Oxytocin
Also known as: OT, OXT
A nonapeptide from paraventricular (and supraoptic) neurons, released from the posterior pituitary, that contracts the uterus during labour and the myoepithelial cells of the breast during milk ejection — both via positive-feedback reflexes.
Overview
Oxytocin differs from ADH at only two of its nine amino acids and is co-synthesised with neurophysin I. It binds the oxytocin receptor, a Gq-coupled GPCR: IP₃ releases Ca²⁺ from stores, Ca²⁺–calmodulin activates myosin light-chain kinase and smooth or myoepithelial cells contract; oxytocin also stimulates prostaglandin production by the decidua.
Labour: estrogen dramatically up-regulates myometrial oxytocin receptors (and gap junctions) towards term, so the uterus becomes highly sensitive. Pressure of the fetal head on the cervix triggers reflex oxytocin release, which strengthens contractions, pushing the head further against the cervix — the Ferguson reflex, a positive-feedback loop terminated by delivery. After delivery it contracts the uterus, compressing spiral arteries and limiting blood loss.
Milk ejection (let-down): suckling stimulates nipple mechanoreceptors; afferent impulses reach the hypothalamus and oxytocin is released within about a minute, contracting the myoepithelial cells around alveoli and ducts. The reflex can be conditioned (a baby's cry) and inhibited by stress or pain. In the brain oxytocin promotes maternal behaviour, pair bonding and social recognition.
- Location
Source: magnocellular neurons of the paraventricular (mainly) and supraoptic nuclei; released from the posterior pituitary. Targets: myometrium, mammary myoepithelial cells, brain.
Function
- Stimulates uterine contractions during labour and after delivery.
- Causes milk ejection by contracting mammary myoepithelial cells.
- Promotes maternal and social bonding behaviour.
- Physiology
Class: nonapeptide. Receptor: OXTR — Gq/IP₃–Ca²⁺. Half-life ≈3–5 min. Myometrial receptor density rises steeply in late pregnancy under estrogen.
- Regulation
Positive feedback: cervical/vaginal stretch (Ferguson reflex) and suckling (milk-ejection reflex). Inhibited by stress, pain, catecholamines and alcohol.
Clinical significance
Synthetic oxytocin
Used to induce or augment labour and, with ergometrine or carbetocin, in active management of the third stage to prevent postpartum haemorrhage from uterine atony. Excessive doses cause uterine hyperstimulation and fetal distress, and — because of weak V2 activity — water intoxication and hyponatraemia with large volumes of hypotonic fluid.
Oxytocin antagonists
Atosiban blocks oxytocin receptors and is used as a tocolytic to delay preterm labour.
Test yourself
Oxytocin release during labour is an example of:
- Negative feedback
- Positive feedback (Ferguson reflex)
- Feed-forward inhibition
- Paracrine signalling only
Answer: Positive feedback (Ferguson reflex)
Stretch of the cervix by the fetal head sends afferent signals to the hypothalamus, increasing oxytocin release; stronger contractions push the head further, which increases stretch — a self-amplifying loop ended by delivery.
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.