Medservatory
USMLE Step 1Pharmacology: principles and autonomic drugsEP 10 of 16

Autonomic receptors: alpha, beta and muscarinic

The same transmitter can do opposite things in different organs, because the receptor and its G protein decide the response. Learn the code and the tissue list once, and the effects follow.

  • 5:33 video
  • 3 min read
  • 10 sources
  • Published 15 Nov 2026
Video lesson · 5:33 · Watch on YouTubeSeries: Pharmacology: Pharmacokinetics & Autonomic Drugs · @medservatory
Chapters (11)
By the end of this lesson

Map alpha-1, alpha-2, beta-1, beta-2 and muscarinic (M1, M2, M3) receptors to their G protein, second messenger and major tissue effects.

The short version

  1. Norepinephrine and epinephrine act on alpha and beta receptors; acetylcholine released by parasympathetic nerves acts on muscarinic receptors.
  2. The receptor decides the response, not the transmitter alone.
  3. Gq activates PLC, which makes IP₃ and DAG, and IP₃ raises calcium; Gs raises cyclic AMP; Gi lowers cyclic AMP.
  4. The code is Q, I, S, S, then Q, I, Q: alpha-1, alpha-2, beta-1, beta-2, then M1, M2, M3.
  5. Beta-1 is the heart, beta-2 the airways and muscle vessels, alpha-1 the vessel squeeze, and M2 the vagal brake.

One drug, two organs, opposite responses

A man in anaphylaxis is wheezing and his blood pressure is falling. An injection of epinephrine opens his airways and lifts his pressure within minutes.[9] One drug, two organs, opposite responses: how?

Epinephrine is the same molecule in both organs. What differs is the receptor it meets, and the G protein that receptor is coupled to. Learn the code and the tissue list once, and many autonomic drug effects become predictions rather than facts to memorise.

The answer is the set of autonomic receptors on each tissue. This lesson maps each receptor to its G protein, its second messenger and its main tissue effects. We return to the patient at the end.

Transmitters and autonomic receptors

Most sympathetic postganglionic nerves release norepinephrine, and the adrenal medulla releases epinephrine. They act on adrenergic receptors: alpha-1, alpha-2, beta-1 and beta-2.[1][3][5]

Parasympathetic postganglionic nerves release acetylcholine, which acts on muscarinic receptors: M1, M2 and M3.[1][3][5]

The same transmitter can do opposite things, because the receptor decides the response.[2][6]

Three G proteins: Gq, Gs and Gi

All of these receptors are G protein coupled receptors, and only three G proteins matter here.[1][5]

The three G proteins and what they do
G proteinPathwayResult
GqActivates phospholipase C, which makes IP₃ and diacylglycerol (DAG)IP₃ releases calcium; the cell contracts or secretes
GsActivates adenylyl cyclaseCyclic AMP rises
GiInhibits adenylyl cyclaseCyclic AMP falls

Put simply: Gq is the calcium pathway, which makes cells contract or secrete. Gs and Gi are the cyclic AMP pathway, one pushing it up and the other pulling it down. The G protein is the link between a receptor and its second messenger.[4][5]

The code: Q, I, S, S, Q, I, Q

The receptor code
α₁ Q · α₂ I · β₁ S · β₂ S · M1 Q · M2 I · M3 Q

Q = Gq, I = Gi, S = Gs

Read the code in order: alpha-1, alpha-2, beta-1, beta-2, then M1, M2, M3. Two Gi entries (alpha-2 and M2) lower cyclic AMP, the two betas raise it, and alpha-1, M1 and M3 raise calcium.

  • Alpha-1 is Gq. Alpha-2 is Gi. Beta-1 and beta-2 are both Gs.[5]
  • M1 is Gq, M2 is Gi and M3 is Gq.[5]

Autonomic receptors in the tissues

Main tissue effects of each receptor
ReceptorG proteinMain tissue effects
Alpha-1GqConstricts blood vessels; contracts the radial muscle of the iris, so the pupil dilates[2][10]
Alpha-2GiSits on the nerve ending and slows norepinephrine release; in the brain it lowers sympathetic outflow[5][10]
Beta-1GsHeart: faster rate, stronger contraction; renin release from the kidney[6][7]
Beta-2GsRelaxes airway smooth muscle; dilates vessels including those in skeletal muscle; promotes glycogen breakdown[6]
M2GiSlows the heart[2][7]
M3GqContracts smooth muscle in the airways and bladder; constricts the pupil; drives gland secretion[8][10]

A check on the heart: sympathetic and vagal input

Using typical textbook numbers: without vagal input, the heart would beat at about 100 times a minute.[2]

  • Sympathetic input, through beta-1, speeds it up, to about 120–140 beats per minute in the textbook example; hard exercise pushes it higher.[2]
  • Vagal input, through M2, slows it to 60–80 beats per minute at rest.

So beta-1 is the accelerator and M2 is the vagal brake.[2][7]

Back to the patient: epinephrine on beta-2 and alpha-1

  1. Beta-2 in the airwaysGs, cyclic AMP up, and the airway smooth muscle relaxes.
  2. Alpha-1 in the vesselsGq, calcium up, the vessels constrict, and the pressure rises.

One drug, two receptors, two second messengers, two opposite effects.[6][9]

Practice question

Original · USMLE style
Your turnQB-MS-S1-PHARM-013-01

An ophthalmologist places a drop of a drug in the eye of a 52-year-old man before a retinal examination. Thirty minutes later, both pupils are widely dilated. The drug acts by activating a receptor on smooth muscle in the iris.

Which receptor and intracellular signal most likely mediate this effect?
Pick one option. Your answers stay on this device.

Common questions

What do the Q, I, S, S, Q, I, Q code letters stand for?

They give the G protein of each receptor in order: alpha-1 Gq, alpha-2 Gi, beta-1 Gs, beta-2 Gs, M1 Gq, M2 Gi and M3 Gq.

Which autonomic receptors signal through Gq?

Alpha-1, M1 and M3 signal through Gq, which activates phospholipase C to make IP₃ and DAG; IP₃ then releases calcium.

Which autonomic receptors raise or lower cyclic AMP?

Beta-1 and beta-2 act through Gs and raise cyclic AMP. Alpha-2 and M2 act through Gi and lower it.

Why does epinephrine open the airways but constrict vessels?

It reaches beta-2 in the airways (Gs, cyclic AMP up, smooth muscle relaxes) and alpha-1 in the vessels (Gq, calcium up, vessels constrict). The receptor decides the response.

Transcript

Read the full narration, by chapter646 words

0:00The hook: one drug, two organs, opposite responses

A man in anaphylaxis is wheezing, and his blood pressure is falling. An injection of epinephrine opens his airways and lifts his pressure within minutes. One drug, two organs, opposite responses. How?

0:14Norepinephrine, epinephrine and acetylcholine: the receptors

Today: map each autonomic receptor to its G protein, its second messenger, and its main tissue effects. Sympathetic nerves release norepinephrine, and the adrenal medulla releases epinephrine. They act on adrenergic receptors: alpha 1, alpha 2, beta 1 and beta 2. Parasympathetic nerves release acetylcholine, which acts on muscarinic receptors: M1, M2 and M3. Sweat glands are the odd one out:

0:44The sweat-gland exception

sympathetic nerves that release acetylcholine onto muscarinic receptors. Some textbooks add skeletal-muscle vessels. The same transmitter can do opposite things, because the receptor decides the response.

0:59Three G proteins: Gq, Gs and Gi

All of these are G protein coupled receptors, and only three G proteins matter. Gq activates phospholipase C, which makes IP3 and diacylglycerol; IP3 releases calcium, and the cell contracts or secretes. Gs activates adenylyl cyclase, so cyclic AMP rises. Gi inhibits adenylyl cyclase, so cyclic AMP falls.

1:27The code: Q I S S Q I Q

The code is Q, I, S, S, then Q, I, Q. Alpha 1 is Gq. Alpha 2 is Gi. Beta 1 and beta 2 are both Gs. M1 is Gq, M2 is Gi, and M3 is Gq.

1:46Alpha and beta receptors in the tissues

Now the tissues. Alpha 1 constricts blood vessels, and in the iris it contracts the radial muscle, so the pupil dilates. Alpha 2 sits on the nerve ending itself and slows norepinephrine release; in the brain it lowers sympathetic outflow. Beta 1 is the heart's receptor: faster rate, stronger contraction, and renin release from the kidney. Beta 2 relaxes airway smooth muscle, dilates vessels including those in skeletal muscle, and promotes glycogen breakdown.

2:19Muscarinic M2 and M3, and the heart-rate check

M2 slows the heart. M3 contracts smooth muscle in the airways and bladder, constricts the pupil, and drives gland secretion. A check on the heart, using typical textbook numbers. Without vagal input, it would beat at about 100 times a minute. Sympathetic input, through beta 1, speeds it up, to 120 to 140 during exercise, for example. Vagal input, through M2, slows it to 60 to 80 at rest.

2:52The patient: epinephrine on beta-2 and alpha-1

Now our patient. Epinephrine reaches beta 2 in the airways: Gs, cyclic AMP up, and the smooth muscle relaxes. It also reaches alpha 1 in the vessels: Gq, calcium up, the vessels constrict, and the pressure rises. One drug, two receptors, two second messengers, two opposite effects.

3:16Your turn: the dilated pupil question

Your turn. An ophthalmologist places a drop of a drug in the eye of a 52-year-old man before a retinal examination. Thirty minutes later, both pupils are widely dilated. The drug acts by activating a receptor on smooth muscle in the iris. Which receptor and intracellular signal most likely mediate this effect? A, alpha 1 receptor; Gq; increased IP3 and diacylglycerol. B, alpha 2 receptor; Gi; decreased cyclic AMP. C, beta 2 receptor; Gs; increased cyclic AMP. D, muscarinic M2 receptor; Gi; decreased cyclic AMP. E, muscarinic M3 receptor; Gq; increased IP3 and diacylglycerol. Pause here and decide. B is out: alpha 2 uses Gi, but its job is braking norepinephrine release, not opening the pupil. C is out: beta 2 uses Gs to relax smooth muscle, the opposite of the contraction needed here. D is out: M2 uses Gi and slows the heart. E is out: M3 also uses Gq, but it contracts the circular muscle and constricts the pupil. So the answer is A.

4:45The answer: alpha-1, Gq, IP3 and DAG

Alpha 1, through Gq, IP3 and calcium, contracts the radial muscle of the iris and dilates the pupil.

4:56High-yield recap and next lesson

Three things to keep. One: the receptor decides the response, not the transmitter. Two: Q, I, S, S, Q, I, Q. Alpha 1, M1 and M3 signal through Gq and raise calcium; alpha 2 and M2 lower cyclic AMP; the betas raise it. Three: beta 1 is the heart, beta 2 the lungs and muscle vessels, alpha 1 the vessel squeeze, M2 the vagal brake. Next, beta blocker selectivity.

Sources

Numbers in the text, e.g. [1], point here
  1. OpenStax Anatomy and Physiology 2e, 15.1 Divisions of the Autonomic Nervous System. openstax.org/books/anatomy-and-physiology-2e/pages/15-1-divisions-of-the-autonomic-nervous-system
  2. OpenStax Anatomy and Physiology 2e, 15.2 Autonomic Reflexes and Homeostasis (effector responses, iris, heart rate figures, sweat glands). openstax.org/books/anatomy-and-physiology-2e/pages/15-2-autonomic-reflexes-and-homeostasis
  3. OpenStax Anatomy and Physiology 2e, 15.4 Drugs That Affect the Autonomic System. openstax.org/books/anatomy-and-physiology-2e/pages/15-4-drugs-that-affect-the-autonomic-system
  4. OpenStax Anatomy and Physiology 2e, 17.2 Hormones (G proteins, phospholipase C, IP3, DAG, cAMP). openstax.org/books/anatomy-and-physiology-2e/pages/17-2-hormones
  5. IUPHAR/BPS adrenoceptor and muscarinic receptor reviews (receptor to G protein coupling), Europe PMC PMC13324819. ebi.ac.uk/europepmc/webservices/rest/PMC13324819/fullTextXML
  6. Recent advances in the management of chronic stable angina II. Vasc Health Risk Manag 2010 (beta-1 and beta-2 effects), PMC2941787. ebi.ac.uk/europepmc/webservices/rest/PMC2941787/fullTextXML
  7. Cardiac autonomic receptor review (M2 slows the heart), PMC5715548. ebi.ac.uk/europepmc/webservices/rest/PMC5715548/fullTextXML
  8. Muscarinic receptor signaling in asthma and COPD. Respir Res 2006, PMC1479816. ebi.ac.uk/europepmc/webservices/rest/PMC1479816/fullTextXML
  9. LITFL, Adrenaline (epinephrine) and Phenylephrine. litfl.com/adrenaline/ · litfl.com/phenylephrine/
  10. Wikipedia, Adrenergic receptor and Muscarinic acetylcholine receptor (secondary cross-checks). en.wikipedia.org/wiki/Adrenergic_receptor · en.wikipedia.org/wiki/Muscarinic_acetylcholine_receptor

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, labelled): "Blood pressure measurement", "Trachea and bronchi", "Syringe", "Sympathetic nervous system", "Arteriole and smooth muscle cells", "Eye view", "Heart", "Liver", "Bladder", "Eye drops", "Cell membrane", "Kidney with adrenal gland". NIAID NIH BioArt: "Sweat Gland Cross Section" (public domain). Original diagrams: receptor and G-protein chips, the QISSQIQ code, the heart-rate bars, the pocket card. No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-PHARM-013.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 09CYP450 inducers and inhibitors: how they change drug levels
  1. EP 01Potency vs efficacy: EC50, Emax and agonist types5:17
  2. EP 02Competitive vs noncompetitive antagonists: effect on EC50 and Emax5:01
  3. EP 03Therapeutic index explained: TD50 / ED505:01
  4. EP 04First-pass metabolism and bioavailability: calculating oral F from AUC5:18
  5. EP 05Volume of distribution: why Vd can be bigger than the body5:17
  6. EP 06Clearance and half-life: t½ = 0.693 × Vd / CL5:01
  7. EP 07Zero-order vs first-order elimination explained5:12
  8. EP 08Loading dose and maintenance dose, explained5:02
  9. EP 09CYP450 inducers and inhibitors: how they change drug levels5:16
  10. EP 10Autonomic receptors: alpha, beta and muscarinic5:33
  11. EP 11Beta blocker selectivity: beta-1, ISA and alpha blockade5:24
  12. EP 12Vasopressors compared: receptors, pressure and heart rate5:46
  13. EP 13Organophosphate poisoning: muscarinic vs nicotinic signs5:36
  14. EP 14Receptor desensitization, downregulation and upregulation5:01
  15. EP 15Beta-lactams and vancomycin: enzyme versus substrate5:04
  16. EP 16Protein synthesis inhibitors: 30S vs 50S antibiotics5:07