Sympathetic Nervous System – Stress (PSY)

00:00 Now, a stress is presented itself, how are you actually dealing with that in terms of how's your body dealing with it.

00:10 So physiologically speaking, we are going to talk about what happens to your body.

00:13 So there's two systems that get activated in parallel.

00:17 I mean they both being activated at the same time.

00:20 Both have different roles and both do it at a different pace.

00:23 Sympathetic nervous system which we had talked about previously.

00:26 And the HPA-axis, we're going to describe what that actually means in the next coming slides.

00:31 So the sympathetic nervous system initiates or moderates something called the fight or flight response.

00:37 Again something we mentioned before and you'll continue to see.

00:40 And this is when your body is presented with an acute stressor and or something that's in front of you right now, a lion attacking.

00:46 As you can see here we have a lion attacking.

00:48 And you're forced to make a decision.

00:53 Am I going to put on my gladiator helmet, get oiled up and fight this tiger.

00:58 Or am I going to turn around and run away so that the tiger does not devour me.

01:03 So you are going to fight or you are going to run away.

01:06 So this is something that's mediated about this sympathetic nervous system.

01:09 And it's usually quite fast.

01:10 Very fast-acting.

01:11 You need to think of a situation that may be happened to you.

01:14 It's not something you mull over the next few minutes.

01:19 So what's happening here is your body is responding to this.

01:23 And the sympathetic nervous system is actually releasing something called a "Stress Hormone." And this comes from the adrenal glands.

01:29 And the adrenal glands actually release something called Adrenaline or Noadrenaline.

01:33 Something that's known as epinephrine norepinephrine.

01:36 And these will mitigate and initiate that response.

01:39 And you've heard that before saying, I had a rush of adrenaline.

01:42 And I was able to these wonderful tasks.

01:45 I was able to beat up a tiger.

01:46 I was able to pick up a car.

01:47 All these wonderful things and that is initiated by this exact response of flight or fight and it's mediated by adrenaline.

01:54 Now the sympathetic nervous system will do things that's gets you ready and ready to fight or flight.

02:02 And that is things like increase heart rate.

02:04 So you see physiological changes.

02:06 Increase heart rate, increase in breathing, changes in blood flow, slow in digestion.

02:11 Because it's going to reallocate those resources to deal with the impending fight or flight.

02:18 And it dulls your pain.

02:19 So you're probably wondering why it's going to dull my pain.

02:22 So if you are about to fight a tiger, you don't want high sense of pain, you want dull to the pain.

02:26 In case that tiger does gives you a couple of shots to your face, you'll feel it, you can keep fighting, okay.

02:31 Now, let's take a look at the other pathway that gets initiated.

02:37 And this is the HPA-axis, or the Hypothalamic Pituitary Adrenal axis.

02:41 It's a manifold.

02:42 And this is a slower process.

02:44 So we're going to walk through some of the main components and then I'm going to subsequently show you something called "Negative Feedback." And how all these pieces tie together.

02:52 So there's a structure that we've mentioned before and we'll come up again called "Hypothalamus." And this is a structure deep in your brain and releases something called "Corticotropin Release Hormone or CRH." CRH then goes on once it gets released to stimulate the pituitary gland to release something called adrenocorticotropic hormone or ACTH.

03:13 ACTH then goes on to signal the adrenal glands to release something called cortisol into the blood stream.

03:19 It's lots of stuff to digest there and put your head around.

03:22 But we're going to break it down into couple of components.

03:24 And so what's happening here is this is a bio-chemical cascade or sequence of things that's happening.

03:29 So think of it as a domino effect.

03:31 So quickly again hypothalamus releases CRH.

03:35 CRH stimulates release of ACTH.

03:37 And ACTH goes on to release cortisol, okay.

03:40 So we'll walk through that in just a moment.

03:43 Now what is cortisol.

03:45 Cortisol, let's look at the definition here, is a glucocorticoid, a hormone that shifts the body from sugar to fat as an energy source.

03:52 So why do we care, why is that important.

03:53 So cortisol is doing something very, very important in that is, it's telling your body, "I'm going to now use my fat which is an energy store which normally you would not use." So let me back track a bit.

04:08 You're sitting here right now watching this video and you have your desk and you got a big bowl of chips besides you.

04:14 You are eating those chips.

04:16 Now your body is actually not using that as energy.

04:18 It's taking those chips and it's converting it into fat.

04:22 Because you have enough energy, it says, "I'm going to use this energy and turn it into fat in case I need it at some other point." Now evolutionary speaking, if we backtrack even further in cave man days, food was a scarce resource.

04:35 They were not sure where actually it was coming from.

04:38 And when they would find food, they would gobble it up and that would get stored as fat.

04:42 And they would use it as it didn't know when it would get it's next meal.

04:46 Fast forward it today, it's the first time in human civilization we have an over abundance of food.

04:50 So it's never really a common situation where you have zero food.

04:55 I'm generalizing, obviously some of we don't.

04:57 But generally speaking, we have unlimited access to food.

05:00 And so we don't really need this fat which is why all have an extra piece of love that we all carry around with us.

05:05 But in this situation when the HPA-axis is activated, it's going to say, "I'm going to go use my fat stores as energy, and I'm not going to use the sugar which I'm normally eating." So food now it's converted into sugar.

05:18 So it's not going to use that.

05:20 Now what that does, is it keeps our blood sugar levels high.

05:24 And the time we actually need it in case we our having to do something like fight.

05:29 Or for a situation of over time we're going to need this energy.

05:33 It's making that energy available and it's using our stores instead right now so that we have access to that high blood sugar when we need it.

05:42 Now, is cortisol all good? Well cortisol is actually quite useful in this situation I have mentioned.

05:49 But long term cortisol is not a good thing.

05:51 So we know that prolonged release of cortisol due to chronic stress can have harmful effects.

05:56 Now in each of these situations that I'm mentioning to you, I'm talking to you about fairly acute stress.

06:01 So a tiger attacking you or your wife yelling at you.

06:05 Small periods of time, if it's alright, may be sometimes it's a little bit longer, but essentially is short lived phenomenon.

06:11 As opposed to, you are that doctor in the ER and you have this prolong stress.

06:15 You are a lawyer, or you are in a very stressful job of all the time you are stress, stress, stress, stress, levels of cortisol will go up because the HPA-axis is being activated.

06:25 And they are not turning off which is not a good thing.

06:27 And so it ends up having long term half effects.

06:31 One of those is it actually inhibits the activity of white blood cells.

06:34 And therefore, also impacts the immune system.

06:37 So once you're impacting white blood cells and you're ready to wore off disease and other things, that's not good.

06:44 It makes you prone to susceptible to getting sick.

06:47 So you do not want prolonged exposure to stress where it causes increase levels of cortisol because it leads to vulnerability to illness.

06:55 Okay, so now let's take a look at this figure where we can make sense of all these crazy parts I just mentioned.

07:03 So like I said, the hypothalamus is going to release corticotropin releasing hormone or CRH, that is done.

07:10 And now we are going to activate the anterior pituitary to release ACTH, okay.

07:15 Now that goes on to activate the adrenal cortex that will go on to release cortisol.

07:19 Now this is a unique situation here and a premise that you will need to understand for the MCAT and that's something called Negative feedback.

07:27 Now everything going from hypothalamus to the adrenal cortex, this way we're talking about a positive feedback.

07:35 One initiates the next, initiates the next.

07:38 Now we need to somehow turn this off.

07:41 The body needs to say, "Hey, I just release the CRH and I activated ACTH." Is that enough or do I need more? ACTH is now initiated to release cortisol, is that enough? Or should I stop? Now what it does is, there is receptors in the body and brain that actually detect levels of ACTH and detect levels of cortisol.

08:00 And when enough cortisol is release.

08:03 The levels in the blood are high enough, it will actually be detected by the hypothalamus.

08:07 And say "Hey, hypothalamus, you should stop releasing CRH because we have enough cortisol." And it will also activate anterior pituitary to not release anymore ACTH.

08:18 So it's the off sort saying, "We have enough, hold off." And that negative feedback will stop that cascade.

08:25 And they'll put the system on pause.

08:28 Until it gets reactivated again, positive feedback continues and then the negative feedback comes back to turn the system off again.

08:35 So we call that whole process negative feedback.

08:37 And this will apply to more than more systems than just HPA-axis.

08:41 This is why I was saying you should probably note this.