Do the Heart Have M3 Receptors?

Muscarinic Receptor Distribution in the Heart

M2 Receptors - The Dominant Cardiac Subtype

• Decrease heart rate (negative chronotropy) - slowing SA node pacemaker activity
• Slow AV conduction (negative dromotropy)
• Directly decrease atrial contractile force
• Exert inhibitory modulation of ventricular contractile force

1. Inhibits adenylyl cyclase (reducing cAMP)
2. Increases K+ conductance (via GIRK channels) → hyperpolarization
3. Modulates Ca2+ channels and If (funny current)

M3 Receptors in the Heart - Where and Why

• Coronary vascular endothelium - stimulation causes release of nitric oxide (NO) → vasodilation
• Coronary vascular smooth muscle - direct relaxation
• Lung, bladder, exocrine glands, smooth muscle are the classic peripheral sites (Lippincott Pharmacology)

• Couples to Gq/11 protein
• Activates phospholipase C
• Generates IP3 (raises intracellular Ca2+) and DAG (activates PKC)

Clinical Relevance

• Anticholinergic drugs (e.g., atropine, glycopyrrolate) block both M2 and M3 receptors. At the heart, M2 blockade is the clinically relevant effect (causing tachycardia).
• Selective M3 antagonists (e.g., darifenacin, used in overactive bladder) are designed to spare M2-mediated cardiac effects. However, since M3 receptors are present in the coronary circulation, they can affect coronary tone.
• The reference to "loss of vagally mediated bradycardia in mice lacking M2 or M3 muscarinic receptors" (from Goodman & Gilman's bibliography) suggests M3 may contribute to vagal bradycardia in some contexts as well.

Summary

The heart has both M2 and M3 muscarinic receptors. M2 is the dominant, clinically important subtype responsible for all classic parasympathetic cardiac effects (bradycardia, reduced AV conduction). M3 receptors are found mainly in the coronary circulation where they mediate vasodilation via NO, and there is some evidence of non-M2 (including M3) receptors in the myocardium itself.

• Miller's Anesthesia (10th ed.), Neural Regulation of Cardiac Function
• Lippincott Illustrated Reviews: Pharmacology, Muscarinic Receptors section
• Goodman & Gilman's Pharmacological Basis of Therapeutics

Effects of Parasympathetic Stimulation ("Rest and Digest")

Organ-by-Organ Effects

Effects on the Heart - In Detail

• ACh acts on M2 receptors at the SA node
• Activates Gi → inhibits adenylyl cyclase (↓cAMP) + increases K+ conductance (GIRK channels) → hyperpolarization
• Slows diastolic depolarization → decreases firing rate
• Result: heart rate falls below 60 bpm (bradycardia)
• Extreme vagal stimulation can even stop the heart temporarily (e.g., carotid sinus syndrome causing syncope)

• ACh slows conduction through the AV node
• Increases PR interval on ECG
• Extreme stimulation can cause complete heart block

• Direct decrease in atrial contractile force
• Indirect inhibitory modulation of ventricular contractility

• Postganglionic fibers extend into coronary arteries
• Direct effect is mild vasoconstriction
• However, M3 receptor stimulation on endothelium releases NO causing vasodilation (as discussed in the previous response)

Mechanism of Action - M2 Receptor Signaling

ACh → M2 receptor → Gi/o protein →
  ├── Inhibits adenylyl cyclase → ↓cAMP → ↓PKA → ↓Ca²⁺ influx
  ├── Opens GIRK (K⁺) channels → hyperpolarization
  └── Modulates If (funny current) → slows pacemaker

Important Clinical Points

• Atropine (muscarinic antagonist) blocks all these effects - used to treat bradycardia and heart block
• Vagal tone at rest: the heart is under dominant parasympathetic influence at rest. Loss of vagal tone (e.g., cardiac vagotomy) increases resting heart rate from ~70 to ~160 bpm
• Vasovagal syncope: sudden, excessive vagal discharge causing profound bradycardia + vasodilation → loss of consciousness
• Carotid sinus massage: increases vagal tone deliberately - used to break certain supraventricular tachycardias

• THIEME Atlas of General Anatomy & Musculoskeletal System, Sympathetic/Parasympathetic Synopsis table
• Histology: A Text and Atlas (Pawlina), Systemic Regulation of Heart Function
• Guyton & Hall Textbook of Medical Physiology, Bradycardia & Vagal Stimulation sections
• Lippincott Illustrated Reviews: Pharmacology, Muscarinic Receptors

Functions of the Ciliary Muscle

1. Accommodation (Primary Function)

When the ciliary muscle contracts, it moves inward and forward, releasing tension on the zonular fibers attached to the lens. The elastic lens then bulges into a more convex shape, increasing its refractive power for near vision. When the muscle relaxes, zonular fibers pull the lens flat for distance vision.

• Innervation: Parasympathetic fibers via CN III (oculomotor nerve), relayed through the ciliary ganglion
• Receptor: M3 muscarinic receptors on ciliary smooth muscle
• Drugs that mimic this: pilocarpine, physostigmine
• Drugs that block this: atropine, cyclopentolate → cause cycloplegia (paralysis of accommodation)

2. Aqueous Humor Drainage - Reduces Intraocular Pressure

• This is the mechanism by which pilocarpine (a muscarinic agonist) lowers IOP in open-angle glaucoma
• Atropine has the opposite effect - blocks ciliary contraction, impairs drainage, can raise IOP (dangerous in glaucoma patients)

3. Aqueous Humor Production (Ciliary Body - Epithelium)

• Beta-adrenoceptors on ciliary epithelium stimulate aqueous humor secretion
• Beta-blockers (e.g., timolol eye drops) reduce secretion and lower IOP - another treatment for glaucoma

Innervation Summary

Parasympathetic (CN III → ciliary ganglion → short ciliary nerves)
  → M3 receptors on ciliary muscle
  → Contraction → accommodation + ↓IOP (trabecular meshwork opening)

Sympathetic
  → β receptors on ciliary epithelium
  → Aqueous humor secretion

Clinical Relevance

• Katzung's Basic & Clinical Pharmacology (16th ed.), Pharmacology of the Eye
• K.J. Lee's Essential Otolaryngology, The Globe
• Gray's Anatomy for Students, Cranial Nerves (CN III innervation table)

Parasympathetic Activation and Defecation

The Two Defecation Reflexes

Reflex 1: Intrinsic Myenteric Defecation Reflex (weak on its own)

• Feces distend the rectal wall
• Afferent signals spread through the myenteric (Auerbach's) plexus
• Triggers peristaltic waves in the descending colon, sigmoid, and rectum
• Relaxes the internal anal sphincter via inhibitory myenteric signals
• By itself this reflex is relatively weak and cannot effectively cause defecation

Reflex 2: Parasympathetic Defecation Reflex (the powerful amplifier)

• Rectal distension signals travel afferently to the sacral spinal cord (S2-S4)
• A reflex signal returns via parasympathetic fibers in the pelvic nerves to the descending colon, sigmoid, rectum, and anus
• These parasympathetic signals:
  • Greatly intensify peristaltic waves
  • Relax the internal anal sphincter
  • Convert the weak intrinsic reflex into a powerful, effective defecation reflex

What Happens Step-by-Step

1. Mass movement pushes feces into rectum
        ↓
2. Rectal wall distension → urge to defecate
        ↓
3. Afferent signals → sacral spinal cord (S2-S4)
        ↓
4. Parasympathetic efferents via pelvic nerves
        ↓
5. Intensified peristalsis in descending colon, sigmoid, rectum
   + Relaxation of INTERNAL anal sphincter (smooth muscle, involuntary)
        ↓
6. Additional signals cause:
   - Deep breath (diaphragm moves down)
   - Glottis closure (Valsalva)
   - Abdominal muscle contraction (↑intra-abdominal pressure)
   - Pelvic floor relaxation
        ↓
7. If EXTERNAL anal sphincter (voluntary, somatic/pudendal nerve) is 
   consciously relaxed → DEFECATION occurs

The Two Sphincters - Key Distinction

Defecation requires both: parasympathetic relaxation of the internal sphincter AND voluntary relaxation of the external sphincter. This is why defecation can be consciously postponed even when the urge is strong.

Gastrocolic Reflex - Why You Feel the Urge After Eating

Clinical Relevance

• Guyton & Hall Textbook of Medical Physiology, Chapter 64 - Defecation