Responses of neurons in feline trigeminal subnucleus caudalis (medullary dorsal horn) to cutaneous, intraoral, and muscle afferent stimuli

Abstract

The extracellular activity of single neurons was recorded in subnucleus caudalis (medullary dorsal horn) of chloralose-anesthetized cats to test the effects of electrical and natural stimuli that activated afferents supplying the jaw and tongue muscles as well as the face, teeth, and intraoral mucosa. Many caudalis neurons that could be functionally classified on the basis of their cutaneous receptive-field properties as low-threshold mechanoreceptive (LTM), wide-dynamic-range (WDR), or nociceptive-specific (NS) neurons could be excited by muscle afferent stimuli. Only five neurons were encountered that received muscle afferent inputs and had no demonstrable cutaneous, dental, or mucosal input. The muscle afferent inputs were a particular feature of the cutaneous nociceptive (i.e., WDR and NS) neurons. Approximately two-thirds of this nociceptive neuronal population (n = 109) could be excited by jaw and/or tongue muscle stimulation, whereas only a small proportion of the LTM neuronal population (n = 247) was activated by muscle afferent stimulation. Neurons with a demonstrated direct axonal projection to the contralateral thalamus as well as nonprojection neurons received muscle afferent inputs. The caudalis nociceptive neurons receiving muscle as well as cutaneous afferent inputs had receptive-field properties comparable to those previously described for caudalis cutaneous nociceptive neurons; they were predominantly located in laminae I/II and V/VI, and many also received convergence of tooth pulp afferent inputs. These neurons generally had larger cutaneous receptive fields than neurons unresponsive to muscle afferent stimulation. The muscle afferent inputs were considered to be predominantly of a nociceptive character for several reasons. These included the long latency and high threshold of most neuronal responses evoked by electrical stimulation of the muscle afferents, the predominance of afferents of small diameter in some of the muscle nerves stimulated, the preferential responsiveness to the muscle afferent stimulation of neurons that were functionally identified as cutaneous nociceptive neurons, and the responsiveness of most of the neurons excited by electrical stimulation of the muscle nerves also to noxious mechanical or thermal stimulation of muscle and the injection of two or more algesic chemicals into small arteries supplying the jaw and tongue muscles. Of the algesic chemicals used in this study (7% NaCl, KCl, bradykinin, histamine, 5-HT), the first two were found to be the most effective and to cause the most rapidly induced excitation.(ABSTRACT TRUNCATED AT 400 WORDS)