The aim of the present study was to demonstrate the convergence of inputs from masseter muscle ( MM) and tooth pulp (TP) onto C-1 spinal neurons and to determine whether the afferent fibers express the functional vanilloid receptor (VR1). Extracellular single-unit recordings were made from 61 C-1 units responding to TP electrical stimulation with a constant temporal relationship to a digastric electromyogram signal in pentobarbital anesthetized rats. Eighty-four percent of C-1 neurons responding to TP stimulation also responded to the ipsilateral MM stimulation. Of these neurons, 61% were considered to be afferent inputs from Ad-fibers and the remaining units (39%) were C-fibers, based on calculation of the nerve conduction velocity. Intramuscular injection of capsaicin (0.05 and 0.1%) produced a reduction in a MM-induced C-1 neuronal activity in a dose-dependent manner and this effect was antagonized by pretreatment with an antagonist of VR1, capsazepine. Some of these units were also excited by noxious heat stimulation (>43degrees C). The trigeminal root ganglion (TRG) neurons that innervated the MM were retrogradely labeled with Fluorogold (FG) and the small-diameter FG-labeled TRG neurons expressed the immunoreactivity for VR1. After intramuscular mustard oil injection ( noxious chemical stimulation), the C-1 neuronal activity induced by both touch and pinch stimuli was enhanced and their receptive field sizes were significantly expanded. These changes were reversed within 15 - 20 min. These results suggest that there may be the convergence of noxious afferents inputs from the MM and TP afferents on the same C-1 neurons in rats, and that the afferent fibers expressing the functional VR1 may contribute to the hyperalgesia and/or referred pain associated with temporomandibular joint disorder.