TRIGEMINAL STRUCTURE-FUNCTION-RELATIONSHIPS - A REEVALUATION BASED ONLONG-RANGE STAINING OF A LARGE-SAMPLE OF BRAIN-STEM A-BETA FIBERS

Prior studies suggest that some classes of thickly myelinated (AP) aff erents have distinct morphologies in the trigeminal (V) brainstem comp lex, and that single fibers have collaterals with different shapes in the four V subnuclei. However, these conclusions are based upon relati vely few and incompletely stained fibers and limited statistical rigor . In the present study, 104 fibers were stained more completely with n eurobiotin in rats to provide within-fiber intersubnucleus comparisons , and between-fiber intrasubnucleus comparisons, of collaterals associ ated with a vibrissa, guard hairs, hairy skin, glabrous skin, or oral structures. Collaterals from all functional categories had similar qua litative features and were distributed somatotopically in the transver se plane according to known maps. Fiber categories were not disproport ionately represented at particular sites along the brainstem's rostroc audal axis, although most fibers adhered to an onion-leaf topography i n caudalis. Surprisingly few structure-function relationships were rev ealed by multivariate analysis of variance and post hoc group comparis ons, as follows: Arbors were larger in caudalis than in any other subn ucleus; collaterals were most numerous in interpolaris; vibrissa affer ents had more collaterals than oral and guard hair afferents; and oral fibers had larger arbors than vibrissa or guard hair afferents in sub nucleus oralis. Peripheral receptor association and response adaptatio n rate failed to predict arbor shapes and terminal bouton numbers in a ny V subnucleus. These data confirm that the locations of V primary af ferent arbors are predicted by their receptive fields. However, collat eral number and morphology are predicted only to a very limited extent by the V subnucleus and peripheral receptor affiliation - a conclusio n that contrasts with those of most prior studies of somatosensory pri mary afferents.