Fig. 5.

The cough reflex desensitizes with repetitive tussive challenge. Coughing was evoked repetitively in 1-min intervals by citric acid challenges (0.1 or 1 M in 100-μl aliquots) or by 5 min continuous electrical stimulation (16 Hz, 1-ms pulse duration) of the tracheal mucosa. A: representative trace showing coughs evoked by 0.1 M citric acid challenges delivered repetitively at 1-min intervals. By the 8th consecutive challenge, few if any coughs were observed. B: mean ± SE coughs evoked by repetitive citric acid challenges (n = 8/treatment group). C: representative trace showing the repetitive coughing evoked initially by electrical stimulation of the tracheal mucosa but the rapid onset of desensitization and cessation of coughing despite continued stimulation. D: mean ± SE coughs evoked in 15- to 30-s epochs during the continuous electrical stimulation of the tracheal mucosa (n = 6). *Significant reduction in response relative to that evoked at the initial time points studied (P < 0.05).