Oxytocin

Oxytocin exerts its major effects on two different target tissue. This hormone stimulates:

• Contraction of uterine smooth muscle
• Contraction of myoepithelial cells

The secretion of oxytocin is regulated by reflexes elicited by cervical stretch and by suckling. Normally, as labor begins, the fetus is positioned head down. This orientation exerts pressure on the cervix and causes it to stretch. Sensory neurons in the cervix are thus activated to transmit signals to the hypothalamus, which will stimulate the release of oxytocin from the neurohypophysis. This hormone then enhances uterine contraction which causes further pressure and stretch oc the cervix, additional oxytocin release, and so on, until pressure has built up adequately so that delivery can take place.

extracted from Essentials of Human Physiology for Pharmacy 2004 By Laurie Kelly, McCorry P 126

If there is nice even and consistent pressure on the cervix the reflex that stimulates the release of more oxytocin is greater. When more oxytocin is generated, stronger contractions occur.