CENTRAL DISTRIBUTION OF SYNAPTIC CONTACTS OF PRIMARY AND SECONDARY JAW MUSCLE-SPINDLE AFFERENTS IN THE TRIGEMINAL MOTOR NUCLEUS OF THE CAT

Little is known about the differences of the terminations of group Ia and group II afferents within the brainstem or spinal cord. The presen t study was performed to classify cat jaw muscle spindle afferents by the use of succinylcholine (SCh) and to examine the morphological char acteristics of the physiologically classified afferents at the light a nd electron microscopic levels through the use of the intra-axonal hor seradish peroxidase (HRP) injection technique. The effects of SCh on s tretch responses of 119 jaw muscle spindle afferents from the masseter were examined. The SCh converted the single skew distribution of the values for dynamic index (DI) into a bimodal one. Fifty-eight and 61 a fferents were classified as group Ia and group II afferents, respectiv ely. The central projections of 17 intra-axonally stained afferents (1 0 group Ia and 7 group II afferents) were examined. The spindle affere nts terminated mainly in the supratrigeminal nucleus (Vsup), region h, and the dorsolateral subdivision of trigeminal motor nucleus (Vmo.dl) but differed in the pattern of projections of group Ia and group II a fferents. The proportion of group Ia afferent terminals was higher in Vmo.dl but lower in Vsup than that of group II afferents. In Vmo.dl, t he proportion of group Ia afferent terminals was higher in the central region but lower in the more outer regions than that of group II affe rents. The ultrastructure of serially sectioned afferent boutons (63 g roup Ia and 72 group II boutons) also was examined. The boutons from t he two groups were distributed widely from the soma to small-diameter dendrites, but the frequency of synaptic contacts on proximal dendrite s was higher in group Ia than group II afferents. The present study pr ovides evidence that the two groups of jaw muscle spindle afferents di ffer in their central projection and the spatial distribution of their synaptic contacts on Vmo.dl neurons. (C) 1998 Wiley-Liss, Inc.