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1.1.Menstrual Cycle
Objectives
Concepts to Learn
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Understand the correspondence between ovarian cycle and menstrual cycle
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Understand the mechanisms estrus and sexual receptivity
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Understand how different hormones regulate the menstrual cycle
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Understand the consequences of dysregulation of the menstrual cycle
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Understand what causes menstruation
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Understand the consequences of FSH and LH surges
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Understand the development of the ovarian follicles during the menstrual cycle
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Understand the role of corpus luteum in maintaining the uterus lining and hormonal release
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Understand the difference in the physiological response for the uterus when a fertilized egg is implanted and not implanted
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Understand the change in menstrual cycle when a fertilized egg is implanted in the uterus
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Understand the hormonal changes leading to premenstrual syndrome
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Understand the similarities and difference between premenstrual syndrome and post-partum syndrome
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Understand the physiological mechanisms for oral contraceptives
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Understand the physiological mechanism of rhythm method for contraception
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Understand how ovulation and pregnancy can be tested by hormonal levels
Q&A
What is estrous cycle?
Estrous cycle is the reproductive cycle of hormonal changes in female mammals that regulates the reproductive system and prepare for the implantation of egg (ovum) into the uterus. It is analogous to menstrual cycle in humans.
The difference in behavior during the reproductive cycle is that other mammals are usually in estrus (in heat) in the middle of the estrous cycle, whereas humans usually do not exhibit this behavior. Furthermore, the phases of estrous cycle are not equal in duration compared to menstrual cycle in humans.
The duration of estrous cycles in mammals varies greatly from species to species, from 2 days to years. For example, the estrous cycle is 2 days for rabbits, and 5 days for rats, while the estrous cycle is a year for other wild animals, which often synchronize the birth of newborns during spring when the food supply is abundant, and the weather is warm, especially for hibernating animals.
What is in estrus?
When a mammal is in estrus, it is in heat (receptive to mating) during the estrous cycle.
Mammals usually exhibit receptive behaviors when in estrus. This behavior is often characterized by lordosis – the reverse arching of the back exhibiting sexual receptivity in female mammals.
Most female mammals are sexually receptive when in estrus during the follicular phase prior to ovulation, which is concomitant with the rise in estrogen level, which regulates the sensitivity of serotonin receptors. Mating often occurs during this time-window.
On the other hand, female mammals are sexually non-receptive during other phases of the cycle, and are often very defensive if approached by the male counterpart.
What is lordosis?
It is the characteristic mating behavior in female quadrupeds, which is characterized by the reverse arching of the back, typical in a sexually receptive position.
This is a reflex mediated by the spinal cord when the genital area is stimulated. Since it is a reflex, it does not necessarily require conscious control.
What is menstrual cycle?
Menstrual cycle is the periodic changes of the uterus that correspond to the changes in reproductive hormonal levels, which regulate the fertility of female human.
Menstrual (menses = month) cycle is the monthly cycle in female human that corresponds to the changes in the uterus lining, in preparation for implantation of fertilized egg. Menstrual cycle is characterized by the changes in the thickness of the uterus lining – endometrium. Normally, the menstrual cycle lasts for 28 days.
Menstrual cycle is defined by the characteristics of the uterus. The same cycle can also be characterized by changes in the ovaries too, which is called the ovarian cycle.
When is defined as the beginning of the menstrual cycle?
The beginning of the cycle is conventionally defined by the onset of menstruation.
This is just a convention that is traditionally defined since the onset of menstruation is a clearly definable event among the various events in the cycle.
When does ovulation occur in the menstrual cycle in human?
Ovulation often occurs at the mid-cycle.
Ovulation occurs when LH is released. The LH surge triggers ovulation.
Although there is no “clock” in the brain that regulates the menstrual cycle, the length of the cycle is determined by the relative phases of release of various hormones, including estrogen, FSH, LH and progesterone. If any of these hormones are out-of-phase, then the cycle can be shifted, and become irregular.
What is ovarian cycle?
Ovarian cycle is equivalent to the menstrual cycle, except that the cycle is characterized by the changes in the ovary rather than the uterus.
Menstrual cycle can be described by the changes in the ovaries uterus too, not just by the changes in the uterus. The changes of the ovaries are resulted from the changes in the reproductive hormonal levels, including FSH and LH.
The maturation of ovarian follicles during the cycle is regulated by numerous hormones, including the anterior pituitary hormones FSH (follicle stimulating hormone) and LH (luteinizing hormone) as well as estrogen levels.
What are the phases of menstrual cycle?
The menstrual cycle phases are:
menstrual flow phase (day 0 – day 5),
proliferative phase (day 5 – day 14), and
secretory phase (day 14 – day 28).
These phases are the changes describing the uterus development during the menstrual cycle. There are also corresponding phases in the ovarian cycle too. The number of days in these phases is approximate; they can vary greatly depending on the synchronization of release of different reproductive hormones in the feedback loop.
What causes menstruation?
The drop in progesterone and estrogen level will induce menstruation.
Because progesterone is essential to promote the proliferation of the uterus lining, a decrease in progest...
