Regulatory relationship between tactile sensation at the vermilion of the lips and lip-closing force

The objectives of this study were to identify the regulatory relationship between tactile sensation at the vermilion of the lips and the output of pursing-like lip-closing force (LCF), as measured by a multidirectional LCF measurement system. Thirty-six healthy subjects were divided into Anaesthesia and Vaseline groups. The tactile detection threshold (TDT) at six sites on the vermilion and the maximum voluntary LCFs in eight directions were recorded before and during partial or whole application of the agent and 6 h after whole application (Recovery). Five per cent lidocaine gel and Vaseline was applied to the vermilion in the Anaesthesia and Vaseline groups, respectively. These agents were applied to either the right part of the vermilion of the lower lip (Partial) or the whole vermilion (Whole). Partial application of 5% lidocaine gel significantly decreased the magnitudes of six of eight directional LCFs, while it only increased the TDT at one site. The subsequent whole application of 5% lidocaine gel did not affect the magnitude of the LCFs in five of these six directions although its application increased the TDTs at all sites. These reductions in LCF were reversed after recovery from surface anaesthesia. Vaseline application did not affect either TDT or LCF in any conditions. We concluded that tactile sensation at the vermilion of the lips is related to the output of LCF, without any particular directional specificity. The present results suggest the presence of a common synaptic drive ordering the pursing of the relevant muscles.