Sympathy. And…parasympathy?

Ever wonder how our body knows how to do things and when to do them? There’s an incredibly complex signaling system in our bodies, working ALL THE TIME, without us having to do anything about it. Isn’t that cool?


Before you answer, I should inform you. It’s cool. It’s really cool.

If you say so.

I do.

Today I want to talk about the autonomic nervous system. “Autonomic” can be broken down into the roots auto = self, and nomos = arrangement or law. So, it’s the branch of the nervous system that is responsible for self-regulation. And when we’re talking about self-regulation, we’re talking about regulation of all kinds of bodily functions: from how quickly you breathe, to how much you salivate, to how big the pupils of your eyes are.

The autonomic nervous system is divided into two opposing forces: the sympathetic influence, and the parasympathetic influence. Both forces are acting upon every bodily system at all times. It’s like playing tug-of-war between two evenly-matched sides. There’s a balance point between the two opposing forces, and a dynamic system that keeps the balance point in place.

You can see an example of this kind of dynamic equilibrium in the first 25 seconds of this video:

So, what do the sympathetic and parasympathetic systems actually do?

I’ll break it down for you.

The sympathetic system is the “fight or flight” system. I like to remember it as, “everything you want to be doing while you’re running away from a lion.”

(well….hopefully more successfully…..)

So, what would be helpful when you’re running away from a lion?

You want your muscles to work really, really well. So you’re going to make sure they get a really good oxygen supply. You’ll dilate your muscular blood vessels, and your heart will beat faster and stronger to make sure enough oxygen is getting to your muscle cells. You’ll also breathe harder and deeper, making sure you have a lot of oxygen in your blood for maximum delivery.

What else? Well, you’ll want to see really well while you’re running away. So your pupils will dilate to a bigger size to let more light in.

And you’ll also want to inhibit a couple of parasympathetic functions, which… well, you’ll see why in a minute.

The parasympathetic system is responsible for the “rest and digest” functions.

Basically, it’s “everything that your body needs to do…unless you’re running away from a lion!”

The parasympathetic system lowers your heart rate and blood pressure. It slows down your breathing. It constricts your pupils and narrows your blood vessels to direct blood flow from your muscles to other important organ systems. It lets your eyes make tears.

The parasympathetic system increases salivation, which helps in the digestion process. It also increases the movement of the digestive tract, allowing you to digest food and excrete waste.

(Helpful tip: Avoid excreting waste while running away from a lion.)

Usually, these systems find an equilibrium point and stick with it, dynamically pulling against each other to maintain it. And they also react to stimuli, changing the equilibrium point as necessary to adapt to daily needs.

Let’s talk about heart rate as an example. A normal heart will contract about 60-100 times per minute. This represents the physiological balance between the sympathetic and parasympathetic influences. At rest, (such as when you’re sleeping), the influence of the parasympathetic function will increase, and your heart rate will slow. When you’re exercising, the influence of the sympathetic function will increase, and your heart rate will speed up.

Another example: your eyes aren’t completely dilated or completely constricted at any one time; they usually hang out around a particular size (that changes slightly according to age). They use the sympathetic and parasympathetic systems to adapt the diameter of the pupil to the amount of light the eye is receiving.

Then, when the particular condition that’s pulling the system in one direction (say, toward the sympathetic side of things) disappears, the other system (the parasympathetic side, in this case) will exert enough influence to bring the systems back into their favorite physiological set point.

How is that not cool?

Your operational definitions are problematic.

It’s still not cool enough? Fine. I hear sex sells.

Fun with autonomics!

The sympathetic and parasympathetic systems are both involved in sexual activity. The parasympathetic system is responsible for arousal, and the sympathetic system takes care of things such as male ejaculation. The mnemonic to remember which does which? “Point and Shoot”. See, doctors are funny!

“Funny”… “Cool”… “Fun”…. You see, this is what I’m talking about. I’m getting you a dictionary for your birthday.

::Sigh:: Another one?

The other ones clearly haven’t helped.

Okay, fine. It’s not cool and doctors aren’t funny. But I still think it’s pretty awesome. So there.

Oh, Dr. G. We really need to get you a life.

It’s going to have to wait till after residency.

Published in: on July 20, 2010 at 1:48 am  Comments (22)  
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22 CommentsLeave a comment

  1. Dr. Grasshopper, that *is* cool!

    Could you tell us some ways in which the sympathetic and parasympathetic systems operate abnormally? From what you’ve said about the parasympathetic system, does it have some influence when the body goes into shock?

    (Yes, this is cool, and yes, I’m a geek, and yes, this will most likely help me with my writing!)

    Thanks for your posts!

    • Sure, BJ!

      My favorite example of autonomics-gone-awry is what happens with vasovagal syncope (Vaso = blood vessels, vagal refers to the parasympathetic influence that the vagus nerve has on the heart, syncope = fainting).

      Vasovagal syncope one reason that someone might faint. Physiologically, the sympathetic influence is overwhelmed by the parasympathetic influence, causing your heart rate to plummet and all of your blood vessels to relax open, letting all of your blood fall with gravity. If you’re upright, that means your brain doesn’t get any blood, which means that you quickly lose consciousness.

      Vasovagal syncope can be induced by pain (which happened to me once, and I’ll probably tell that story later for illustrative purposes) or emotional shock. This is why people faint when they see blood, or get stuck with needles, or get scared witless.

      But (and this is my favorite/least favorite part), it can even happen because the autonomic nervous system gets bored! Medical students faint all the time in the operating room, since their job is often to stand for HOURS holding retractors without moving.

      My little brother has a very strong vasovagal reaction. We were both in a children’s choir for years when we were younger, and he would often faint from standing still for hours, singing under bright lights. Recently, he came to me because he passed out while urinating, which is also a very, very common vasovagal event. (Remember: urination is parasympathetic!)

      So, yeah! The autonomic nervous system can definitely go screwy sometimes!

      Shock is actually an interesting, complex example….I might do a post on it in the near future. But for now, I’m going to answer another question or two, and then go to bed. Hope that’s okay. 😀

      Glad you’re enjoying this as much as I do!

  2. (Helpful tip: Avoid excreting waste while running away from a lion.)

    I attended a psychology lecture about that a while ago… The lecturer said that excreting when scared was the body’s way of reducing weight to make flight more effective. Unfortunately, instinct hadn’t kept up with fashion, and underwear rendered that particular evolutionary trick ineffective.

    On the sexual arousal thing… This means it’s possible for men–and women?–to climax without being aroused? Yeah, sex sells 🙂

    Also, this *is* cool and bold-type-guy is boring and wrong.

    • No kidding! I hadn’t heard that part. Pretty nifty!

      And to be honest, I have no idea how to answer your question. And I don’t know any sex physiologists that I could ask.

      Glad you’re enjoying the posts!

      Science is fun!

  3. And is there any way to artifically stimulate/control either system? (Not related to my above comment about sex, just so you know.)

    • Absolutely! As a matter of fact, an overwhelming part of my job is actually manipulating these two systems against each other to preserve function.

      The way you do this? Well, there are some biological manipulations you can do. But mainly, we use drugs.

      I’m going to beg off going through different drugs and their effects on the autonomic nervous system for now, since it was a really, REALLY big chunk of my pharmacology class, which was like 1/3 of the Step 1 exam of the medical boards. But if you’d like to go here, there are a couple of examples of drugs types and their actions. It’s not very reader-friendly, though. If I have more energy tomorrow, and if I can figure out a good way to present it, I’ll try writing a post on the different types of substances and how they affect the autonomic nervous system.

  4. Good question, Dylan Fox! I can imagine any number of scenarios when someone may want to either make someone’s heart pump harder, or put them to sleep…

  5. So does that mean that when I get told that I should create sympathetic characters, they mean that I should have them chased by lions? OK, I can do that.

    On a slightly more serious note, I’ve noticed that as I’ve gotten into better shape, my resting heart rate has gone down. I assumed that was because my heart and related systems are working more efficiently, but does that also mean that the equilibrium point has shifted as well?

    • I have a lot of sympathy for people who have to run away from lions. 😀

      Well, your equilibrium point is determined by your general state of health. Remember that it’s a balance between the two systems, which basically tug against each other until they find their favorite set point. If your heart isn’t perfusing your organs well enough (for example, if you were a bit out of shape), the sympathetic system will kick in to raise blood pressure and cardiac output in general, making sure everything gets enough oxygen. The set point for heart rate will increase, since the influence of the sympathetic system is needed to properly perfuse the organs.

      You’re right in that exercising makes your heart and related systems work more efficiently. (It has a lot of wonderful effects on your body.) This means (among other things) that your heart doesn’t have to work as hard, and so the sympathetic system doesn’t put quite as much influence into the heart rate. As a matter of fact, the heart rate of well-trained athletes is heavily influenced by the parasympathetic system, giving them far lower heart rates than the general population.

      Exercise is wonderful for you. Way to go on getting in shape!

  6. […] This post was mentioned on Twitter by BJ Muntain, John Murphy. John Murphy said: Doc Grasshopper discusses sympathy and parasympathy: […]

  7. […] round! Which branch of the autonomic nervous system predominated in every single person in that story? Especially the teen? Picture from: […]

  8. Dear Reader,

    This message is to notify you that your comment has been removed from this site. Reasons for moderation may include impropriety, inappropriateness of content for the forum (including the giving or solicitation of medical advice and/or services), or concerns raised by other readers. This does not bar you from posting further messages, but please consider the other readers and the nature of the forum when you post.

    Thank you for reading, and have a great day.

    Dr. Grasshopper

  9. […] There’s some evidence that a significant increase in the levels of circulating catecholamines (the “fight-or-flight” hormones released by the sympathetic nervous system) may contribute to the development of the syndrome through any one of a number of proposed […]

  10. […] what happens. The sympathetic and parasympathetic influences are in their tug-of-war balance, keeping heart rate, blood pressure, breathing rate, etc. all […]

  11. Dear Reader,

    Sorry to have to do this, but I’ve deleted your post because it either gave or asked for medical advice. Since you likely showed up here from the interwebzes at large, you probably missed this post: which should explain why this comment was inappropriate for this forum. I wish you the best of luck, and I’m sorry again for deleting your post. Hope you’re having a great day otherwise!

    Dr. Grasshopper

  12. […] Sympathy. And…parasympathy? « How To Kill Your Imaginary FriendsJul 20, 2010 … Vasovagal syncope one reason that someone might faint. … I have a lot of sympathy for people who have to run away from lions. . Well, your … […]

  13. Pathetic that yanks can’t use words other than cool, awesome, and such like rubbish, and finishing off everything with “hope you have a good day” – nauseating ! You people destroy the English language !

    • We do indeed, having risen up and conquered the mewling and feckless old guard time and again, and it is OUR language now, thou random curmudgeon, ours to do with as we please, and destroy as we please, while you, toothless and nonpuissant, only can watch. Witness our awesome power over your language, surely, but stay: let it cool your ardor to remember that again the reins must be passed, lately to those who will make us in our turn grit our teeth and bemoan its fate.

      Have a nice day.

  14. One way I remember the difference between the sympathetic (fight or flight) and parasympathetic (rest and digest) is by a pneumonic I made up for myself. SFF – Special Forces Fight and PRD – Public Relations Department. Special Forces Fight is pretty self explanatory, but the Public Relations Department I kind of think of them as it’s their job to keep things calm.

    • Cute! I always remembered it this way: I’d feel very sorry….a lot of sympathy…..for anyone who had to run away from a lion.


  15. This is very good. Its clear and concise.

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