Welcome to the main event!
After weeks of talking about our reproductive parts and how they work, today we finally
discuss what happens when those parts come together, to do what they were born to do
- the sex.
But first, let’s talk about what we won’t be talking about:
We’re not going to give any advice. We’re not going to talk about STIs - even though
some STIs can affect conception.
We’re also not going to talk about sexual behavior, or mate selection and there will
be no discussion of birds or bees.
And while people have sex in lots of different ways and for lots of different reasons, we’re
only going to be talking about the kind of sex that could lead to fertilization.
We are going to talk about contraception, though, in terms of how, and where, and when
different kinds can prevent fertilization.
And, we are going to look at what exactly is going on in your body before, during, and
after sex, in the most very safe for work, clinical terms possible.
Humans have been experiencing coitus for as long as they could be called humans. And actually
before that, because obviously there would be no humans without the sex.
And although it’s something that comes pretty naturally, it wasn’t until the late 1950s
and 60s that anyone thought to look at the physiological science behind the act of sex.
So when American gynecologist William Masters and sexologist Virginia Johnson kicked off
their unprecedented -- and controversial -- research into the so-called human sexual response cycle,
they began by inviting hundreds of male and female volunteers to come into the lab and
have sex in the name of science.
Though it wasn’t as voyeuristic as it might sound.
Outfitted with loads of wires and heart monitors, volunteers were periodically interrupted in
the middle of the act to be measured.
10,000 trials or so later, Masters and Johnson found out some interesting stuff. Namely,
that a typical complete sexual cycle runs through four distinct phases: excitement,
plateau, orgasm, and resolution.
During the excitement phase, things are just getting going. Sexual arousal triggers the
parasympathetic reflex that causes arterioles in the external genitalia to dilate and fill
with blood, which is actually one of the only times the parasympathetic system controls
any arteriole action.
The plateau phase is marked by increased muscle tension, heart rate, blood pressure, and breathing
rates. Here the male urethral sphincter contracts to make sure urine doesn’t mix with semen.
During orgasm, pelvic and other muscles around the body contract rhythmically as an anatomical
male ejaculates, releasing the sperm that might just get a chance to fertilize an egg,
depending on the situation.
Female orgasm is not required for conception, and some may never experience one. But if
it does happen, the resulting uterine contractions and increased lubrication can help draw up
and hold sperm in the uterus.
As the body relaxes after all that excitement, it enters the resolution phase.
Here we see males entering a refractory period that may last several minutes to a day or
two, during which they are usually unable to reach orgasm again.
Anatomical females are thought not to need this time out physiologically -- though they
may need it physically. Instead some females are actually capable of serial orgasms, one after the other.
Masters and Johnson’s work has its share of critics, many of whom point out that not
everyone fits into their four-stage mold, and that things are not always quite so linear
in the heat of the moment.
But overall, their core physiological framework is still accepted.
Now, regardless of exactly how sex happens, if it involves a fertile anatomical female
and male, and the timing is right, it could ultimately end with fertilization.
So how do a single little ovum, and an even tinier sperm come together to make a fertilized
egg, or zygote, that divides and multiplies and grows into the trillions of cells that
make up the glorious, complex individual that is you?
You’ll remember that during ovulation, about once a month, an ovary releases a secondary
oocyte that heads down the fallopian tube and toward the uterus.
That oocyte has a window of about a day, or less, to meet a sperm before it’s no longer
viable. But, that being said, sperm can persist in the female reproductive tract for as much as 3-5 days.
Which means, for fertilization to occur, they have to either be in place during ovulation,
or they have to arrive no more than a day after.
But now I feel like I’m getting a little ahead of myself.
Let’s go back to how sperm typically get to the anatomical female body in the first place.
During sex, if a male ejaculates into a female's vaginal canal, millions of hopeful sperm will be on their way.
Despite the fact that they only have about a dozen or so centimeters to go, their journey
isn’t for the faint of heart. Luckily they don’t have hearts.
The truth is, most sperm never reach an oocyte.
Some go in the wrong direction. Others are killed in the strange new acidic environment
of the vaginal canal. And millions more fail to penetrate the gauntlet of mucus that surrounds the cervix.
Even the ones that swim hard enough and manage to reach the uterus may then be culled by
defending resident white blood cells.
In fact, sometimes only a dozen or so of the original millions make it through all those
trials to actually reach an oocyte.
But even if they do it’s still not over for them.
The sperm that do make it to the uterus, and then the fallopian tube, have another hurdle
to overcome, because they can’t actually penetrate an oocyte. Not yet.
They could literally hang out right next to one for hours, and be unable to make a move
until they are what’s known as “capacitated,” and go through one last set of changes.
As they swim through the cervix, uterus, and fallopian tube, female secretions start to
degrade some of the sperm’s protective proteins until the the cap on the tip of its little
head -- called the acrosome -- is fragile enough it can leak special hydrolytic enzymes.
Those enzymes are the key to getting through the oocyte’s protective outer layers.
But before that it has to weave through the granulosa cells that form the oocyte’s outermost
layer, known as the corona radiata.
Once it’s past that, it runs into the zona pellucida, a layer of glycoprotein that covers
the oocyte’s plasma membrane.
And this is where it starts to get a warmer reception: This layer has specific sperm receptors
waiting for it to bind to, and when it does, it opens a bunch of calcium channels, which
flood the sperm with calcium ions.
These ions trigger the final acrosomal reaction that releases the enzymes the sperm needs
to work through the zona pellucida.
Now, a number of sperm make it to this phase. And in terms of strategy or luck, it’s best
to arrive on the scene after your compadres have already been hard at work digesting holes
through the pellucida.
Because once a path is cleared, a lucky sperm can wiggle against the now-exposed oocyte
membrane until it locks onto another set of sperm-binding receptors. And when a sperm
docks into one of those, the membrane of the sperm fuses with the innermost membrane of
the oocyte, and the contents of the sperm enter the oocyte proper.
Then something pretty cool happens.
Remember that the “egg” is still just a secondary oocyte here, and hasn’t completed
meiosis. As the sperm delivers its payload, it causes a flash of calcium ions from the
oocyte’s endoplasmic reticulum.
This tells the secondary oocyte to get ready to complete its second meiotic division. But
it also triggers a reaction that suddenly seals the egg by destroying all the leftover
sperm receptors in the membrane, preventing any other sperm from weaseling in.
The oocyte then quickly completes meiosis II, leaving two daughter cells: the large
ovum and a puny second polar body.
Meanwhile, the sperm’s detached nucleus swells up to five times its original size, forming
the male pronucleus, one of two sort of proto-nuclei that contain each gamete’s genetic information.
The other one, the female pronucleus, takes shape from the nucleus of the ovum, and then
the two then start to approach each other, pulled along a protein scaffolding, or
mitotic spindle, between the two.
When the membranes of the two pronuclei rupture, chromosomes go flying, and boom, you officially
have a diploid zygote, which almost immediately starts mitosis.
And that, folks, is how you fertilize an egg.
But even though our bodies are made to have lots of sex and babies, there are plenty of
people who are interested in enjoying the sex part, without enjoying the baby part.
And given all of the things that usually need to go right to achieve fertilization and implantation,
a little disruption at some stage along they way is usually all you need to remain baby-free.
So folks have been experimenting with various herbal, chemical, physical, and behavioral
methods of contraception for centuries -- for millennia.
Some of which have been pretty nutty, like drinking cocktails laced with lead, or wearing
weasel testicle amulets or rabbit butts, or shoving herbs in places where herbs should not go.
But, of course, contraception has come a long long way in recent years. In cultures that provide
access to it, folks have wide range of contraception options -- you know, in addition to abstinence.
First, if you’re really sure you don’t want kids, or more kids, sterilization is an option.
During a tubal ligation or vasectomy procedure, a doctor snips, blocks, burns, or ties up
the fallopian tubes or vas deferens, effectively ending the gamete express by permanently preventing
sperm and egg from ever getting to the penis or uterus.
But those who prefer a less permanent option also have lots of choices.
We’ve got barrier methods that work mostly by physically keeping sperm and egg on either
side of a wall, which is usually made of latex.
Condoms, of course, cover the penis and catch ejaculated sperm before it can go anywhere,
while diaphragms, sponges, cervical caps, and female condoms are inserted into the vagina
where they form a wall over the cervix, turning sperm away.
Hormonal methods of contraception -- like the pill, or any number of newer shots, patches,
and rings -- use synthetic estrogen and progestin to prevent the progression of the ovarian
cycle to ovulation, and sometimes make changes to the functional uterine layer, the endometrium,
and cervical mucus to help deter sperm.
An IUD, or intrauterine device, is a small T-shaped object a medical provider inserts
into the uterus to prevent zygotes from implanting.
And the implantation is key.
Because, even if a sperm and egg manage to complete fertilization, in order for gestation
to take place, the zygote has to find a home for itself in the uterus pretty quickly, so
it gets the nutrients it needs to continue developing.
So that’s what we’re gonna be talking about next time. It takes a long time to talk
about all the ways to make a baby -- but it’s important!
But for now you learned about the four phases of the human sexual response, how a sperm
finds and fertilizes an egg, creating a zygote, and how different types of contraception work
to prevent that from happening.
Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon
patrons whose monthly contributions help make Crash Course possible, not only for themselves,
but for everybody, everywhere. If you like Crash Course and you want to help us keep
making videos like this one, you can go to patreon.com/crashcourse.
This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written
by Kathleen Yale. The script was edited by Blake de Pastino. Our consultant is Dr. Brandon
Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer
is Michael Aranda, and the Graphics team is Thought Cafe.