Reproduction – parturition, neonatal physiology, lactation

Parturition – control • Parturition requires myometrial contractions and cervical softening. • The myometrium is a functional syncytium (coupled by gap junctions) so contractions are coordinated. Contractions occur because action potentials cause Ca influx; oestrogen-primed myometrium shows pacemaker potentials, which initiate APs if they are large enough. Prostaglandins liberate Ca from intracellular binding sites. Oxytocin increases the rate of Ca influx and lowers the excitation threshold of the myocyte. • Cervical softening. The cervix has a high connective tissue content – collagen bundles in a proteoglycan matrix. Cervical softening occurs because there is a loss of collagen and an increase in glycosoaminoglycans (GAGs), specifically keratan sulphate. (This does not bind collagen well; dermatan sulphate does, so the increased keratan:dermatan sulphate ratio may explain the “loosening” of the collagen.) Prostaglandins (PGE2; PGF ) increase cervical compliance. • The endometrium is probably the most important site of prostaglandin (PG) synthesis. Remember that PGs are local hormones. The enzyme PLA2 is critical in PG synthesis. The oestrogen:progesterone ratio controls both PG synthesis (high O:P KLJK 3* V\QWKHVLV DQG DOVR 3* UHOHDVH RHVWUDGLRO ↑oxytocin receptors in the endometrium PRUH 3* UHOHDVH • Oxytocin is a peptide synthesized in hypothalamic neurons and transported along their axons to the posterior pituitary. It is released in response to tactile stimulation of the reproductive tract, esp. the cervix, and increases myometrial contractility (the Ferguson reflex). This reflex is facilitated by a high O:P ratio. • Relaxin is a polypeptide hormone made by the corpus luteum and secreted throughout most of pregnancy. It causes relaxation of the interpubic ligament, so the pubic symphysis can separate to some extent. (This can cause considerable discomfort during pregnancy.) Its precise action in the human is not certain, but in addition to ligamentous relaxation it may also inhibit myometrial activity, inhibit oxytocin secretion and facilitate cervical softening. It therefore helps to maintain pregnancy but gets things ready for parturition.