Functional properties of low-threshold mechanoreceptive afferents in the human labial mucosa

We used microneurography to investigate the functional properties of low-threshold mechanoreceptive afferents innervating the oral mucosa of the inside of the lower lip. Impulse responses were recorded from the inferior alveolar nerve of four human subjects. The threshold force and receptive field boundaries were identified for 19 single mechanoreceptive afferents using thin filaments (von Frey hairs) that applied known forces to the mucosa. Most of the receptive fields were located close to the corners of the mouth. Twelve of the afferents were slowly adapting (SA) and the remaining seven units were fast adapting (FA). Two types of slowly adapting responses were observed, SA I and SA II. Four of the six SA II units were spontaneously active. The geometric mean value of the receptive field sizes was 4.20 mm(2) for the SA I units, 5.65 mm(2) for the SA II units, and 5.60 mm(2) for the FA I units. None of the FA afferents showed response properties characteristic of Pacinian-corpuscle type afferents (FA II units). All afferents showed low force threshold between 0.06 and 1 mN. The properties of the mechanoreceptors supplying the human labial mucosa appear more similar to those of the vermilion and facial skin of the lower lip than those supplying the mucosa of the dorsal tongue.