Physiological properties of periodontal mechanosensitive neurones in the trigeminal (Gasserian) ganglion of the rat.

Periodontal mechanosensitive (PM) primary afferent neurones were recorded from the rat trigeminal ganglion and their response to mechanical tooth stimulation studied. The majority (95%) of PM afferents were single-tooth units and most were sensitive to mechanical stimulation of the incisor. They had a sustained response to pressure applied to the tooth, and showed a directional selectivity to the stimulation. Only a small number (5%) of units were multi-tooth sensitive; their receptive fields were restricted to the molars. When the incisor was stimulated mechanically in 12 directions in a plane perpendicular to the axis of the tooth crown, the predominant response fields of the PM units were of the medium type (90-180 degree response angle) or the broad type (180-360 degrees). Each unit had single optimal stimulus direction oriented predominantly in the rostrocaudal or caudorostral direction. About 60% of the PM units responded to tooth stimulation at irregular spike intervals, whereas the remaining 40% fired at relatively regular intervals. When the tooth was stimulated at a force of 0.05 N, the mean spike interval and rate of spike-interval fluctuation [percentage of the standard deviation (SD) of the interval distribution to the mean interval (mean); SD/mean x 100] were 29.2 ms and 38.1% for the regular-interval units, and 15.1 ms and 8.8% for the irregular-interval ones. The mean spike interval of the regular units was significantly longer than that of the irregular ones.