Modulation of the extrinsic tongue muscle activity in response to bronchopulmonary C-fiber activation following midcervical contusion in the rat.

This study was designed to investigate extrinsic tongue muscle activity in response to bronchopulmonary C-fiber activation following midcervical spinal contusion in the rat. Esophageal pressure and electromyogram of the extrinsic tongue muscles (genioglossus and hyoglossus) were monitored before and after inhalation of capsaicin (25 and 100 µg/mL) at the acute (3 days), subchronic (12-16 days), and chronic (52-65 days) injured stages following unilateral midcervical spinal contusion. Three days after injury, the preinspiratory burst amplitude of the extrinsic tongue muscle at baseline was significantly greater in midcervical spinal-contused animals than in sham animals. At this time, capsaicin induced a significant reduction in both preinspiratory and inspiratory activity of the extrinsic tongue muscle in sham but not contused animals at the acute stage. During the chronic injured stage, capsaicin at 100 µg/mL induced stronger suppression of preinspiratory genioglossus muscle activity in the contused animals than in sham animals. These results demonstrated that cervical spinal cord injury alters upper airway motor outputs and their reflex modulation by bronchopulmonary C-fibers. The compensatory increase in respiratory activity of the extrinsic tongue muscle early after cervical spinal cord injury may help to maintain upper airway patency. However, under the condition of chronic cervical spinal cord injury, the increased suppression of genioglossus muscle activity by bronchopulmonary C-fiber activation may increase the risk of airway obstruction following chronic cervical spinal cord injury.NEW & NOTEWORTHY Tongue muscle activity plays an important role in the regulation of upper airway patency. This study aimed to investigate the respiratory activity of the extrinsic tongue muscle in response to capsaicin-induced bronchopulmonary C-fiber activation following cervical spinal cord contusion. Midcervical spinal-contused animals exhibited a greater baseline preinspiratory burst amplitude of the extrinsic tongue muscle and were resistant to inhaled capsaicin-induced reduction of respiratory tongue muscle activity at the acute injured stage. However, inhalation of capsaicin caused a more severe attenuation of preinspiratory activity of the extrinsic tongue muscle at the chronic injured stage. These results suggest that the upper airway may be predisposed to collapse in response to bronchopulmonary C-fiber activation following chronic cervical spinal cord injury.