Summary
Highlights
Oxytocin is produced by the posterior pituitary gland, specifically by the paraventricular nucleus in the hypothalamus. This section introduces the hormone and its origin.
During childbirth, the stretching of the cervix activates stretch receptors, signaling the hypothalamus to release oxytocin. Oxytocin then binds to receptors on smooth muscle cells in the myometrium, leading to increased intracellular calcium levels and uterine contractions, which help push the baby out. This is a positive feedback loop.
Oxytocin binds to its receptor, activating a Gq protein, which in turn activates phospholipase C. This enzyme breaks down PIP2 into DAG and IP3. IP3 releases calcium from the sarcoplasmic reticulum, and DAG activates protein kinase C, increasing calcium entry from the extracellular space. The overall effect is an increase in intracellular calcium, leading to muscle contraction.
Suckling by the baby activates mechanoreceptors on the nipple, which send signals to the hypothalamus, triggering oxytocin release. Oxytocin acts on myoepithelial cells surrounding milk glands, causing them to contract and eject milk, a process known as the milk letdown reflex.
Oxytocin also plays a role in male sexual orgasm, causing contraction of the vas deferens to expel semen, and is associated with feelings of love, compassion, and social bonding (the 'cuddle hormone'). Hypersecretion (too much) of oxytocin is rare but can be induced by synthetic oxytocin (Pitocin) to augment labor. Hyposecretion (too little) can lead to uterine inertia during childbirth and, rarely, can be caused by postpartum hemorrhaging affecting the hypothalamus.