The relationship between coughing and spinal monosynaptic reflexes (SMR) in extensors and flexors was investigated in eight healthy subjects by using the Hoffmann technique. Coughing exerted major depression (81% in extensor and 83% in flexor SMR). This depressant effect began simultaneously with the first cough and lasted through the coughing phase. Complete recovery to initial values occurred an average of 40 s thereafter. 2. In comparison, mental tasks induced only a slight decrease in SMR amplitude. In contrast, the Mueller and Valsalva manoeuvres induced facilitation. 3. During coughing, large variations in extensor amplitudes were observed that were coupled with the ventilatory cycle. Slight facilitation was observed during inspiration, but expiration induced pronounced depression, occurring 0.20 s after the beginning of the expiratory period. Experiments performed during baseline breathing failed to show any change throughout inspiratory or expiratory phases. 4. Chemical stimulation of irritant receptors (inhalation of citric acid) produced no specific modification of extensor responses compared with the effect of placebo inhalation (distilled water). 5. These data suggest that coughing exerts a major depressant effect on motor activity via a loop that possibly includes cardiopulmonary receptors and inhibitory supraspinal descending pathways.
