Fig. 9.

CO₂ concentration and muscle sympathetic nerve activity and their coupling during one fast-to-slow breathing ramp from one subject. Top three rows: continuous recordings of CO₂ (black) and muscle sympathetic nerve activity (red) during one entire fast-to-slow breathing ramp. Almost without exception, muscle sympathetic nerve firing occurred during expiration, the envelope inscribed by the increases and decreases in CO₂. Bottom: red and red-gray shaded areas indicate significant information transfer in the two directions. The large red area documents major information transfer from breathing to muscle sympathetic nerve oscillations and suggests, moreover, that transfer does not begin until a frequency threshold is reached (in this subject, at about 0.2 Hz, a breathing interval of 5 s).