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Fight or Flight Response: Definition, Physiology & Examples

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  • 1:40 Autonomic Nervous System
  • 2:58 HPA Axis
  • 3:45 Adrenaline
  • 5:20 Animal Fight or Flight…
  • 6:14 Human Fight or Flight…
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Lesson Transcript
Instructor: Ellie Green

Ellie holds a B.A. with Honors in English from Stanford University. She is pursuing a Ph.D. in English Literature at Princeton University.

A squirrel caught in headlights or the cock that fights to the death? Do you know what makes us choose between freezing, running or fighting back? And what's happening inside of you all the while? This lesson answers all of your fight-or-flight questions.

You might have heard of the fight or flight response. That's something that has gotten into our vocabulary a little bit. And it's basically something that's super observable in animals so that's the best way to start, I think. A way to think about it is, you know when people tell you that a certain animal is more afraid of you that you are of it? You might encounter a scary raccoon or something like that - 'No, it's more afraid of you than you are of it.' Well, it usually is and the reason why this is a good thing for you is that, when threatened, a lot of animals will just run away. And that's usually true if the animal can get away, so if it's not cornered. Sometimes though, this sort of a threat, like a person standing in front of it, will actually provoke a fight response instead. The reason this is true is because both of these responses - running away and fighting - are basically the result of what's called an aroused sympathetic nervous system. And when this happens, when your sympathetic nervous system is aroused, you basically get pepped up to do one of two things: you either are going to fight, or you're going to flee. You're going to run away.

And to understand this a little better, we have to go over what the sympathetic nervous system is. In brief, it's a part of the autonomic nervous system and this is basically the part of your brain's communication with your body that isn't conscious. It's not like when you decide, 'I'm going to move my arm. Look, it moved.' That's not your autonomic nervous system. Autonomic nervous system is things that you don't even know are going on, like your breathing and digestion. And this autonomic part is, again, divided into two parts within autonomic. We got parasympathetic and sympathetic.

Your parasympathetic is dealing with things in a resting state. Things like digesting your food, salivating, if you're going to tear productions, you keep your eyes all moist and happy. Your sympathetic nervous system is what gets going when you're ready for action, and that's why it's important for fight or flight because what it does is it readies your muscles to go. And if think about it, fighting and running away are basically requiring the same muscle preparation. You might crouch down or tense up, and your sympathetic nervous system is sending out all sorts of signals to help you do this.

It's mediated by certain parts of your body known as the HPA axis. It's called an axis because it's three things that work all together, so you can think of it as a little system. One of them is called the hypothalamus and that's a part of your brain that you might have heard about in a lesson about the brain. The pituitary gland, this is also in the brain. And also, your adrenal glands - these are not in the brain, these are near your kidneys, it's kind of random. We can just remember, you got two in the brain - hypothalamus, pituitary - and one random guy just hanging out down by your kidneys. That's your HPA axis.

And what this does is it sends out hormones like adrenaline - that probably sounds familiar, like adrenaline - to get you going. So when you're in a situation where you need to fight or flee, your HPA axis is sending out these adrenaline hormones because your sympathetic nervous system is getting going. And people say this, when you're about to do something intense in sports, that is what's happening when you're trying to get all excited to do something important in a sports game.

And your body has a lot of interesting reactions to this besides just getting ready to fight or flee. You may have noticed some of these as well. What it does is it gets your heart racing, that's probably pretty obvious. You've probably felt that. It actually stops digestion. This is something kind of interesting because basically, what your body is doing when it gets into this aroused sympathetic state, is it's just saying, 'You know what, if there's anything I don't need to be doing right now, I'm just going to stop.' So anything that your parasympathetic was taking care of that was maintenance, long-term things, sympathetic says, 'No, we're going to stop digesting.' It even cuts off your peripheral vision because you don't need to see what's going on the side, you just need to see what's right in front of you. It basically shuts down your body to do anything but focus on the problem at hand and whether you're going to run or you're going to stand there and fight it.

Animals do some pretty crazy things when their fight or flight responses are activated. Octopuses and squids, they shoot ink - there's that Finding Nemo scene: 'You guys made me ink!' - that's basically what's going on there; their sympathetic nervous systems are active and camouflage is sort of a manifestation of the fight or flight response. It's saying, 'How can I get away?' basically, and hiding is one of those things. Also, puffer fish, when they get all big - they're so cute when they're small and they get all big - that's, again, that's the fight or flight response in action because it's trying to help you evade capture by looking scary. The animal world gets pretty crazy with this stuff.

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