Morphology of central terminations of low-threshold trigeminal primary afferents from facial skin in the cat — intra-axonal staining with HRP

Abstract

Intra-axonal recording and horseradish peroxidase (HRP) injection techniques were employed the response properties of low-threshold mechanoreceptive afferents and the morphological characteristics of their axon arbors in the main sensory nucleus (Vp) and oral nucleus (Vo). Thirteen afferents were characterized and recovered. One gave fast or rapidly adapting (FA) and 3 slowly adapting (SA) responses to mystacial vibrissa deflection, 5 were sensitive to deflection of non-vibrissae hairs or hair (4 were guard hair afferents and the other responded to deflection of a long hair in slowly adapting fashion) and two were responsive to indentation of the hairy skin. The remainder were responsive to indentation of the glabrous skin on the lower lip: one was of FA type and the other of SA type. All of the axons had bifurcating fibers that ascended in the ascending tract (ascending fiber) and descended in the trigeminal spinal tract (descending fiber). The main collaterals given off from the ascending fiber and rostral segment of the descending fiber terminated in the Vp, and the other collaterals from the descending fiber projected to the Vo. Terminal arbors produced by the main collaterals formed a rostrocaudally continuous column, but generally the adjacent arbors did not overlap except when pairs of collaterals arose near each other on the ascending and descending fibers. Projections of collaterals to Vp and Vo were organized topographically. The head was represented in an inverted fashion with its anteroposterior axis in a mediolateral sequence, but the lower glabrous lip was represented more dorsally than the other mandibular facial regions. Vibrissa afferents formed a rostrocaudally continuous, densely packed terminal column throughout the length of Vp and Vo. SA vibrissa afferents gave rise to more dense and roundish arbors in Vp than the FA afferent, while the Vo.c arbors were more compact and smaller than those of the FA afferent. Guard hair afferents had arbors that were highly variable throughout the nuclei and were characterized by less developed arbors in Vp than in Vo. Unlike vibrissa afferents, hairy skin afferents gave rise to sparseand wide spread arbors characterized by a string-like appearance, while the Vo collaterals were more stringly. Facial lip afferents were characterized by a great difference in collateral morphology between FA and SA type. The FA fiber had a strong resemblance to those of hairy skin, while the SA fiber gave rise to more dense and larger arbors in Vp, contrary to the Vo collaterals with less developed and stringlike arbors. Statistical comparison of data from distinct functional types indicated trends that supported the above-described qualitative observations. The total number of boutons (Vp + Vo), and numbers of boutons and boutons per collateral in Vp were significantly larger in SA fibers than in FA fibers had significantly larger boutons than SA fibers. The average size of boutons became successively smaller in the following fiber groups, vibrissa, guard hair, hairy skin and lip afferents. The present findings that a distinctive morphology in the terminal arborizations has a good correlation with a functional segregation among the cytoarchitectonically different zones of the trigeminal sensory nuclear complex (TSNC) were consistent with our previously described data on periodontal and tooth pulp afferents (see ref. 34).