NEET 2021 Zoology Question 189 Solution

Parturition (the process of delivering the foetus through labor) is governed by an integrated neuroendocrine mechanism known as the foetal ejection reflex: - Signals for parturition originate from the fully developed foetus and placenta, inducing mild uterine contractions. - (1) Estrogen-to-Progesterone Ratio: In late pregnancy, placental estrogen synthesis rises while local progesterone dominance declines. This high estrogen/progesterone ratio upregulates oxytocin receptors on myometrial smooth muscle cells and promotes gap junction formation between myocytes. - (3) Oxytocin Release: Maternal posterior pituitary secretes oxytocin, which acts directly on the myometrium to cause vigorous contractions. - (2) Prostaglandin Synthesis: Uterine decidua and fetal membranes synthesize prostaglandins ($PGF_{2\alpha}$ and $PGE_2$), which further amplify uterine contractions and soften/dilate the cervix in a positive feedback loop (Ferguson reflex).

Prolactin (PRL) is an anterior pituitary peptide hormone that stimulates the differentiation of mammary alveolar epithelium and drives milk synthesis during the puerperium (lactation). During pregnancy, high circulating levels of placental steroids (estrogen and progesterone) block the lactogenic action of prolactin on the breasts. Following parturition and placental expulsion, steroid levels plummet, disinhibiting prolactin to initiate copious milk production. Prolactin has zero stimulatory effect on myometrial contraction or labor initiation.