We observed striking differences in respiratory muscle electromyogram activity when active expirations were interrupted in rapid succession, depending on the mode of interruption. When the interruptions were produced at the level of the glottis (utterances, uh-uh-uh-uh, at 5-8 Hz) there were synchronous bursts of activity from expiratory muscles in all three subjects during the periods of no flow and rapid bursts of diaphragmatic activity during the flow phases in one subject. In contrast, when similarly rapid interruptions of active expirations were produced with the tongue on a mouthpiece (utterance, te-te-te-te) or with an external valve, no synchronous bursts were observed. Since all interruptions would have been mechanically similar at expiratory muscular and pulmonary levels, we reasoned that the bursts with glottic interruptions were either programmed centrally or driven reflexly at the laryngeal level.
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Mead et al. (1988) studied this question.