Motor reactions during the antenatal period correlated with the periodic change in the activity of the cardiovascular system.

A connection between the periodic appearance of vagal bradycardia and of intrauterine large-amplitude respiratory movements, as well as the appearance of generalized jerky-type and more sustained extensor motor reactions, has been established in experiments on dog fetuses connected with the mother through the umbilical cord. Brief vagal bradycardia also arises immediately after completion of these reactions and is an analogue of the vagal phase in adults after motor activity. These correlated phenomena are caused by periodic reversible mild hypoxemia which through the chemoreceptors of the sinocarotid zone stimulates the centers of vagal innervation of the heart and the centers of innervation of skeletal muscles. These phenomena disappear when the mother inhales a gas mixture containing less than 14-15% oxygen or greater than 50-60% oxygen, as well as when the mother is under deep anesthesia.