What mechanism is responsible for contractions during the menstrual cycle in the non-pregnant state?

The correct answer is based on the understanding of the hormonal changes and their role in the menstrual cycle. While estrogen and progesterone receptor activity primarily influences the preparation and maintenance of the endometrial lining, during menstruation, the decline in progesterone leads to a decrease in its receptor activity, triggering the shedding of the endometrium.

Prostaglandins play a crucial role by mediating contractions of the uterus, especially during menstruation, which leads to cramping. They are released in response to the shedding of the endometrial tissue, promoting uterine contractions to expel this tissue. Hence, while estrogen and progesterone receptor activity is generally important for the cycle's modulation, the acute cause of menstrual contractions specifically stems from the action of prostaglandins.

Oxytocin contributes to uterine contractions but is more relevant in the context of childbirth and labor rather than the normal menstrual cycle. Increased gap junction expression can help facilitate coordinated contractions, but prostaglandins are the primary mediators triggering the muscle contractions associated with menstruation.

Thus, the mechanism primarily responsible for contractions during the menstrual cycle in the non-pregnant state is the stimulation from prostaglandins, not solely the receptor activity of estrogen and progesterone.