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.