Proprioceptive, chemoreceptive and sleep state modulation of expiratory muscle activity in the rat

The purpose of this study was to assess the respiratory and tonic activity of the abdominal muscles and the postinspiratory activity of the diaphragm (stage 1 expiration) in rats during sleep while they breathed air, hypercapnic, and hypoxic gas mixtures. ECoG and neck EMG recordings enabled the differentiation to be made between nonrapid eye movement sleep (nREMS) and rapid eye movement sleep (REMS). EMGs of the rectus abdominis, internal and external oblique, and diaphragm muscles were displayed on a CRT and polygraph. During nREMS the rectus abdominis showed no respiratory activity, whereas the oblique muscles showed activity confined to stage 2 expiration. This activity was modulated by proprioceptive (sleep postures) and chemoreceptive activation (5% CO2 in air and 10-12% O2 in nitrogen): tonic activity was not consistently affected by such inputs. During REMS tonic activity disappeared, whereas phasic activity either remained unchanged or was abolished. If phasic activity ceased it could reappear periodically during the same REMS epoch. While breathing air, rats in nREMS showed postinspiratory diaphragmatic activity which was sustained or slightly increased while breathing a hypoxic gas mixture but was virtually abolished during hypercapnia. In REMS postinspiratory discharges almost disappeared. The data support the conclusion that the diaphragm provides expiratory braking and that the external and internal oblique muscles contribute to active exhalation during nREMS as well as priming the diaphragm for the next inspiration by improving its length-tension relationship. A three-phase neural respiratory pattern generator operates in nREMS: it changes temporarily to a two-phase system while breathing CO2 and during REMS due to the inhibition of the postinspiratory phase.