Tracheobronchial muscle tonus and its reflex control

Airway smooth muscle tone is reinforced during the inspiratory phase of the breathing cycle and depends largely from neurogenic motor drive carried by the vagus nerve. This muscle tone seems to be produced mostly by a vago-vagal reflex loop initiated by the tonic discharge of tracheo-bronchial and/or alveolar receptors connected to thin sensory vagal fibres (non-myelinated or C-fibres). Inhibitory influences carried by large myelinated vagal fibres connected to tracheobronchial stretch receptors and also numerous afferents from the upper airways, systemic and pulmonary circulation, digestive tract and skeletal and respiratory muscles participate to the modulation of airway tone. The identification of neurotransmitters specific of the motor or sensory pathways helps to understand the peripheral modulation of airway motor drive and also the central integration of some peripheral informations.