Figure 7.

A series of photomicrographs illustrating the projections of the Kolliker-Fuse nucleus to respiratory-related sites. Panels A-C show c-Fos-activation with CO₂ exposure in the projection to the ventrolateral medulla; D-F show projections to the phrenic motor level of the spinal cord; and G-I demonstrate projections to the hypoglossal motor nucleus. Injection sites into the brain (A, G) or spinal cord (D) of animals exposed to CO₂ are shown on the left. The middle panels show a low magnification photomicrograph through the KF nucleus (B, E, H), and a higher magnification image on the right (C, F, I) showing the area in the box in B, E, H from a hypercapnic animal in each series. Note the large percentage of doubly-labeled neurons (indicated by arrows, brown cytoplasm demonstrating CTb, black nucleus stained for c-Fos) in each case. Few if any doubly labeled neurons were seen in control brains (Figs 6, 8, 9). Abbreviations: C4, fourth cervical spinal segment; VLM: ventrolateral medulla; 12 hypoglossal motor nucleus; vsct: ventral spinocerebellar tract.