Fig. 5. The effects of chronic hypercapnia (CHC) (C,D) or control normocapnia (A,B) on the breathing frequency response to acute hypercapnia in vivo with the olfactory nerves intact (A,C) or denervated (B,D). The letters `a', `b' and `c' indicate that the three values in each panel are significantly different from one another. The asterisk in B indicates a significant difference from the first air value (white bar).