Listen to this article about how a strong understanding of the autonomic nervous system ties into understanding nursing pharmacology straight from here, or look for episode #500 of the Straight A Nursing podcast on your favorite podcast player.

If you’ve ever sat down to study pharmacology and felt like you were trying to memorize an encyclopedia, I get it. You’ve always studied lists of facts by memorizing them and you’re good at it. But what if there was another way…

Recalling pharmacology facts at test time will feel a lot easier if you shift your focus to understanding foundational concepts really well.  In fact, foundational concepts are the key to understanding a lot of nursing school, not just pharmacology. 

You have probably heard the phrases “fight or flight” and “rest and digest” so many times, but have you ever thought about what they really mean? Understanding the differences between the sympathetic and parasympathetic nervous systems gives you a strong foundation to build on in your nursing pharmacology class.  

For example, when you truly understand how the sympathetic and parasympathetic nervous system works, you can lean on that knowledge as you encounter new medications to predict side effects and think through clinical scenarios without scrambling to recall everything you’ve memorized about each medication.

This is what I mean when I talk about having a strong foundation. With a solid grasp of foundational concepts, you’ll be able to connect new topics to something you already know, instead of feeling like you’re learning a brand new topic every single time. If you’re curious about why this is beneficial, then you should listen to episode #485 on my podcast, it’s all about Cognitive Load.

By the end of this article, you are going to understand what the sympathetic and parasympathetic nervous systems actually do, why they matter at the bedside, and how understanding them will make a lot of nursing school (especially pharmacology) feel way less overwhelming.  

🧠 When you know a core concept like this inside and out, everything downstream gets easier. 

What Is the Autonomic Nervous System?

The autonomic nervous system is the part of your nervous system that runs on autopilot. Think: autonomic, autopilot. It controls all the things your body does without conscious thought — heart rate, blood pressure, respiratory rate, digestion, pupil size, bladder function. All of it is humming along in the background 24/7 without you having to make a single decision about it.

The ANS has two divisions: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). You’ve probably heard these described as opposites — one on, one off — but that’s a significant oversimplification that can actually get you into trouble clinically.

Here’s the more accurate picture: both systems are active all the time. What changes is the balance between them, depending on what the body needs in any given moment. Think of it less like an on/off switch and more like a steering wheel. It can turn one way or the other — but it’s always in motion, always negotiating.

Most organs receive input from both systems, and whichever one is dominant at any moment determines what that organ does. Your resting heart rate, for example, is literally the result of these two systems working out a deal with each other in real time.

The Sympathetic Nervous System: Fight or Flight

The sympathetic nervous system is also called the fight-or-flight system, and while that phrase is familiar, what triggers it may be broader than you think. Yes, it fires in response to obvious threats and danger — but it also activates during exercise, illness, pain, significant blood loss, and anything else the body interprets as a stressor, including anxiety. Your body cannot always tell the difference between a real physical threat and psychological stress, and it responds to both in essentially the same way.

The Chemical Messengers: Catecholamines

When the sympathetic system activates, it releases chemical messengers called catecholamines — primarily norepinephrine and epinephrine. Norepinephrine is released directly at nerve endings in target organs. Epinephrine is released from the adrenal glands into the bloodstream for more widespread, systemic effects. Both of these bind to a family of receptors called adrenergic receptors, and the specific receptor they bind to determines exactly what happens in that tissue.

The Three Receptors You Need to Know

Alpha-1 receptors are found primarily in blood vessels. When stimulated, they cause vasoconstriction — blood vessels narrow, resistance increases, and blood pressure goes up. This is one of the most clinically significant things the sympathetic nervous system does. Think of the medication norepinephrine, one of the first vasoactive drugs used in septic shock: it works by stimulating alpha-1 receptors to cause vasoconstriction and support blood pressure.

Beta-1 receptors are found primarily in the heart. A simple memory trick: I have one heart — beta-1. When beta-1 receptors are stimulated, heart rate increases and force of contraction increases. More beats per minute, more force per beat — cardiac output goes up. That makes perfect sense if you think about what the body needs during a threat. It needs more blood pumping to the muscles and vital organs, fast. 

Beta-2 receptors are found primarily in the smooth muscle of the airways. Another memory trick: I have two lungs — beta-2. When these receptors are stimulated, the bronchioles dilate and the airways open up, allowing more oxygen in. This is the exact mechanism behind albuterol, one of the most commonly used medications in nursing. Albuterol is a beta-2 agonist — it mimics the sympathetic effect on the airways to reverse bronchospasm in a patient having an asthma exacerbation. (Note: beta-2 receptors are primarily in the lungs, but there are some in the heart as well, which is why tachycardia is a potential side effect of albuterol.)

What Sympathetic Activation Looks Like Across the Body

When the SNS fires — let’s say someone is in a genuinely threatening situation — here is what happens throughout the body:

• Heart rate increases and contractions become more forceful
• Blood vessels constrict and blood pressure rises
• Airways dilate, bringing in more oxygen
• Pupils dilate to take in more visual information
• Blood flow is redirected away from the gut and toward the muscles
• GI motility slows — digestion is not a priority right now
• The bladder sphincter tightens and urination is suppressed
• The liver releases stored glucose to fuel the response
• Sweat glands activate
• The immune system is temporarily suppressed

Every single one of these responses makes sense when you understand the goal: the body is preparing to fight or flee. It’s prioritizing systems that support survival and suppressing everything else.

Recognizing Sympathetic Activation at the Bedside

This is where the concept really starts clicking. Think about what a patient in shock looks like: They’ve got pale or mottled skin (blood is being shunted away from the periphery toward vital organs), tachycardia, and tachypnea. Their blood pressure holds for a while (because tachycardia, increased contractility, and vasoconstriction are all working together to compensate) before ultimately falling apart when those mechanisms fail.

That patient is not just presenting a list of symptoms to memorize. That patient’s sympathetic nervous system is firing at maximum capacity, doing everything it can to keep them alive. When you understand the mechanism, the clinical picture tells a story. And stories are a lot easier to remember than random lists.

The Parasympathetic Nervous System: Rest and Digest

The parasympathetic nervous system is the counterbalance to the sympathetic — the steering wheel turning in the opposite direction. If the SNS is activated by threat and stress, the PNS is dominant during safety, rest, recovery, and digestion. This is the mode your body runs in when there’s nothing urgent to respond to, when it can direct energy toward maintenance and repair instead of just survival.

The key neurotransmitter here is acetylcholine, which binds to muscarinic receptors at the target organs. You’ll also hear about nicotinic receptors, but those are at the nerve junctions and are common to both systems so for now, focus on the end-organ effects at muscarinic receptors.

What Parasympathetic Activation Looks Like Across the Body

When the PNS is dominant:

• Heart rate decreases — the vagus nerve releases acetylcholine at the heart, causing it to slow. This is called vagal tone, and it’s clinically relevant in more ways than you might expect. Patients who bear down during a bowel movement, vomit, or have pressure applied to certain arteries can trigger a vagal response that dramatically slows the heart. Code blues in the bathroom are not unheard of — and now you know exactly why.
• Airways constrict — the opposite of the sympathetic effect. This is why patients with asthma or COPD can experience worsening symptoms when parasympathetic activity is elevated, and why some medications for those conditions work by blocking PNS activity in the airways.
• Pupils constrict for near vision — you don’t need to scan a wide environment when you’re safe and relaxed.
• GI motility increases — digestion activates, sphincters relax, peristalsis picks up, the gut gets the blood flow and signals it needs to do its job. Hence: rest and digest.
• Salivary glands produce watery secretions and lacrimal glands produce tears.
• The bladder’s detrusor muscle contracts and the urethral sphincter relaxes — in plain terms, the parasympathetic system is what allows urination to happen.

A useful frame: if the sympathetic system is about spending energy, the parasympathetic is about conserving and restoring it — building glycogen stores, activating the immune system, digesting food, clearing waste. All the maintenance work the body does when it’s not in crisis mode.

Why This Matters So Much for Nursing Pharmacology

Here is where understanding the ANS becomes a genuine cheat code for pharm. When you block one system, the other one dominates. That’s it. That one principle explains an enormous number of drug mechanisms and side effects.

Anticholinergic Medications

Anticholinergic medications block muscarinic receptors — meaning they block parasympathetic effects throughout the body. Common anticholinergics include atropine, diphenhydramine (Benadryl), benztropine mesylate (Cogentin), and scopolamine.

When you remove parasympathetic effects, here’s what you see:

• Heart rate increases (the vagal brake is removed)
• Dry mouth (salivary glands aren’t getting the PNS signal)
• Urinary retention (bladder detrusor isn’t contracting)
• Dilated pupils
• Slowed GI motility and constipation

If you’ve ever taken Benadryl and woken up in the middle of the night with an unbearably dry mouth — that’s anticholinergic activity. Every single one of those side effects is completely predictable once you know what the parasympathetic system normally does and that these drugs are blocking it.

Atropine is the classic example of an anticholinergic used clinically. It blocks the parasympathetic effect on the heart — removing the vagal brake — which allows the sympathetic tone to push heart rate back up. That’s why atropine is used in symptomatic bradycardia. It’s not making the heart go faster. It’s removing the thing that was holding it back.

Beta Blockers

Beta blockers block the sympathetic beta-1 receptors in the heart. Remove that sympathetic stimulation, and heart rate slows and the heart contracts less forcefully. That’s why beta blockers lower heart rate and lower blood pressure.

Some beta blockers are cardioselective, meaning they primarily target beta-1 receptors in the heart and have minimal effect on beta-2 receptors in the lungs. Metoprolol is a key example. This distinction matters enormously for patients with asthma or COPD. A non-cardioselective beta blocker in a patient with reactive airway disease could worsen bronchoconstriction — because it would block beta-2 receptors in the airways along with beta-1 receptors in the heart. When you see a beta blocker ordered for a patient with a respiratory condition, this is exactly the kind of safety check you should be running.

Sympathomimetic Medications

These are medications that mimic or stimulate sympathetic effects.

Epinephrine stimulates all three adrenergic receptor types — alpha-1, beta-1, and beta-2. That means you get vasoconstriction, increased heart rate and contractility, and bronchodilation all at once. This is why epinephrine is the drug of choice for anaphylaxis, which causes massive vasodilation, bronchospasm, and cardiovascular collapse. One drug, three simultaneous mechanisms, all directly reversing what anaphylaxis is doing in the body.

Norepinephrine is primarily an alpha-1 and beta-1 agonist. Its main clinical purpose is vasoconstriction to improve blood pressure, which is why it’s commonly used in shock. At lower doses you’ll see some cardiac effects; the primary driver is that alpha-1-mediated vascular tone.

Albuterol, as discussed above, is a beta-2 agonist used to increase bronchodilation for asthma and reactive airway disease. Watch for tachycardia as a side effect, especially in patients who are already tachycardic.

Parasympathomimetic Medications

These mimic or stimulate parasympathetic effects. For example, bethanechol is a muscarinic agonist used for urinary retention. It stimulates the muscarinic receptors on the bladder detrusor muscle, causing it to contract so the patient can urinate. Logical once you know what the PNS normally does to the bladder.

Putting It All Together: Three Clinical Scenarios

Scenario 1: Anaphylaxis. Your patient was stung by a bee. They’re hypotensive, wheezing, and their throat is tightening. The drug of choice is epinephrine. Alpha-1 stimulation causes vasoconstriction to restore blood pressure. Beta-1 stimulation increases heart rate and cardiac output. Beta-2 stimulation causes bronchodilation to open the airways. Three receptor types, one drug, one critical intervention.

Scenario 2: Symptomatic bradycardia. Your patient has a heart rate of 38, is dizzy, and is diaphoretic. You administer atropine. It blocks muscarinic receptors at the heart, removing the parasympathetic brake and allowing sympathetic tone to push the rate back up. Atropine doesn’t speed the heart up directly — it releases the thing that was slowing it down.

Scenario 3: Septic shock. Your patient is tachycardic, hypotensive, initially warm and flushed, then pale and cool as things progress. After a fluid bolus, dynamic assessment reveals they won’t respond to additional fluids. The physician orders norepinephrine. Alpha-1 stimulation increases vascular tone, blood pressure improves. Sometimes norepinephrine is all that’s needed; in severe cases, additional vasoactive agents such as vasopressin or epinephrine may follow.

In each scenario, you’re not recalling a memorized protocol.
You’re following the logic of a system you understand.

TL;DR

The autonomic nervous system has two divisions, both active all the time. The balance between them shifts based on what the body needs. Most organs receive input from both and respond to whichever one is dominant in the moment. 

Sympathetic Nervous System (fight or flight):

• Triggered by stress, threat, illness, exercise, pain, anxiety
• Key neurotransmitters: norepinephrine and epinephrine
• Key receptors and their effects: alpha-1 (vasoconstriction), beta-1 (heart: increased rate and contractility), beta-2 (airways: bronchodilation)
• Purpose: prepares the body to respond to a threat

Parasympathetic Nervous System (rest and digest):

• Dominant during rest, safety, and recovery
• Key neurotransmitter: acetylcholine at muscarinic receptors
• Effects: slows heart rate (vagal tone), bronchoconstriction, stimulates digestion, allows urination
• Purpose: conserves and restores energy

Nursing pharmacology shortcut:  

Every time you encounter a new medication in nursing school, one of the first questions worth asking is, “Does this drug stimulate or block the sympathetic system, or does it stimulate or block the parasympathetic system?” That one question alone will answer a lot of questions you have about specific medications or classes of medications, and you’ll be able to answer it quickly because you have solid foundation.

When you understand both systems, medications stop being a list of facts and start being a set of logical consequences. You know what blocking a receptor will do. You know what stimulating one will do. You can predict side effects, understand mechanisms, and think through clinical scenarios instead of scrambling to remember a bunch of random facts you memorized.

This is what it means to think like a nurse rather than memorize like a student. It starts with strong foundations. When you get a concept like this solidly under your belt, it doesn’t just help you now. It makes everything you learn moving forward significantly easier because you’re not starting from scratch every time.

Want to Go Deeper on Pharmacology?

If pharm feels overwhelming, Fast Pharmacology was built exactly for that. Each concept is broken down in five minutes or less, in a format you can listen to from your podcast player on the go — and every lesson comes with a downloadable study guide. Whether you’re in nursing school, prepping for NCLEX, or just want to feel more confident administering medications at the bedside, it’s worth a look.

And if you’re just getting started and want a framework for approaching nursing school as a whole, grab the free Nursing School Survival Blueprint.

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References:

LeBouef T, Yaker Z, Whited L. Physiology, Autonomic Nervous System. In: StatPearls [Internet]. StatPearls Publishing; 2023. PMID: 30860751. URL: https://www.ncbi.nlm.nih.gov/books/NBK538516/

Open RN Nursing Pharmacology. Chapter 4: Autonomic Nervous System. NCBI Bookshelf, NIH. URL: https://www.ncbi.nlm.nih.gov/books/NBK595003/

Hafen BB, Bhimji SS. Beta 1 Receptors. In: StatPearls [Internet]. StatPearls Publishing; 2023. URL: https://www.ncbi.nlm.nih.gov/books/NBK532904/

Tindle J, Tadi P. Neuroanatomy, Parasympathetic Nervous System. In: StatPearls [Internet]. StatPearls Publishing; 2022. URL: https://www.ncbi.nlm.nih.gov/books/NBK553141/

Fink M et al. Understanding the Autonomic Nervous System: A How-To Guide for Designing Engaging Pathophysiology and Pharmacology Courses for Nursing Students. Nursing Forum. 2023. DOI: https://doi.org/10.1155/2023/2397564

McCorry LK. Physiology of the autonomic nervous system. Am J Pharm Educ. 2007;71(4):78. PMID: 17786266. DOI: https://doi.org/10.5688/aj710478

Last Updated on July 19, 2026 by Maureen Osuna, MSN, RN