Changes in ventral medullary light reflectance during hypercapnia in awake and sleeping cats

Activity within rostral and intermediate ventral medullary surface areas, measured as 660 nm scattered light changes, was examined in six cats, (five experimental, one control site) following 5% CO(2) challenges during waking, quiet sleep, and rapid eye movement (REM) sleep states. Activity declined to hypercapnia in all states, with a smaller decline in quiet sleep compared to waking, and intermediate values in REM sleep. The decline occurred more rapidly, with a shorter latency, during wakefulness, but with a much slower return to baseline than during quiet sleep. During REM sleep, the latency to nadir and recovery were greater than in other states. Regional patterns of activation emerged which differed in extent of activation between states.