the dog

The present study investigates the nature of tensor veli palatini muscle (TVP) and levator veli palatini muscle (LVP) as accessory respiratory muscles. In the first part of the study, the relation between the muscles' activities as revealed by EMG activities and respiration rhythm was analysed under various combinations of partial pressures of O2 and CO2 in the arterial blood. Furthermore, the effect of sodium cyanide (NaCN) perfused through the carotid sinus was examined. During resting breathing, no EMG activity was recorded from either muscle. In hypercapnic or hypoxemic conditions produced by rebreathing, TVP exhibited a phasic EMG activity during inspiration. LVP showed a phasic EMG activity during expiration in hypoxic conditions (PaO2 less than 40 mmHg). NaCN perfused bilaterally through the carotid sinus induced the phasic EMG activities similar to those observed in hypercapnia and/or hypoxemia. TVP was more sensitive to NaCN than LVP. The second part of the study examined specific roles of the muscles in altered states of breathing. At the time of onset of LVP activity induced by rebreathing, the oral proportion of airflow markedly increased. On the other hand, TVP activity greatly increased in amplitude when negative pressure was applied to the upper airway. The results suggest that both muscles are accessory respiratory muscles and are regulated by chemogenic inputs including those from the carotid body; TVP is an accessory inspiratory muscle that contributes to the maintenance of upper airway patency, and LVP is an accessory expiratory muscle that increases the portion of expiratory airflow through the oral cavity.