Structure-function relationships in rat medullary and cervical dorsal horns. I. Trigeminal primary afferents.

Intracellular recording and horseradish peroxidase (HRP) labeling were used to examine structure-function relationships in the medullary dorsal horn (MDH) and rostral cervical dorsal horn. In Nembutal-anesthetized rats, 78 trigeminal (V) primary afferent fibers were physiologically characterized and injected with HRP. Axons were sufficiently well stained to reconstruct all of their collaterals in the MDH. Many also extended into the cervical dorsal horn. Except for four axons, which responded best to noxious stimuli, all responded at short (mean = 0.50 ms) latencies to V ganglion shocks and to innocuous stimulation. Forty-five of our recovered fibers were associated with facial vibrissae and responded in either a rapidly adapting, slowly adapting type I, slowly adapting type IIa, or slowly adapting type IIb fashion. The adequate stimuli consisted of either slow deflection, high-velocity deflection, or a noxious pinch of the vibrissa follicle. The collaterals of all of the above-described mystacial vibrissa primary afferents proceeded directly to their region of arborization in a plane perpendicular to the lateral border of the medulla to collectively form a largely continuous, circumscribed terminal column. This longitudinally oriented column of terminal and en passant boutons angled from lamina V rostrally to lamina III caudally. In the magnocellular laminae of the MDH, all mystacial vibrissa primary afferents gave rise to similarly shaped arbors, regardless of their functional classification. While morphological variability was observed both within and between individual axons, variance between functional classes was no greater than that within a class. Moreover, number of collaterals, number of boutons, or bouton size did not distinguish functional classes. Nonmystacial vibrissa afferent arbors, with more caudal peripheral fields, had their primary arbor focus in C1 and C2 dorsal horn. These arbors had relatively little rostrocaudal overlap with mystacial vibrissa afferents, though they exhibited the same lamina V-to-III shift as they descended through the cervical cord. Unlike mystacial vibrissa afferents in the MDH, their collaterals followed a tortuous course and often occupied laminae II-V in one transverse section. The relative location of each vibrissa afferent's terminal field could be predicted by the particular vibrissa innervated. Dorsal vibrissae afferents had ventrolateral terminations and ventral vibrissae afferents terminated dorsomedially. Rostral vibrissae were represented in the rostral MDH, whereas caudal vibrissae were represented in the caudal MDH and rostral cervical dorsal horn.(ABSTRACT TRUNCATED AT 400 WORDS)