The present study tested whether electrical stimulation increases the number of voluntary repetitive swallows in humans and if summation of peripheral inputs increases the number of swallows. In addition, the potential of initiating both voluntary and involuntary swallowing was compared and the effect of chewing behaviors on the initiation of swallowing evoked by the electrical stimulation was evaluated. Facilitatory effect of both the mid- and hypo-pharyngeal stimulation was much larger than that of mid-pharyngeal stimulation. The longer the pulse duration was, the larger the number of swallows was, suggesting temporal and spatial summation of peripheral inputs into the swallowing center. There was a wide variation in the number of swallows among subjects. The number of reflexively evoked swallowing (i.e., involuntary swallow) by pharyngeal stimulation also varied greatly, and there was a significant linear correlation in the number of swallows between voluntary and involuntary swallowing, which suggests that the swallowing central pattern generator is a common component of both neuronal networks and therefore is responsible for inter-individual variations. The chewing strongly inhibited the initiation of swallows. Although it should be clarified how the chewing behaviors modulate swallowing function, these data suggest the functional interaction between chewing and swallowing centers.
