Hexon

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i was reading in kaplan how oestrogen starts off in the menstrual cycle as having a negative feedback system then switches to positive feedback just before the spike in LH.

My question is when exactly does this occur, or am i missing something here? Meaning, days, hours or seconds before the LH surge?

and what sort of physiological effects would be observed from this change in typical oestrogen activity?

Thanks guys
 
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MedPR

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i was reading in kaplan how oestrogen starts off in the menstrual cycle as having a negative feedback system then switches to positive feedback just before the spike in LH.

My question is when exactly does this occur, or am i missing something here? Meaning, days, hours or seconds before the LH surge?

and what sort of physiological effects would be observed from this change in typical oestrogen activity?

Thanks guys

Usually estrogen has a negative feedback on LH release (not LH production). At some point estrogen reaches a certain level and switches to positive feedback on LH release. There is a surge in LH because there is a huge LH store that hasn't been released (due to estrogen).

I don't know what day it is, but it is the reason for ovulation.
 

Hexon

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thanks for that MedPR;

btw what exactly is it about oestrogen that makes it 'flip' its role? do conditions in the ovaries change somehow? and in what way does it change in order for the effects to flip from negative to positive??
 
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MedPR

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Dec 1, 2011
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thanks for that MedPR;

btw what exactly is it about oestrogen that makes it 'flip' its role? do conditions in the ovaries change somehow? and in what way does it change in order for the effects to flip from negative to positive??
I don't know, and I don't think you have to know for the MCAT. I'm sure it has something to do with receptor up/down regulation, but I don't have a real answer. There are a couple of med students that help out on this forum a lot. I'm sure one of them will wander in here and provide a better answer.
 

Morsetlis

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thanks for that MedPR;

btw what exactly is it about oestrogen that makes it 'flip' its role? do conditions in the ovaries change somehow? and in what way does it change in order for the effects to flip from negative to positive??
Low estrogen triggers Adrenergic neurons in the hypothalamus, which represses parvocellular release of GnRH. High estrogen triggers neuropeptide-Y neurons to stimulate the parvocellular release of GnRH from the hypothalamus. It is possible that the neuropeptide-Y neurons only respond to high amounts of estrogen.

Of course, after the follicle ejects its ovum, it becomes a corpus luteum and slowly secretes progesterone as the granulosa cells begin to express FSH-receptors. This progesterone activates beta-endorphin neurons in the hypothalamus and begins to block GnRH release again. With lowered LH/FSH release from the AP due to lowered GnRH, the corpus luteum begins to decay and progesterone level eventually falls. As progesterone level tapers off, it cannot maintain the endometrium and menses occurs.

Even though there is a decreasing level of LH/FSH, the growing SIZE of the primary follical (and the associated internal thecal/granulosa cells) means that estrogen level is steadily rising.

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But why on earth do you need to know this? :p
 
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