DISTINCT MORPHOLOGICAL-CHARACTERISTICS OF TOUCH, TEMPERATURE, AND MECHANICAL NOCICEPTIVE NEURONS IN THE CROTALINE TRIGEMINAL GANGLIA

Intrasomal recording and horseradish peroxidase injection techniques w ere employed in vivo to determine the morphological characteristics of touch, temperature, and mechanical nociceptive neurons in the trigemi nal ganglia of crotaline snakes. The touch neurons, with a peripheral axon conducting at the A-beta range, could be subdivided into tactile and vibrotactile neurons according to their response properties, but t here were no morphological differences between them. These neurons exh ibited a large and oval soma and possessed a set of large stem, periph eral, and central axons which were all myelinated and equal in diamete r with a constriction at the bifurcation. The temperature neurons, whi ch conducted peripherally at the A-delta range, were physiologically s eparated into thermosensitive and thermo-mechanosensitive neurons, whi ch were also morphologically indistinguishable. The temperature neuron s had a round soma of medium size and a set of medium axons with varie d axonal bifurcation patterns. All axons of these neurons were myelina ted, but the central axon was thinner than the stem and peripheral axo ns. The mechanical nociceptive neurons, which had a peripheral axon co nducting at the A-delta range, were morphologically heterogeneous base d on their conduction velocities. The neurons conducting at the fast A -delta range were morphologically similar to the temperature neurons i n the ganglion excepting their thinner central axons, whereas those at the slow A-delta range had a thinner myelinated stem axon that gave r ise to a thinner myelinated peripheral axon and an unmyelinated centra l axon with a bifurcation of either a triangular expansion at the bifu rcating point or a central axon arising straightforwardly from the con stant stem and peripheral axons. This study revealed that distinct mor phological characteristics do exist for the touch and temperature neur ons and the subtypes of mechanical nociceptive neurons in the trigemin al ganglion, but not for the subfunctional types of touch neurons or t emperature neurons. (C) 1995 Wiley-Liss, Inc.