Chapter 53.4, Problem 1CS

Core Skill: Connections In what other context have you learned about positive feedback? Refer back to Figures 51.9 and 51.11 for help.

Figure 51.9 The ovarian and uterine cycles in a human female. The ovarian cycle is divided into the follicular and luteal phases. The uterine cycle is divided into menstruation and the proliferative and secretory phases.

Figure 51.11 Hormonal control of parturition. Birth relies on neural signals from the uterus and maternal hormones that act on the uterus. In response to sensory neural input arising from the push of the fetus on the cervix, the maternal posterior pituitary releases oxytocin into the blood, which stimulates uterine smooth muscle contractions. The secretion of prostaglandins by the uterus also increases the strength of the contractions. Sensory receptors in the uterus detect the more forceful contractions and signal the mother's posterior pituitary to secrete more oxytocin, thus completing a positive feedback loop that further strengthens the contractions.