ORIGIN OF TONIC ELECTRICAL-ACTIVITY PATTERNS IN THE DIAPHRAGM OF THE FETAL LAMB

The respiratory pattern generator in fetal and postnatal life activate s the phrenic nucleus and diaphragm muscle with phasic bursts of activ ity. In the fetus, diaphragmatic activity is also characterized by ton ic activity patterns of unknown origin. We have examined whether such activity is diaphragmatic, or radiated from nearby ribcage muscles, by placing two sets of electrodes side-by-side in the costal portion of the diaphragm in five fetuses. The rationale for this approach is that if tonic activity radiates to the diaphragm it should be recorded by both sets of electrodes and there should be no delay between the actio n potentials from each set of electrodes. Of 24 single tonic units ide ntified, 15 were recorded from only one of the two sets of electrodes in the diaphragm. In the 9 tonic units recorded from both sets of elec trodes, there was a time delay between the appearance of the action po tentials in the two recordings (mean +/- S.E.M. 1.6 +/- 0.2 ms). This is the expected conduction delay along the muscle fibres separating th e two electrodes. Since tonic diaphragmatic activity persisted in fetu ses with the spinal cord transected rostral or caudal to the phrenic n ucleus, we conclude that the spinal cord alone is sufficient to produc e the tonic activity recorded from the fetal diaphragm but that the br ain may also generate such.activity.