Experimental modulation of the cough reflex

This article provides an overview of the current understanding of the experimental modulation of the cough reflex. The effect of various extrapulmonary (outside the lungs, larynx and the tracheobronchial tree) afferent inputs on mechanically and chemically induced cough has been carried out in experimental animals. Mechanisms generating cough are less resistant than inspiratory drive to the direct hypoxic and hypercapnic depression of medullary neurones. The role of the input from peripheral chemoreceptors to centrally regulating mechanisms of the cough reflex is supported by the decreased cough response to mechanical stimuli either after carotid body resection or their functional inhibition by pure oxygen inhalation. Stimulation of afferent cardiac and abdominal nerve endings leading to depression of breathing is accompanied by marked inhibition of the cough response to a mechanical stimulus. In health, stimulation of oesophageal mucosal afferents cannot mediate cough or modulate its intensity. It is likely that a decreased threshold of the nerve endings mediating cough due to chronic stimulation of oesophageal mucosal receptors and probably laryngeal afferents is important for cough in patients suffering from gastro-oesophageal reflux disease. Increased cough sensitivity occurs in patients with seasonal allergic rhinitis. Increased sensitivity of nerve endings in the larynx and lower airways is mainly responsible for this effect. Sensitivity could be increased either by the movement of nasal mucus or by a reflex effect of activated nasal receptors on the reflex arc of coughing. The authors conclude from their results that the acute inputs from different afferent nerve endings affect the ventilatory and cough responses in a similar way.