Write a short note on oxytocin.

Source and Nature

• Differs from ADH in only 2 of its 9 amino acids — hence their overlapping actions at high dose
• Bound to neurophysin I during transport
• Half-life 3–5 minutes

Actions

Milk Ejection Reflex

Suckling at the nipple → Touch receptors stimulated → Afferents → spinal cord → hypothalamus → Oxytocin from posterior pituitary → Myoepithelial cells contract → milk ejection (let-down)

• It is a neuroendocrine reflex — neural afferent, hormonal efferent
• Conditioned — the cry or sight of the baby can trigger it
• Inhibited by stress, fear, pain and embarrassment through adrenaline, which constricts breast vessels — an important cause of apparent lactation failure

Role in Labour — the Ferguson Reflex

Descent of the fetal head → Stretches the cervix → Afferents to hypothalamus → ↑ Oxytocin → Stronger uterine contraction → Further cervical stretch → positive feedback → delivery

• One of the few useful positive feedback mechanisms in the body
• After delivery, oxytocin contracts the uterus → compresses the spiral arteries → controls postpartum haemorrhage

Applied Aspects

• Oxytocin infusion is used to induce or augment labour, and routinely in the active management of the third stage to prevent postpartum haemorrhage
• Overdose → uterine hyperstimulation, fetal distress, rupture; and water intoxication from its ADH-like action
• Suckling suppresses GnRH → lactational amenorrhoea → a natural though unreliable contraceptive
• Oxytocin challenge test — formerly used to assess placental reserve before labour
• Comparison with ADH — both are nonapeptides from the posterior pituitary, but ADH comes chiefly from the supraoptic nucleus and acts on the kidney, while oxytocin comes chiefly from the paraventricular nucleus and acts on uterus and breast