MORPHOLOGICAL DIFFERENCES BETWEEN FAST AND SLOWLY ADAPTING LINGUAL AFFERENT TERMINATIONS IN THE PRINCIPAL AND ORAL NUCLEI IN THE CAT

Previous studies indicated that fast-adapting (FA) and slowly adapting (SA) mechanoreceptive afferents innervating the facial or intraoral s tructures give rise to morphologically distinct terminal arbors in the individual subdivisions of the trigeminal sensory nuclear complex. Th e present study examined the collateral morphologies of lingual affere nts in the nuclei principalis (Vp) and oralis (Vo) of the cat. Seven F A and six SA lingual afferents were physiologically characterized and stained by the intra-axonal horseradish peroxidase (HRP) injection tec hnique, The two types of afferents established terminal arbors in the dorsomedial subdivision (Vpd) of the Vp, and the rostrodorsomedial (Vo .r) and dorsomedial subdivisions (Vo.dm) of the Vo, but the collateral morphologies are different between the two types. The FA afferents ga ve rise to mediolaterally extended oblong arbors in each subdivision, but the arbors were better developed in the Vo.r than in the Vpd and V o.dm. The number of collaterals, intercollateral distance, number of b outons per collateral, and bouton size were also different among the s ubdivisions. The SA afferents were divided into two subtypes; one had a preferential projection into the Vpd or the Vo.r and Vo.dm, and othe rs lacked a selected projection. Although the shape of their arbors va ried from a stringy form to a roundish form, the general profile was d enser, better developed, and rounder than that of FA afferents in each subdivision. The intercollateral distance and bouton size were differ ent among the subdivisions. The number of boutons per collateral, bout on density, and bouton size were larger in SA than FA afferents in eac h subdivision. The present study demonstrated that two functionally di stinct lingual afferents manifest unique morphological differences in the Vpd and Vo. (C) 1998 Wiley-Liss, Inc.