Sympathetic modulation of the jaw-opening reflex in anaesthetized rabbits

The influence of sympathetic nerve stimulation (SNS) on the digastric muscle response to tapping on a front tooth was investigated in 14 anaesthetized rabbits. The reflex response to transverse taps was measured as changes in EMG activity of the ipsilateral digastric muscle. With taps of 1.1-2.2 times the threshold, the amplitude of the digastric EMG response was reduced by stimulation of the cervical sympathetic trunks (0.5-10.0 Hz) in 35 out of 40 test series. Maximum effect was observed at 6 Hz, which produced an average inhibition of 82%. The sympathetic effect appeared a few seconds after the start of stimulation, and in the range 0.5-6 Hz increased with the stimulation frequency. It was blocked by phenoxybenzamine. The reflex was triggered by receptors in or around the tooth, since it disappeared after sectioning of the inferior alveolar nerve (IAN) or local anaesthesia of the tooth. Afferent electrical stimulation of the IAN evoked a digastric excitation that was insensitive to SNS. We conclude that the sympathetic inhibition of the reflex was mediated by a local action in the receptor area, probably related to the evoked vasoconstriction.