Analysis of respiratory neuronal activity in fetal sheep

We developed a new method to monitor fetal medullary respiratory neurons utilizing a two-stage approach. At 129-133 days of gestation, sheep were anesthetized, and a window was placed over the area of the fourth ventricle. After a recovery period of 3-5 days, the fetus was exteriorized into a saline bath under maternal spinal anesthesia, and the head was connected rigidly to a stereotaxic frame. Microelectrodes were inserted into the area of the nucleus tractus solitarius during rapid-eye-movement sleep, and extracellular recordings of 223 respiratory neurons were analyzed: 76% were inspiratory, 9% expiratory, and 15% phase spanning, as classified by visual and computer correlation to diaphragmatic activity. More detailed analysis of 100 neurons was done to assess the respiratory component (eta 2) by use of a modification of the method developed by Orem and Dick (J. Neurophysiol, 50: 1098-1107, 1983). With use of cohorts of 25 breaths, fetal respiratory neurons were found to frequently change their phase relationship to diaphragmatic activity. The eta 2 statistic of fetal respiratory neurons was not a stable characteristic but changed over time. This could be a reflection of an immature central respiratory system before birth or the lack of major sensory inputs.