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.
