Projections from the commissural subnucleus of the nucleus of the solitary tract: An anterograde tracing study in the cat

The commissural subnucleus (COM) of the nucleus of the solitary tract (NTS) is known to receive primary afferents from the lungs and other viscera innervated by the vagus nerve, and thus to participate in central autonomic and respiratory control. The aim of the present study was to identify the areas of terminal arborizations of COM neurons in order to examine brainstem sites which may be involved in reflex responses mediated by these neurons. The projections were studied in cats, using biocytin as an anterograde tracer. Labeled fibers and terminal boutons were visualized by horseradish‐peroxidase histochemistry, 2–3 days after microinjection of the tracers into the COM 1–2 mm caudal to the obex. Labeled axons were examined in the brainstem from the rostral pons to the caudal medulla and were found bilaterally, with an ipsilateral predominance, mainly in the following regions: (1) The dorsolateral rostral pons. Terminal boutons were observed in the lateral and medial parabrachial nuclei, Kölliker‐Fuse nucleus, and around the mesencephalic trigeminal tract. This area corresponds to the pontine respiratory group also known as the “pneumotaxic center.” (2) The pontine area dorsolateral to the superior olivary nucleus. This region contains the A5 noradrenergic cell group; (3) Near the ventral surface, below the facial nucleus. This area overlaps with the ‘retrotrapezoid nucleus.’ (4) Respiration‐related areas of the medulla, including the dorsal and ventral respiratory groups, and the Bötzinger complex. (5) The dorsal motor nucleus of the vagus. These results suggest that the COM is involved in reflex arcs, which have both respiratory functions and autonomic functions. The pathway to the dorsolateral pons, which has been identified in our recent electrophysiological study is likely to play a role in mediating respiratory responses from pulmonary rapidly adapting receptors. Other pathways may represent additional projections from second‐order neurons receiving input from this group of lung receptors, or projections from as yet unidentified neurons that relay information from different afferents terminating in the COM.