This study was designed to investigate extrinsic tongue muscle activity in response to bronchopulmonary C-fiber activation following mid-cervical 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 (three days), subchronic (12-16 days) and chronic (52-65 days) injured stages following unilateral mid-cervical spinal contusion. Three days after injury, the pre-inspiratory burst amplitude of the extrinsic tongue muscle during at baseline was significantly greater in mid-cervical spinal contused animals than in sham animals. At this time, capsaicin induced a significant reduction in both pre-inspiratory 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 pre-inspiratory genioglossus muscle activity in the contused 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.

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