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 particula r, previous studies suggest that, besides sensory inputs arising from the o rofacial region, extratrigeminal information may modulate jaw muscle functi on. In the present study the actions exerted on masseter and digastric musc les 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 ne rves, at a threshold intensity for the activation of group II afferent fibr es, were studied. The stimulation induced an excitatory response in both ma sseter and digastric muscles bilaterally. Ipsi- and contralateral radial ne rve 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 respo nses 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 mot or units were excited by both radial and sciatic nerve stimulation; this su ggests a convergence of somatosensory inputs arising from fore- and hind li mbs on the same pool of masseter and digastric motoneurones. Paired stimula tion of the two nerves did not induce any summation of the responses; this finding suggests that the two inputs, reaching a common relay station, coul d give rise either to occlusion or to inhibitory interactions. Spinotrigemi nal relationship evidenced in this study may be involved in the coordinatio n of jaw and limb movements.