04.04 Autonomic Nervous System (ANS)

In this lesson we’re going to talk in a little bit more detail about the autonomic nervous system.

Remember from the neuro anatomy lesson that the nervous system is divided into Central and Peripheral Nervous Systems. Then the Peripheral Nervous System is divided into Somatic and Autonomic. Remember we said Somatic is our voluntary actions and Autonomic is our Automatic or involuntary actions in the body - so the things we don’t have any control over. The autonomic nervous system is then split again into the Sympathetic Nervous System and the Parasympathetic Nervous System. Sympathetic is our Fight or Flight response, and Parasympathetic is our Rest and Digest response. So - let’s dive in a little deeper to each of these two systems and then understand a bit better how they work together.

So - the Sympathetic Nervous System is our Fight or Flight response. It’s what’s going to help us be ready to fight off a bear or run away screaming! So some of our more vital functions are going to be heightened - our heart rate and blood pressure are going to increase so that our cardiac output increases. We have the make sure we’re getting good blood flow to our brain and our lungs and our muscles if we’re going to fight off a bear, right? Our air passages are going to dilate and we’re going to breathe faster so we can get oxygen where it needs to go! The pupils in our eyes are going to dilate or open wider so we can see better and take in our surroundings more clearly. And our metabolic rate is going to increase - our temperature is going to go up and our body is going to start sweating if it needs to to make sure we don’t overheat. So all of these things together are going to make it easier for us to handle fight or flight responses, right? But - what do we NOT need to do while we’re trying to fight off a bear? Well - even though you may be scared enough to pee your pants, your body is going to SLOW down your urinary and excretory systems so that you DON’T - it’s just not important enough right now! Our reproductive systems will be slowed down as well - no need for uterine contractions right now. Digestion is going to be slowed - the body doesn’t need to be focusing its energy on digesting food - it needs to keep you alive, right!? So we see decreased peristalsis - which is that smooth muscle contraction that moves food along the intestines. And finally, the LAST thing we need when we’re trying to fight off a bear is a mouth full of saliva, right? So we will see decreased output from our salivary glands.

So let’s review - what happens with the stomach and digestive system - it’s decreased. Less peristalsis, everything slows down. What about our kidneys and our urinary and reproductive systems - slowed down, too, right? And, of course - no saliva! We’re trying to fight off a bear, not eat it! So our pupils are going to dilate, our lungs are going to open up and breathe faster, our heart is going to beat faster - we also see some vasoconstriction to help increase the return of blood to the heart and lungs.

Now - this metaphor of fighting off a bear is just a way to remember what the sympathetic nervous system does - it doesn’t mean we ONLY see it when we’re actually fighting or fleeing. As we’ll see in a minute, signals from the sympathetic nervous system are always coming! So - we need a way to counterbalance it, right?

That’s where the Parasympathetic Nervous System comes in! This is our “Rest and Digest” system. So - we beat the bear! We’re all good - we’re out of danger. Our body can relax and resume some of its normal functions again. Like… going to the bathroom. Have you ever been through a haunted house and you’re absolutely terrified and freaking out and then you finally get out to the other side and what happens? You have to pee, right!? That’s your Parasympathetic Nervous System trying to bring you back to normal! This is also why we actually see a lot of women go into labor after a traumatic experience - their parasympathetic nervous system kicks in a little too hard to bring them back down and they start experiencing uterine contractions. And, of course, now that we’re resting and digesting - we can have increased peristalsis and increased salivation again. Again, the goal of the Parasympathetic Nervous System is to bring you down from that fight or flight high and let you rest and digest - so we’re going to see the heart rate and blood pressure come back down, because we don’t need as much cardiac output anymore. The respiratory rate comes back down and we actually see some constriction of the airways. Our pupils will constrict back down and our metabolic rate and temperature come back down as well.

So let’s recap - the digestive system starts back up again and we see increased peristalsis. The kidneys, urinary, and reproductive system kick back on - so we may see some urination or uterine contractions. And we can start salivating again! We don’t need that extra sharp vision, so our pupils constrict, and our heart rate, blood pressure, and respirations come back down to normal. Again - the whole goal here is to bring you down off that fight or flight high, right?

So that brings me to my next point - which is that the #1 goal of the autonomic nervous system is to maintain homeostasis. When one system brings you up, the other brings you down, and vice versa. These systems are working all the time to help keep you right at that sweet spot of homeostasis. Now - one big point to know is that certain organs or tissues could have single innervation or dual innervation. Single innervation means it only gets signals from ONE of these systems - usually the sympathetic. So you’d get the sympathetic ramping it up, and then you just remove the sympathetic stimulus and that’s what allows it to come back down - passively. OR you could have dual innervation which means both systems are involved. So the sympathetic ramps it up and the parasympathetic brings it back down actively. In this case we see what is known as antagonism and dominance.

So let’s look at this in terms of something like heart rate. We know that the sympathetic signals are going to speed up the heart rate and the parasympathetic signals are going to slow it down. Well, if you have equal signal or input from both systems, then the heart rate just kind of chills right where it’s at - that’s called Antagonism. They’re equally antagonizing, or opposing each other - so we don’t see any change - again, maintaining homeostasis. But, if we see stronger sympathetic than parasympathetic signals, then we have what’s called sympathetic dominance. So basically the sympathetic nervous system has taken over - so we end up with an overall increase in the heart rate. If it’s the other way around and we see stronger parasympathetic signals than sympathetic ones, then we have parasympathetic dominance and we’ll see an overall decrease in the heart rate. So the body is constantly going through these ebbs and flows of antagonism and dominance in order maintain normal homeostasis.

Now, super quick I just want to touch on neurotransmitters. Remember these are the chemicals that work to help send signals throughout the nervous system. First - remember this is part of the peripheral nervous system, so these nerves all come off of the spinal cord. What happens is one neuron comes off the spinal cord and comes into what’s called a ganglion - which is just a group of neuron cell bodies all in one place. Then it synapses with another neuron and goes out to the target cell or organ. SO - we call this nerve before the ganglion the preganglionic neuron and this one after the postganglionic neuron. SO - our two main neurotransmitters in the autonomic nervous system are going to be norepinephrine and acetylcholine. Norepinephrine is also called noradrenaline - so think adrenaline, fight or flight, sympathetic. So the majority of our postganglion sympathetic neurons use norepinephrine to send their signals out. The receptors for norepinephrine, or noradrenaline, are called ‘adrenergic’ receptors. We have alpha, beta 1, and beta 2 adrenergic receptors. Alpha is in our blood vessels, beta 1 is in the heart, and beta 2 is in the lungs - I remember this because you have one heart and two lungs. And remember - with sympathetic dominance we see that vasoconstriction, heart rate up, respirations up, etc. - so this is how they make that happen.

Now, acetylcholine is found in ALL preganglionic neurons - both sympathetic and parasympathetic. So essentially all of the neurons coming off the spinal cord are using acetylcholine. These go to what are called cholinergic receptors - choline, cholinergic. There are two types - nicotinic, which is found on these postganglionic neurons, and muscarinic, which is found on the target cells. So this is how I remember this - we know acetylcholine is coming down here and it’s also going over here in the parasympathetic system, right? So the receptor that’s on the neuron is nicotinic (see the N), and the receptor on the target cell - which could be a muscle - is muscarinic.

So these neurotransmitters are the major players in our autonomic nervous system. As you learn more about various disease processes and medications, you’re going to see how powerful of an effect it can have when either of these two things are altered.
Okay - I know that was a bunch of information, but let’s recap quickly. There are two divisions of the autonomic nervous system - sympathetic and parasympathetic. Sympathetic is fight or flight and parasympathetic is rest and digest. So the two of these are constantly antagonizing each other and working to maintain homeostasis. But if one overpowers another, it’s called dominance. The two main neurotransmitters we see in the autonomic nervous system are norepinephrine or noradrenaline in the sympathetic nervous system, which binds to adrenergic receptors. And acetylcholine, which binds to cholinergic receptors for the parasympathetic nervous system.
So that’s it for our lesson on the autonomic nervous system. Make sure you check out all of the resources attached to this lesson. Now, go out and be your best self today. And, as always, happy nursing!