Effect of abdominal compression on diaphragmatic tendon organ activity.

Previous studies suggest that afferents in the diaphragm participate in the reflex reduction in phrenic nerve efferent activation when the length of the diaphragm is increased by abdominal compression. The present study determined the response of tendon organ afferents in the diaphragm to increases in abdominal pressure. Five cats were anesthetized with thiopental sodium (60 mg/kg ip to induce, supplemented intravenously). Extracellular recordings from nine individual tendon organ afferents were made from right cervical dorsal root ganglia 5 and 6. Right crural electromyographic activity was recorded. The right extrathoracic phrenic nerve was isolated and stimulated to identify tendon organs on the basis of conduction velocity and response to twitch. The response to ramp-and-hold stretch of the diaphragm was used as an additional test to differentiate tendon organs from muscle spindles. The mean level of activity of the tendon organs during the 1st s of the inspiratory phase was 47 +/- 10 (SD) Hz. Abdominal compression was associated with a significant increase in the activity of these afferents to 61 +/- 11 Hz. Results indicate that increases in the activity of diaphragmatic tendon organs are associated with moderate increases in abdominal pressure and are likely the result of elevations in the active tension developed by the diaphragm. Combined with results from previous studies, it is possible that diaphragmatic tendon organs may play a role in the attenuation of respiratory muscle activation when abdominal pressure is increased.