what actually triggers uterine contractions?

[yoyohomieg5432]

So far between UW and an NBME i've gathered that estrogen prior to start of labor upregulates oxytocin receptors and gap junctions between myometrial cells... and that increased prostaglandins can stimulate uterine contractions (in the case of premature rupture of membranes)... but what is causing the oxytocin release to bind the upregulated oxytocin receptors?

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[Burnt Almonds]

Isn't there a mechanical stretch mechanism involved?

 

[drarnav]

increased prostaglandins perhaps?