Effects of repeated volitional swallowing on the excitability of submental corticobulbar motor pathways

The aim of this study was to examine the effects of repeated volitional saliva swallowing on corticobulbar excitability recorded during two muscle preactivation conditions of the submental muscle group. Motor-evoked potentials (MEPs), elicited by transcranial magnetic stimulation (TMS), were assessed in ten healthy volunteers prior to and at 5, 30, 60, and 90 min after 60 volitional saliva swallows (Protocol A). To control for intrinsic fluctuations in corticobulbar excitability during this assessment period, MEPs were also recorded, on a different day, at 30-min intervals across a 2-h period (Protocol B). At each assessment, 15 MEPs were recorded during two submental muscle preactivation conditions: Volitional contraction and contraction associated with the pharyngeal phase of volitional swallowing. There were no significant effects of repetitive volitional swallowing or time on MEP measures (p > 0.05). We conclude that volitional saliva swallowing does not have immediate effects on the excitability of corticobulbar projections to the submental musculature during volitionally initiated swallowing motor tasks. These results provide no evidence for use-dependent potentiation of corticobulbar excitability through repetitive saliva swallowing. The lack of effects of time on mean MEP measures supports previous reports of good intrasession reliability of MEPs as a measure of corticobulbar excitability.