So first, we're going to talk about the HPO cycle.
Not to be confused with the popular cable station.
Again, HPO stands for
hypothalamus, anterior pituitary, and the ovaries.
Let's look at what's going on in the hypothalamus.
The hypothalamus controls lots of things,
not just reproduction.
I want you to take a moment and jog your mind
and see if you can think of all the things the hypothalamus controls.
You got it.
When we think about things like being hungry,
and when we've had enough to eat,
the hypothalamus lets us know.
When we think about the fact that we might be cold or hot,
the hypothalamus lets us know.
But here today, we're going to talk about
how the hypothalamus controls the menstrual cycle.
Within the menstrual cycle,
there is a neurotransmitter released about every 60 minutes,
and that neurotransmitter is called GnRH.
GnRH stands for Gonadotropin-releasing hormone.
Try to say that three times fast.
The GnRH acts on the anterior pituitary.
Hopefully you remember that there are two parts of the pituitary.
There's the anterior side, and the posterior side.
The posterior pituitary is going to be important to us later,
because it actually releases a hormone called oxytocin.
And oxytocin is responsible for things like uterine contractions.
But today we're focused on the anterior pituitary.
So under the influence of GnRH,
it actually releases two hormones known as
FSH or Follicle-stimulating hormone, and LH, Luteinizing hormone.
Those two neurotransmitter hormones are responsible
for acting on the ovaries.
And they help to build the follicle from which the egg emerges.
Part of the function of the ovary is to release two hormones
that should be well known to you.
And those are estradiol and progesterone.
An estradiol is just a type of estrogen.
So, don't get confused by that at all.
So, this is the HPO cycle.
The next thing we want to talk about in this cycle
is this idea of a feedback loop.
So let's talk for a second about what a feedback loop actually is.
A feedback loop can be positive or negative.
When we have a positive feedback loop,
what that means is when a substance is present,
it elicits more of another substance to be produced.
A negative feedback loop is exactly the opposite.
When that substance is present,
it sends a message to decrease production of another substance.
So, estrogen and progesterone actually form
positive and negative feedback loops
with both the anterior pituitary, and the hypothalamus.
Now, this is important, because as the follicle grows,
and the egg ovum inside grows,
we don't want to keep growing the egg,
because that would be a really big egg.
And that would be a problem. So, we don't want to do that.
We want to send a message when there is enough
when the egg is perfect in terms of size
to stop the production.
So that's what the feedback loop is about.
Let's talk about what it looks like with the anterior pituitary
and with the hypothalamus.
So, in the beginning,
FSH is stimulating the production of the follicle and the egg inside.
So when we have enough FSH, and think about the word
follicle-stimulating hormone,
when we have sufficient stimulation
from the anterior pituitary to the ovary,
and estrogen is produced, it sends a message
back to the anterior pituitary that there's enough FSH,
so slow down.
So the feedback loop goes from a positive feedback loop
to a negative feedback loop, okay.
The same thing happens with the hypothalamus.
Initially, as the hypothalamus produces GnRH,
and it stimulates the anterior pituitary
to release FSH, that's great
because the ovary needs to produce the follicle and the ovum,
but once the egg is sufficient size,
then it needs to convert to a negative feedback loop
to slow down the stimulation of GnRH and FSH
because our egg is ready.
That's the feedback loop.