Isolated perfused carotid body preparation

To compare O₂ sensitivity of the spinal cord and the carotid body, we used the artificially perfused carotid body (fig. S1). We constructed an oxygen dose-response curve for the carotid body and plotted with the dose-response curve for the artificially perfused thoracic in situ spinal cord preparation. The carotid body preparation has been described elsewhere (80, 81). Please note that the carotid body is densely irrigated with blood vessels that are thought to minimize tissue O₂ gradients and thereby accounting for its ability to respond to PO₂, not SO₂. As the carotid body PO₂-response curve of the artificially perfused carotid body preparation is similar to that of carotid body afferents measured in vivo (82), we assume that tissue PO₂ in this preparation is similar to that of the perfusate. To generate a hypoxic dose-response curve for the carotid body, we used a similar protocol as used for the in situ spinal cord preparation (Fig. 2D). To aid comparison, relative change in activity of both preparations was determined and normalized to that when both tissues were assumed to be at 100-torr PO₂.