Jaw muscle response to stimulation of type II somatosensory afferents of limbs in the rat.

Convergence of various afferent inputs onto brainstem neurones may play an important role in the regulation of trigeminal motor activity. In particular, previous studies suggest that, besides sensory inputs arising from the orofacial region, extratrigeminal information may modulate jaw muscle function. In the present study the actions exerted on masseter and digastric muscles by the activation of somatosensory afferents coming from fore- and hind limbs were examined. The electromyographic activity (EMG) of masseter and digastric muscles was recorded in 20 anaesthetised rats, and EMG responses to single and paired electrical stimulation of common radial and sciatic nerves, at a threshold intensity for the activation of group II afferent fibres, were studied. The stimulation induced an excitatory response in both masseter and digastric muscles bilaterally. Ipsiand contralateral radial nerve stimulation evoked masseter responses at latencies of 13.8 +/- 2.4 ms and of 18.0 +/- 2.6 ms, respectively, and digastric responses 1.6 +/- 0.4 ms later. Ipsi- and contralateral sciatic nerve stimulation elicited masseter responses at latencies of 21.4 +/- 2.6 ms and of 23.3 +/- 2.0 ms, respectively, and digastric responses 2.0 +/- 0.2 ms later. The same masseter and digastric motor units were excited by both radial and sciatic nerve stimulation; this suggests a convergence of somatosensory inputs arising from fore- and hind limbs on the same pool of masseter and digastric motoneurones. Paired stimulation of the two nerves did not induce any summation of the responses; this finding suggests that the two inputs, reaching a common relay station, could give rise either to occlusion or to inhibitory interactions. Spinotrigeminal relationship evidenced in this study may be involved in the coordination of jaw and limb movements.