Chemical Senses Affecting Cough and Swallowing

Dysfunction of swallowing and coughing leads to life-threatening aspiration pneumonia, especially in the elderly. In order to induce the cough and swallowing reflexes efficiently, sensory inputs to trigger the reflexes are essential.

Both the cough and swallowing reflexes respond to mechanical and chemical stimuli. However, the mechanisms of action of the two reflexes are not homogeneous. Some substances stimulate both reflexes, but others stimulate one of the reflexes and inhibit the other one.

Capsaicin, a TRPV1 agonist, stimulates both the cough and swallowing reflexes. Menthol, a TRPM8 agonist, stimulates the swallowing reflex, but it inhibits the cough reflex, especially if applied to the nose. Acid stimulates the cough reflex but its effect on the swallowing reflex is complicated. Theophylline inhibits the cough reflex by decreasing the excitability of sensory nerves, whereas it stimulates the swallowing reflex by antagonizing adenosine receptors. In smoking, cigarette smoke and nicotine have different effects. Cigarette smoke stimulates the cough reflex, while it inhibits the swallowing reflex. Nicotine inhibits the cough reflex but does not affect the swallowing reflex.

Whenever you prescribe for an abnormality of one of the reflexes, you should think about the effect of the prescription on the other reflex.