An efferent pathway mediating reflex tracheal dilation in awake dogs

Area of Science:

• Respiratory physiology
• Neuroscience

Background:

• Lung inflation typically causes reflex relaxation of tracheal smooth muscle (TSM) and upper airway dilation.
• The precise efferent pathway mediating this response remains unidentified.

Purpose of the Study:

• To investigate the specific efferent pathway responsible for reflex tracheal smooth muscle relaxation during lung inflation.

Main Methods:

• Experiments were conducted on four trained, awake dogs with a permanent tracheostomy.
• Tracheal smooth muscle tone was assessed by measuring cuff pressure changes in an endotracheal tube during lung inflation.
• Pharmacological interventions included beta-adrenergic blockade (propranolol) and efferent parasympathetic blockade (atropine sulfate).

Main Results:

• Lung inflation induced apnea and a significant decrease in cuff pressure (37.4 cmH2O).
• Beta-adrenergic blockade did not alter the response.
• Parasympathetic blockade abolished TSM relaxation, which could be partially restored by serotonin infusion, but lung inflation no longer relaxed TSM.

Conclusions:

• Reflex tracheal dilation in response to lung inflation is mediated by an efferent cholinergic (parasympathetic) pathway.
• This pathway is distinct from the one mediating the Hering-Breuer inflation reflex (apnea).