ABSTRACT
Intrasomal recording and horseradish peroxidase injection techniques were employed in vivo to determine the morphological characteristics of touch, temperature, and mechanical nociceptive neurons in the trigeminal ganglia of crotaline snakes. The touch neurons, with a peripheral axon conducting at the A-ß range, could be subdivided into tactile and vibrotactile neurons according to their response properties, but there were no morphological differences between them. These neurons exhibited a large and oval soma and possessed a set of large stem, peripheral, and central axons which were all myelinated and equal in diameter with a constriction at the bifurcation. The temperature neurons, which conducted peripherally at the A-δ range, were physiologically separated into thermosensitive and thermo-mechanosensitive neurons, which were also morphologically indistinguishable. The temperature neurons had a round soma of medium size and a set of medium axons with varied axonal bifurcation patterns. All axons of these neurons were myelinated, but the central axon was thinner than the stem and peripheral axons. The mechanical nociceptive neurons, which had a peripheral axon conducting at the A-δ range, were morphologically heterogeneous based on their conduction velocities. The neurons conducting at the fast A-δ range were morphologically similar to the temperature neurons in the ganglion excepting their thinner central axons, whereas those at the slow A-δ range had a thinner myelinated stem axon that gave rise to a thinner myelinated peripheral axon and an unmyelinated central axon with a bifurcation of either a triangular expansion at the bifurcating point or a central axon arising straightforwardly from the constant stem and peripheral axons. This study revealed that distinct morphological characteristics do exist for the touch and temperature neurons and the subtypes of mechanical nociceptive neurons in the trigeminal ganglion, but not for the subfunctional types of touch neurons or temperature neurons. ® 1995 Wiley-Liss, Inc.
Indexing terms: functional morphology, intracellular horseradish peroxidase, soma area, axon diameter, axonal bifurcation
