Morphology of physiologically identified slowly adapting lung stretch receptor afferents stained with intra‐axonal horseradish peroxidase in the nucleus of the tractus solitarius of the cat. I. A light microscopic analysis

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Abstract
The present series of experiments was designed to study the organization of preterminai processes and synaptic boutons of single physiologically identified slowly adapting receptor (SAR) pulmonary stretch afferent fibers. Intra-axonally injected horseradish peroxidase-wheat germ agglutinin (HRP-WGA) conjugate was used as the label. In the first paper, we describe the pattern of arborization of axon collaterals from single physiologically identified SAR afferent fibers evident in the various subnuclei of the nucleus of the tractus solitarius (nTS). In the second paper, details are presented regarding the ultrastructure of these synaptic boutons and axon collaterals. A number of significant findings resulted from this study: (1) A single lung stretch SAR afferent fiber arborized over a considerable distance rostrocaudally in the brain stem (1,700–2,100 μm). (2) A single lung stretch SAR afferent fiber terminated as hundreds of bouton terminals (650–1,180). (3) There was a remarkable consistency in the subnuclei of the nTS that received these terminal arborizations of SAR afferents. (4) The ventral (vnTS), intermediate (nI), ventrolateral (vInTS), and interstitial (ni) subnuclei of the nTS were the only regions of the nTS receiving bouton terminals of SAR afferent fibers. (5) Under the light microscope the pattern of termination of SAR afferents was similar in all the axons studied in this series. (6) The injected parent axon in each case could be followed in the TS at all levels and remained consistent with regard to position and orientation and could be traced rostrally to levels as far as 3.5 mm rostral to the obex whereas the region of terminal arborization was located around 1.7–2.1 mm rostral to the obex. This pattern indicates that a single lung stretch SAR afferent fiber descends caudally upon entering the nTS. In the cat vagal afferent fibers are known to enter the medulla at levels between 0.5 mm and 3.2 mm rostral to the obex (Kalia and Mesulam, '80a). The results of the light microscopic analysis presented in this article indicate that lung stretch (SAR) afferents from the lungs and tracheobronchial tree have distinctly localized patterns of distribution in the nTS. In addition, these findings support the concept that representation of pulmonary afferents in the medulla is constituted by a differentiated distribution of nerve terminals in the various subnuclei of the nTS. Modality-specific localization (SAR afferents in this case) appears to be predominant in the n'1'S. The widespread rostrocaudal distribution of the terminal field of a single lung stretch SAR afferent is remarkable. In addition, the finding that the terminals of different SAR afferents are localized in only a few subnuclei of the nTS suggests that specificity exists in this afferent system which could provide the morphological substrate for focussed physiological effects.