Autonomic Nervous System: INBDE Review
Sympathetic vs parasympathetic anatomy, neurotransmitters, receptor subtypes (α/β, muscarinic), and dental-relevant drug interactions including epinephrine + β-blockers. 11 board-style MCQs.
Concept summary & clinical relevance.
Quick-reference structure first, then nerve-by-nerve detail. Mnemonics in amber, clinical pearls in blue.
The autonomic nervous system is high-yield for the INBDE because it directly governs how dental drugs and emergencies behave: epinephrine in local anesthetic, atropine for dry-field surgery, vasovagal syncope in the chair, β-blocker interactions, and xerostomia from anticholinergics. Get the neurotransmitter pairs and receptor subtypes locked in, and the clinical scenarios fall out of them.
| Sympathetic (fight or flight) | Parasympathetic (rest & digest) | |
|---|---|---|
| Origin | Thoracolumbar (T1–L2) | Craniosacral (CN III, VII, IX, X + S2–S4) |
| Preganglionic fiber | Short, myelinated | Long, myelinated |
| Preganglionic NT | ACh → nicotinic | ACh → nicotinic |
| Postganglionic fiber | Long | Short |
| Postganglionic NT | Norepinephrine (sweat glands = ACh) | ACh → muscarinic |
| Receptor | Location / target | Effect |
|---|---|---|
| α1 | Vascular smooth muscle | Vasoconstriction (basis of epinephrine vasoconstriction) |
| α2 | Presynaptic neurons | Inhibits NE release (negative feedback) |
| β1 | Heart (“1 heart”) | ↑ Heart rate, ↑ contractility |
| β2 | Lungs, vascular smooth muscle (“2 lungs”) | Bronchodilation; vasodilation in skeletal muscle |
| M2 | Heart | ↓ Heart rate (parasympathetic vagal) |
| M3 | Glands, smooth muscle, eye | Salivation, GI motility, pupil constriction (miosis) |
| Nicotinic (NN) | All autonomic ganglia (SNS & PNS) | Fast excitatory transmission |
| Organ | Sympathetic | Parasympathetic |
|---|---|---|
| Heart | ↑ HR, ↑ contractility (β1) | ↓ HR (M2) |
| Bronchi | Bronchodilation (β2) | Bronchoconstriction |
| Pupil | Mydriasis (α1) | Miosis (M3) |
| Salivary glands | Thick, viscous saliva | Watery, copious saliva |
| GI motility | Decreased | Increased |
| Vasculature | Vasoconstriction (α1) | Minimal direct effect |
Sympathetic nervous system
- Origin: thoracolumbar cord (T1–L2).
- Preganglionic fibers are short and myelinated; release ACh onto nicotinic receptors at sympathetic chain ganglia.
- Postganglionic fibers are long; release norepinephrine onto α and β adrenergic receptors. Sweat glands are the exception (ACh onto muscarinic receptors).
- Adrenal medulla is a modified ganglion: preganglionic fibers release ACh, the chromaffin cells release epinephrine into the blood.
Parasympathetic nervous system
- Origin: craniosacral (CN III, VII, IX, X + S2–S4 spinal segments).
- Cranial outflow: CN VII → submandibular and sublingual glands; CN IX → parotid; CN X → thoracic and abdominal viscera.
- Preganglionic fibers are long and synapse close to the target organ; postganglionic fibers are short; both use ACh (nicotinic at the ganglion, muscarinic at the effector).
Dental-relevant drug interactions
- Local anesthetic + epinephrine: α1 vasoconstriction prolongs anesthesia and reduces bleeding.
- Atropine / scopolamine: muscarinic blockade → reduced salivation (helpful for dry-field surgery, but causes xerostomia long-term).
- Non-selective β-blockers + epinephrine: unopposed α1 → hypertensive crisis risk.
- α1-blockers (e.g., for BPH): orthostatic hypotension when patient stands from chair.
- β2 agonists (e.g., albuterol): allow asthmatic patients to use their inhaler before procedures.
11 board-style MCQs.
Active recall is the highest-yield study method for the INBDE. Pick an answer, check it, and read why every distractor is wrong, that's where the learning compounds.
The MCQs above are Core Recall, testing what you've memorized. The book adds a full Clinical Integration set: board-style patient scenarios where you apply this anatomy to real clinical reasoning. That's the section the INBDE actually weights heaviest.
- Question 1EasyWhich division of the nervous system is responsible for “fight or flight” responses?
- Question 2EasySympathetic fibers originate from which spinal cord region?
- Question 3EasyMost sympathetic postganglionic neurons release which neurotransmitter?
- Question 4ModerateParasympathetic fibers arise from which cranial nerves?
- Question 5EasyWhich neurotransmitter is released at all autonomic ganglia (sympathetic and parasympathetic)?
- Question 6EasySympathetic stimulation of the pupil produces:
- Question 7EasyParasympathetic activation of the salivary glands produces:
- Question 8ModerateWhich adrenergic receptor mediates vasoconstriction?
- Question 9ModerateWhich receptor subtype increases heart rate and contractility when activated?
- Question 10EasyWhich receptor subtype produces bronchodilation when stimulated?
- Question 11EasyParasympathetic activation of the eye produces:
900 INBDE-style MCQs with full explanations across 18 chapters — Core Recall plus board-style Clinical Integration scenarios — alongside Learning Summaries, Integration Bridges, and Review Boxes. Built by Dr. Isaac Sun for dental students who want to think like a clinician, not just memorize.
Founder, KYT Dental Services · Author, KYT INBDE series. These MCQs and Learning Summaries are part of a structural-thinking framework Dr. Sun uses with patients in the chair.