Physiological properties and morphological characteristics of cutaneous and mucosal mechanical nociceptive neurons with A-delta peripheral axons in the trigeminal ganglia of crotaline snakes.

Primary A-delta nociceptive neurons in the trigeminal ganglia of immobilized crotaline snakes were examined by intrasomal recording and injection of horseradish peroxidase in vivo. Thirty-four neurons supplying the oral mucosa or facial skin were identified as A-delta nociceptive neurons which responded exclusively to noxious mechanical stimuli and had a peripheral conduction velocity ranging from 2.6 to 15.4 m/s. These neurons were subdivided into a fast-conducting type (FC-type) and a slowly conducting type (SC-type). Neurons of both types had a receptive field limited to a single spot which responded to pin prick stimulus with a threshold of more than 5 g. The FC-type neurons had a narrow spike followed by a shorter after-hyperpolarization. In contrast, SC-type neurons exhibited a broad spike with a hump on the falling phase and a longer after-hyperpolarization. The diameters of the stem, central and peripheral axons of the FC-type neurons were significantly thicker than those of the SC-type neurons, but there was no statistical difference in the soma size of the two types. Central axons of both types of neurons were thinner than their stem and peripheral axons. Dichotomizing fibers of peripheral axons were observed within the ganglion on 3 neurons. Central axons of the FC-type neurons terminated ipsilaterally in the nucleus principalis, the subnucleus oralis, interpolaris and caudalis and the interstitial nucleus, whereas those of the SC-type neurons generally projected only to the caudal half of the subnucleus interpolaris, subnucleus caudalis and interstitial nucleus ipsilaterally. The present data showed for the first time the physiological and morphological heterogeneity of the primary trigeminal A-delta nociceptive neurons and revealed that the trigeminal nucleus principalis and all the subdivisions of the trigeminal descending nucleus are involved in nociception as relay nuclei, but the subnucleus caudalis and the caudal half subnucleus interpolaris are the essential relay sites of the primary nociceptive afferents supplying the oral mucosa and facial skin. The interstitial nucleus also appears to play an important role in orofacial nociception.