Somatic-respiratory reflex and onset of regular breathing movements in the lamb fetus in utero.

Breathing activity of six mature lamb fetuses (greater than 135 days of gestation) in utero was monitored from recordings of intraesophageal pressure, intratracheal pressure, and tracheal circumference from a mercury strain gauge before, during, and after stimulation of the central end of a cut sciatic nerve. Stimuli were either low (0.5-2.0 cps) or high (66 cps) frequency, 6-15-V square wave pulses of 0.6-1.25 msec duration. The fetuses remained in utero throughout the experiments in which ambient temperature, paO2, paCO2, arterial pH, mechanical stimulation, and spontaneous respiratory center activity could be ruled out as primary stimuli of the breathing movements observed. In one-third of the trials a "somatic-respiratory reflex" was elicited in which breathing coincided with the period of stimulation: in over 85% of these trials with low frequency stimulation, breathing movements were synchronous with the stimuli; in the rest the synchrony was broken during the period of stimulation. In two-thirds of the trials the "reflex" response was followed by spontaneous regular breathing movements ("onset of regular breathing") which continued for 1 min to 2 hr 30 min after the stimulation was stopped. Thiopental administration to the ewe (5 mg/kg) seemed to depress respiratory responsivity for about 60 min. Changes of tracheal circumference reflected both transmural pressure gradients and possibly also rhythmic vagal activity associated with breathing.