Influence of lip sensation on lip-closing force in humans

Objectives: Lip closure is essential for oral function. It has been known that lip sensation plays an important role in the control of lip movement. However, it is unknown whether this sensory-motor integration has regional specificity or not. We hypothesized that the change of partial lip sensation alters the lip-closing force on the specific direction. We developed an apparatus to measure the multidirectional lip-closing force in 8 directions. This study aims to clarify the influence of partial change of lip sensation on the directional distribution of lip-closing force using this new apparatus. Moreover, the involvement of cortical control in lip movements was assessed by magnetoencephalography (MEG).

Methods: Eleven healthy adult subjects with natural dentition were studied. To induce the change of partial lip sensation, we applied topical anesthesia on the lower-right part of lip. First, the multidirectional lip-closing forces with maximum effort and electromyograms (EMG) of bilateral upper and lower lip muscles were recorded with and without anesthesia. Second, the MEG signals were recorded during light lip closing to investigate the change of cerebral activities using partial anesthesia.

Results: The directional distribution of the lip-closing force was not altered with or without anesthesia. The forces in 6 directions, except for the forces in the transverse direction, were significantly smaller with anesthesia than without, whereas the EMG activities in the four muscle parts were not different in both cases. The magnetic signals were observed preceding the lip movement on the lip area of the primary motor cortex. The anesthesia decreased the dipole moment significantly.

Conclusions: These results suggest that the change of partial lip sensation does not cause the change of the directional distribution, but decrease the lip-closing forces. The cortical activities may be involved in the decrease of the lip-closing force.