To understand the modulation of periodontal mechanoreceptor activity o
n mastication, the jaw-opening reflex (JOR) evoked by electrical stimu
lation of the inferior alveolar nerve (IAN) was studied during chewing
in freely behaving rabbits. Spontaneous chewing movements were intact
from the IAN stimulation below two times the threshold (T) which was
measured at rest. As the stimulus intensity was increased to more than
2.5 T, the amplitude of the masseter activity decreased or it was abo
lished; however, the chewing rhythm was still maintained up to 3.0 T.
When the low-threshold primary afferents were tested while the rabbit
chewed pellets rhythmically, the amplitude in the JOR was inhibited be
low the prior control level. The inhibitory effect on the JOR was larg
er in the opening phase than that in the closing phase. In conclusion,
the JOR was tonically depressed during the masticatory cycle and the
inhibition of the JOR was rhythmically modulated in a phase-linked man
ner that was larger in the opening phase than in the closing phase.