Rapidly adapting pulmonary receptor afferents: I. Arborization in the nucleus of the tractus solitarius
Authors: Kalia Madhu , Richter Diethelm 
Published: January 2005

The organization of axon collaterals, preterminal processes, and presumptive synaptic boutons of single physiologically identified rapidly adapting receptor (RAR) pulmonary afferent fibers was examined following the intraaxonal application of wheat germ agglutinin conjugated with horseradish peroxidase (WGA‐HRP). The RAR axons were injected 200–300 μm lateral to the nucleus of the tractus solitarius (nTS) at a number of different rostrocaudal levels in seven individual experiments. The trajectories of the stained axons were reconstructed from individual 50‐μm‐thick serial sections. The rostrocaudal extent, as well as the distribution of the trajectory of each RAR afferent, was reconstructed from every section by using a camera lucida attachment. In this first of two papers, we describe the pattern of organization of bouton terminals of RAR afferents related to cytoarchitectonically distinct subnuclei of the nTS. In the companion paper, morphological details of the fine structure of these synaptic boutons and axonal branches are described in different subnuclei in order to illustrate morphological differences in these functionally distinct regions.

A number of significant findings have resulted from this light microscopic study. The central process of a single RAR afferent fiber arborized in the medulla oblongata over a considerable distance in the rostrocaudal plane (2.5 mm rostral to 1.4 mm caudal to the obex). A single RAR afferent fiber terminated in numerous bouton terminals (range 500–1,050), and these terminals arose from over 400 segments of branches of the parent injected axon. A small number of en passant bouton terminals were found. There appeared to be a remarkable degree of consistency in the subnuclei of the nTS where these terminals arborized. The dorsal and dorsolateral subnuclei of the nTS received 144–647 bouton terminals. The second‐largest concentration of bouton terminals of RAR afferents was found in the intermediate (nI) subnucleus of the nTS. No labeled bouton terminal was found in the ventral and ventrolateral subnuclei of the nTS. This finding is in sharp contrast to the terminations of SAR afferents which terminated predominantly in the ventral and ventrolateral nuclei of the nTS, the interstitial nucleus of the nTS, and the nI.

The parent RAR axon could be traced as far rostrally as 2.5 mm, even though the region of terminal arborization could not be followed beyond 0.8 mm. The destination of this rostrally projecting RAR afferent could not be determined in this study. This pattern indicates that a single RAR afferent fiber ascends rostrally in the tractus solitarius giving off branches at a number of different levels.

Since the rostrocaudal location of the randomly selected RAR afferent impaled in the medulla varied considerably in all seven cases examined, it is likely the location of their peripheral endings was different in each case. In spite of this viscerotopic difference between the injected axons, the patern of distribution of the axon collaterals and synaptic boutons in the nTS was very similar in all cases.

This illustrates that modality specificity, and not viscerotopic specificity, is an important feature of the central organization of these afferents. These findings are similar to the modality specificity observed with SAR afferents (Kalia and Richter: J. Comp. Neurol. 233: 308‐332, '85a). In addition, these results further support the concept of functional organization of the nTS along cytoarchitectonic boundaries.