00:01
So first, we're going to talk about
the HPO cycle.
00:04
Not to be confused
with the popular cable station.
00:07
Again, HPO stands for
hypothalamus, anterior pituitary,
and the ovaries.
00:15
Let's look at what's going on
in the hypothalamus.
00:18
The hypothalamus controls
lots of things,
not just reproduction.
00:23
I want you to take a moment
and jog your mind
and see if you can think of all the
things the hypothalamus controls.
00:34
You got it.
00:35
When we think about things
like being hungry,
and when we've had enough to eat,
the hypothalamus lets us know.
00:42
When we think about the fact
that we might be cold or hot,
the hypothalamus lets us know.
00:47
But here today,
we're going to talk about
how the hypothalamus controls
the menstrual cycle.
00:53
Within the menstrual cycle,
there is a neurotransmitter released
about every 60 minutes,
and that neurotransmitter
is called GnRH.
01:03
GnRH stands for
Gonadotropin-releasing hormone.
01:08
Try to say that three times fast.
01:11
The GnRH
acts on the anterior pituitary.
01:14
Hopefully you remember that there
are two parts of the pituitary.
01:17
There's the anterior side,
and the posterior side.
01:22
The posterior pituitary is going
to be important to us later,
because it actually releases
a hormone called oxytocin.
01:29
And oxytocin is responsible for
things like uterine contractions.
01:33
But today we're focused on the
anterior pituitary.
01:36
So under the influence of GnRH,
it actually releases
two hormones known as
FSH or Follicle-stimulating hormone,
and LH, Luteinizing hormone.
01:48
Those two neurotransmitter
hormones are responsible
for acting on the ovaries.
01:53
And they help to build the follicle
from which the egg emerges.
01:58
Part of the function of the ovary
is to release two hormones
that should be
well known to you.
02:04
And those are
estradiol and progesterone.
02:07
An estradiol
is just a type of estrogen.
02:09
So, don't get confused
by that at all.
02:12
So, this is the HPO cycle.
02:16
The next thing we want
to talk about in this cycle
is this idea of a feedback loop.
02:20
So let's talk for a second about
what a feedback loop actually is.
02:25
A feedback loop
can be positive or negative.
02:28
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.
02:37
A negative feedback loop
is exactly the opposite.
02:41
When that substance is present,
it sends a message to decrease
production of another substance.
02:47
So, estrogen and progesterone
actually form
positive and negative
feedback loops
with both the anterior pituitary,
and the hypothalamus.
02:57
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.
03:08
And that would be a problem.
So, we don't want to do that.
03:11
We want to send a message
when there is enough
when the egg is perfect
in terms of size
to stop the production.
03:18
So that's what the
feedback loop is about.
03:20
Let's talk about what it looks like
with the anterior pituitary
and with the hypothalamus.
03:26
So, in the beginning,
FSH is stimulating the production
of the follicle and the egg inside.
03:33
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.
03:54
So the feedback loop goes
from a positive feedback loop
to a negative feedback loop, okay.
04:02
The same thing happens
with the hypothalamus.
04:05
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.
04:30
That's the feedback loop.