Oxytocin - essential facts

• small peptide (eight amino acids)
• involved in two endocrine reflexes :
  • milk ejection-reflex
  • uterine contraction during labour

Additional Information

milk ejection-reflex

• oxytocin secreted in response to baby suckling at nipple (activation of touch receptors).
• sensory nerves synapse with hypothalamic nuclei.
• increased frequency of action potentials in supra-optic and paraventricular neurones cause release of oxytocin from nerve endings in the posterior lobe into the bloodstream.
• oxytocin causes contraction of myoepithelial cells of the alveoli and ducts of the mammary glands.
• causes ejection of milk which triggers further suckling - positive feedback.

Uterine contraction during labour

• oxytocin secreted in response to activation of uterine stretch receptors.
• sensory nerves synapse with hypothalamic nuclei.
• increased frequency of action potentials in supra-optic and paraventricular neurones cause the release of oxytocin from nerve endings in the posterior lobe into the bloodstream.
• oxytocin indirectly causes contraction of uterine smooth muscle by stimulating the release of prostaglandins (PGE and PGF).
• oestrogen released from the placenta during pregnancy stimulates the expression of oxytocin receptors on uterine myometrial cells.
• another example of positive feedback.