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If cells and organs never talked to each other, an animal would fall apart faster than a boy band after the popular one goes solo. Thankfully, that’s where the nervous and endocrine systems come in! In this episode of Crash Course Biology, we’ll learn how they use neurons, hormones, and more to keep animals’ bodies coordinated.

Introduction: Internal communication in animals 00:00
Why communication matters 01:01
The nervous system 01:58
Fight or Flight 08:01
Review & Credits 09:07

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CC Kids:
Your heart is racing.

Your breathing is fast.  Are you stressing over a big test? Rooting for your favorite team?  Getting ready for the most  epic rap battle in history!?  As it turns out, our bodies have pretty similar  responses for both anxiety and excitement,   which is why you might feel tense  during any of those situations.  Those physical reactions are caused by our  bodies communicating within themselves.  And the more we learn about this internal  communication, the better we are at understanding   how we’ve survived and evolved  long enough to invent pop quizzes.  I mean, you’ve gotta be pretty secure as a  species to introduce that kind of stressor.  Hi!

I’m Dr. Sammy, your friendly neighborhood  entomologist, and this is Crash Course Biology.  Also, do you feel that tingle  on the back of your neck.  That’s your body communicating that  you’re ready for this funky theme music! [THEME MUSIC] We multicellular organisms  are a lot like boy bands.  In a boy band, you’ve got singers that need to  work together to make one super catchy song.  They can’t all be solo artists doing  whatever makes each of them look best.  If everyone starts singing in their  own key things are gonna get weird.  No one will be in sync, or going in One Direction. Hehe.  The same is true if you’re an  organism made of lots of cells.  To grow and develop, maintain an immune system,  and respond to the ever-changing world around you,   your cells and organs have to create some  beautiful harmonies, working together for   the good of the group – in this case, your body.

Otherwise, things will fall apart faster than   the career of a solo Jonas Brother. Vertebrates, or animals with backbones   like yours truly, have two main  systems for making this happen:   the nervous system and the endocrine system. You can learn more about the nitty-gritty of   how they work back in episode 25, but right  now we’re gonna take more of a bird’s eye view.

Let’s start with the nervous system,  which includes the brain, the spinal   cord, and the spidery collection of nerves  traveling from an animal’s head to its toes.  Or its tailfin. This system has   three main jobs: Collecting information about  the environment inside and outside the body.  Processing that information in the central  nervous system — aka the brain and spinal cord.  And responding to that information  by sending signals to various cells.  Consider this scenario: You walk into class,  and see “pop quiz” written on the board.  The sense of sight is one of the ways  your nervous system collects information.  That information goes from your eyes to your  brain, which processes those words and the fact   that you definitely didn’t do the reading. In response, your brain sends signals to   the rest of your body that it’s panic time,  and suddenly, your heart is going so fast,   that your smartwatch thinks you’re exercising.

In real time, though, we almost never   notice this happening. We read “pop quiz” and,   boom: It’s all panic, no disco. That’s because the signals in the   nervous system can travel fast — anywhere from  half a meter per second to 120 meters per second.  This is also why you can usually catch  your phone the second it starts to fall,   and why a fish doesn’t need to stare at a shark  for long before realizing they should bolt.  That said, the nervous system  is mostly for short-term   responses in the body, not long-term ones.

It’s great for a high-five or a thumbs up,   not so useful when we need to  regulate our hormonal cycles.  Additionally, not every cell  can talk to the nervous system.  This group text is limited to special cellular  messengers called neurons, and any cell that can   get signals from neurons, like muscle cells. And even then, the neurons are basically   whispering: so to actually hear the  message, a cell has to be right next door.  Let’s see this in action,  over in the Thought Bubble…  You’ve just stepped on a LEGO. My condolences.  The moment your foot landed on  that colorful torture device,   sensory receptors that branch from neurons  in your feet sounded the alarm: This hurts.  A lot.

It can seem instantaneous,   but for a merciful fraction of a second,  your brain doesn’t know about the LEGO.  Cue the slo-mo. The signal from your   sensory receptors has to first travel along  your neurons from your foot to your brain.  When the message reaches the end of one neuron,  it encounters a tiny gap called a synapse.  To keep the message going, neurons release  molecules called neurotransmitters,   which carry the message across  the gap and on to the next neuron.  And the next and the next, like a line of dominoes  falling over, until the signal hits your brain.  Once that last domino falls,  it’s goodbye bliss, hello pain.  Your brain uses that pain as data and tells your  body to move to safety by sending a signal to your   muscles, also known as a motor output. And as fast as you can, you escape   to a clear patch of floor.

Let’s just hope no one has left   a rogue banana peel behind. Thanks, Thought Bubble!  Our sensory receptors are always taking in  information about what we see, hear, taste,   and touch — plus information about things like  balance and what’s going on /inside/ our bodies,   from our temperature to our blood pressure. Some animals even have sensory receptors that   give them information about electric  and magnetic fields, like the internal   magnetic compass that helps monarch butterflies  navigate nearly 3,000 miles each year to Mexico!  Though we’re pretty sure humans don’t  have those…but, research is ongoing.  In any case, this information is all  used in more or less the same way.  The signals from the receptors  travel along neurons, jump synapses,   get processed by the central nervous  system, and then trigger a response.  This kind of communication helps animals survive  in some obvious ways, like letting us know if   we touch a hot surface or that danger is  around if we hear the roar of a predator.  And some less obvious ways, like figuring out what  foods are safe to eat based on how they taste.  So, sweet foods tell us something  is rich in energy-packed carbs.  Salty equals minerals.

Sour means acids, like in rotting food.  And many plant poisons are bitter. When the central nervous system processes   these sensations, our brains learn what’s  safe to eat and what’s probably bad for us.  While the nervous system is all about getting  things done fast, like tasting a flavor as   soon as it hits your tongue, we know that  it isn’t so good at playing the long game.  Thankfully, that’s where the  endocrine system comes in handy.  In humans, it includes glands and organs  throughout the body, from the hypothalamus   in the brain to the adrenal glands on top  of the kidneys, to reproductive organs.  It works more slowly than the nervous system,  but the results often stick around a lot longer.  This is because the endocrine system  doesn’t primarily rely on neurons:   it sends messages using hormones, instead. Hormones are basically just molecules   secreted into the blood, which carries them  to organs and tissues to make things happen.  You may have heard about estrogen or  testosterone, human reproductive hormones – but   those are just the tip of the hormonal iceberg.

Others make you sleepy, some respond to stress,   and still, others regulate how much glucose and  calcium are kickin’ around your bloodstream.  Like with the nervous system, not every cell  can communicate with every type of hormone.  But the ones that can interpret  the signal are called target cells.  It’s like how if you speak to me in Dothraki  instead of English, I just won’t get the message,   no matter how many times you repeat yourself. But unlike with the nervous system,   a target cell doesn’t have to be right next to the  gland releasing the hormone to get the message.  Hormones can hop on the bloodstream highway  and reach a target cell anywhere in the body.  This system takes a little longer  to ramp up, but once it does,   the changes tend to last longer, too. For instance: the hormone adrenaline   helps get your body ready for stressful  events by sending a Big Time Rush of   blood to up your heart rate and prep your  muscles for exertion – among other things.  As you’re being strapped in to a roller  coaster, it might take a minute for your   body to realize what’s about to happen and start  sending out adrenaline — and on the flip side,   even once you’re off the ride and toddling  toward the nearest bathroom, it’ll take your   heart a few minutes to come back down.

That’s hormonal communication at work! And you might be wondering, “Dr. Sammy,  How does my endocrine system know I’m   about to get on that roller coaster anyway?

There aren’t endocrine glands in my eyeballs.”  Well, this is where the endocrine  and nervous systems team up.  They work together to send both fast  and slow messages around the body.  For example, some hormones can act  like neurotransmitters, and vice-versa:   the nervous system has cells that can  make endocrine glands release hormones.  So, say a gazelle smells a lion on the wind. The signal hits the gazelle’s brain and its   central nervous system shoots a  message to the muscles to get out   of there. But a signal also goes to the gazelle’s adrenal glands in their endocrine system, giving them the cue   to release adrenaline and get their heart rate  and breathing up so they’re ready to escape.  Unfortunately, it also kicks in when you  have to take a test you haven’t studied for.  Your body thinks you’re in danger,  and fully opens the adrenaline taps.  You might even hear this referred to as  your body’s “fight or flight” response.  But, uh, don’t try and fight your  test, you got this, you got this.

The good news is, like we mentioned in the intro, this system also kicks in  when you’re super excited.  In fact, some psychologists have suggested that  one way to fight anxiety is to think about stuff   you’re nervous about as if it’s exciting instead. Our bodies’ internal communication systems,   from the nervous to the endocrine, have helped  us be aware of, and respond to, our environments,   keeping us from feeling like the new kids  on the block every time something changes.  And we can also learn to use  these systems to our advantage,   like understanding how to stop pop quizzes  from freaking us out… even if only a little.  In our next episode, we’re going  to find out where babies come from.  Yup, we’ll be talking reproduction. I’ll see ya then.  Peace!

This series was produced in   collaboration with HHMI BioInteractive. If you’re an educator, visit for classroom  resources and professional development related   to the topics covered in this course. Thanks for watching this episode of   Crash Course Biology which was filmed at our  studio in Indianapolis, Indiana, and was made   with the help of all these nice people.

If you want to help keep Crash Course   free for everyone, forever, you  can join our community on Patreon. ---