Voluntary and reflex influences on the initiation of swallowing reflex in man

The electrophysiological features of voluntarily induced and reflexive/spontaneous swallows were investigated. In normal subjects, swallows were elicited by infusing water either into the mouth (1-3 ml) or directly into the oropharyngeal region through a nasopharyngeal cannula (0.3-1 ml). For water infused orally, subjects were either requested to swallow voluntarily or instructed to resist swallowing and maintain the horizontal head position until swallowing occurred reflexively. Spontaneous saliva swallowing was investigated in patients with severe dysphagia who had a prominent clinical picture of suprabulbar palsy. Comparisons between different swallowing types were made by measuring the time interval between the onset of submental electromyographic activity (SM-EMG) and the onset of the upward movement of the larynx recorded by a movement sensor. This interval was less than 100 ms, even frequently less than 50 ms, in reflexive/spontaneous swallows, while in voluntarily induced swallows it was substantially longer. The rising time of submental muscle's excitation was also shorter in reflexive/spontaneous swallows. It was suggested that the triggering of voluntarily induced swallows commences more than 100 ms before the onset of swallowing reflex and that this mechanism is under the control of corticobulbar-pyramidal pathways. If the swallowing reflex is triggered within such a short period of time following the onset of SM-EMG, the central control by the bulbar swallowing center should be effective until the end of oropharyngeal swallowing.