How do contractions during the non-pregnant phase of the menstrual cycle primarily occur?

During the non-pregnant phase of the menstrual cycle, contractions of the uterus primarily occur due to the activation of estrogen and progesterone receptors. In this phase, the levels of estrogen and progesterone fluctuate as the ovaries go through their cyclical changes.

Estrogen is involved in the build-up of the uterine lining (endometrium) during the follicular phase, while progesterone plays a crucial role during the luteal phase. As the levels of these hormones decline towards the end of the menstrual cycle, it triggers the shedding of the endometrial lining, known as menstruation. The activation of estrogen receptors promotes uterine contractions, helping to facilitate the process of shedding the lining.

The decline in progesterone near the end of the cycle also leads to contractions, as progesterone's role is primarily to maintain the endometrial lining when pregnancy does not occur. Therefore, these hormonal changes and their respective receptor activations are the driving force behind the contractions experienced during menstruation. This highlights the essential role of hormonal signaling rather than physical stress or other stimuli in regulating uterine activity during this phase.