2003 Volume 37 Issue 1 Pages 65-73
Using cats anesthetized with urethane and chloralose, we studied the responses evoked by electrical stimulation of tooth pulp (TP), muscle nerves and visceral afferent input through the greater splanchnic nerve, by noxious and innocuous mechanical stimulation of muscle, cutaneous and subcutaneous tissues, and by injection of algesic chemicals (hypertonic saline) into the masseter and splenius muscles. The responses were investigated in single neurons located in the nucleus ventralis posteromedialis proper (VPM) and the intralaminar nuclei (IL). Using chemical and noxious mechanical stimuli of the masseter muscle, we were able to excite 17 VPM neurons that could be functionally classified on the basis of their cutaneous receptive field properties as nociceptive specific (NS), tooth pulp specific (TPS), or wide dynamic range (WDR) neurons. We were able to excite 23 IL neurons, which had a complex receptive field, sometimes covering half of the body surface, using the same stimuli. The VPM and IL nociceptive neurons receiving muscle as well as cutaneous or TP afferent inputs had receptive field properties comparable to those previously described for thalamic nociceptive neurons. The VPM nociceptive neurons were located around the periphery of the posterior half of the VPM. These neurons generally had larger cutaneous receptive fields than neurons unresponsive to muscle afferent stimulation, and WDR neurons were located more anteriorly than NS neurons. The IL neurons had a more extensive receptive field than the VPM neurons, and the receptive fields were predominantly located in subcutaneous rather than cutaneous tissues. The muscle afferent inputs were considered to be predominantly of a nociceptive character for three reasons. They had a high threshold for most of the neuronal responses evoked by electrical stimulation of the muscle afferents. They had a preferential responsiveness to the muscle afferent stimulation of neurons that were functionally identified as cutaneous nociceptive neurons. In addition, there was responsiveness of the neurons excited by electrical stimulation of the muscle nerves, by noxious mechanical stimulation of the muscle, and by injection of algesic chemicals into the muscles. This study provided the first detailed analysis of the response of functionally identified thalamic neurons to natural and electrical stimulation of masseter muscle afferent input. The data indicate that many VPM and IL nociceptive neurons also relay high-threshold muscle afferent inputs and so may play a role in deep craniofacial pain as well as in superficial pain. We concluded that there are regions within the VPM and IL of cats that are capable of processing nociceptive information from the masseter muscle. (J Osaka Dent Univ 2003 ; 37(1) : 65-73)
