---

M2 and M4 Muscarinic Receptors: Effect on Calcium

Excitatory Receptors (M1, M3, M5) - INCREASE calcium

• Coupled to Gq protein
• Activate phospholipase C → generate IP3 and DAG
• IP3 triggers release of calcium from the endoplasmic reticulum → increases intracellular calcium

Inhibitory Receptors (M2, M4) - DECREASE/inhibit

• Coupled to Gi protein (inhibitory G protein, pertussis toxin-sensitive)
• Inhibit adenylyl cyclase → decrease cAMP
• Do not generate IP3, so they do not mobilize intracellular calcium
• They actually attenuate calcium-dependent signaling pathways

Summary Table

Clinical relevance

• In the heart, M2 receptors (predominant subtype) slow the rate via Gi → reduced cAMP → reduced Ca²⁺ influx through L-type channels → negative chronotropy/dromotropy
• In the bladder detrusor, M3 (not M2/M4) mediates the main contractile response because it is the calcium-mobilizing subtype

---

Sympathetic vs Parasympathetic Nervous System - Complete MBBS Comparison

---

1. OVERVIEW / NICKNAME

---

2. ANATOMICAL ORIGIN (Outflow)

Mnemonic - PSNS cranial nerves: "Some Say Marry Money But My Brother Says Big Brains Matter More" (or simply: CN III, VII, IX, X)

---

3. GANGLIA - LOCATION AND DISTANCE

---

4. PRE- AND POSTGANGLIONIC FIBERS

---

5. NEUROTRANSMITTERS

Key fact: The adrenal medulla = modified sympathetic ganglion. Chromaffin cells = modified postganglionic neurons that secrete into blood instead of a synapse.

---

6. RECEPTORS

Adrenergic Receptors (Sympathetic targets)

Cholinergic Receptors (Parasympathetic targets + autonomic ganglia)

---

7. ORGAN-BY-ORGAN EFFECTS

Diagrammatic Overview

Detailed Organ Effects Table

Memory trick for erection/ejaculation: "Point and Shoot" - Parasympathetic = erection (Point), Sympathetic = ejaculation (Shoot)

---

8. STRUCTURES WITH ONLY ONE DIVISION

---

9. NEUROTRANSMITTER SYNTHESIS, STORAGE AND TERMINATION

Norepinephrine (Sympathetic)

• Synthesis: Tyrosine → DOPA → Dopamine → NE (in vesicles) → Epi (adrenal medulla only, via PNMT enzyme)
• Termination: Mainly reuptake into presynaptic terminal (uptake-1 = NET transporter); also COMT (extraneuronal), MAO (intraneuronal)
• Drugs blocking reuptake: Cocaine, tricyclic antidepressants

Acetylcholine (Parasympathetic)

• Synthesis: Choline + Acetyl-CoA → ACh (via choline acetyltransferase, ChAT)
• Termination: Rapid hydrolysis by Acetylcholinesterase (AChE) in synaptic cleft
• Drugs blocking AChE: Neostigmine, physostigmine, organophosphates

---

10. SECOND MESSENGER SYSTEMS

---

11. COMPARISON TABLE - STRUCTURAL SUMMARY

---

12. PHARMACOLOGICAL HANDLE (High-yield for MBBS)

---

---

How M3-Acting Drugs Cause Hypotension

The Short Answer - A Receptor Correction First

Important note: Katzung's Pharmacology specifically states the vascular vasodilation by muscarinic agonists is mediated via M2 receptors on endothelium (not M3) in intact vessels. However, M3 receptors are present on endothelial cells in some vascular beds (e.g. skeletal muscle via M1/M3), and the mechanism of NO release is the same. For exam purposes in many MBBS curricula, this effect is attributed to M3. The mechanism is identical regardless of which muscarinic subtype is cited.

---

Step-by-Step Mechanism

Step 1 - Muscarinic Agonist Activates Endothelial Receptors (M2/M3)

• Receptor subtype: M2 (classic) or M3 (in some beds)
• G protein: Gq → activates phospholipase C → ↑ IP3 → ↑ intracellular Ca²⁺ in endothelial cells

Step 2 - eNOS Activation in Endothelium

• Ca²⁺ binds calmodulin
• Ca²⁺-calmodulin complex activates endothelial Nitric Oxide Synthase (eNOS)
• eNOS converts L-arginine → Nitric Oxide (NO)

NO is also known as EDRF - Endothelium-Derived Relaxing Factor (Furchgott & Zawadzki, 1980 - Nobel Prize discovery)

Step 3 - NO Diffuses to Vascular Smooth Muscle

Step 4 - Activation of Guanylyl Cyclase → ↑ cGMP

• NO activates soluble guanylyl cyclase (sGC)
• sGC converts GTP → cyclic GMP (cGMP)
• ↑ cGMP activates Protein Kinase G (PKG)

Step 5 - Smooth Muscle Relaxation → Vasodilation

• Phosphorylation of myosin light chain kinase (MLCK) → inactivated
• ↓ intracellular Ca²⁺ (Ca²⁺-ATPase activated, Ca²⁺ channels inhibited)
• Result: smooth muscle RELAXES → vessel DILATES

Step 6 - Vasodilation → ↓ Peripheral Vascular Resistance → Hypotension

---

Summary Flowchart

Muscarinic agonist (M2/M3)
        ↓
Endothelial cell: Gq → ↑Ca²⁺ → Ca²⁺-calmodulin → eNOS activated
        ↓
L-arginine → NITRIC OXIDE (NO)
        ↓  (diffuses into vascular smooth muscle)
sGC activated → GTP → ↑ cGMP → PKG activated
        ↓
MLCK inhibited + ↓Ca²⁺ in smooth muscle
        ↓
VASODILATION
        ↓
↓ Total Peripheral Resistance
        ↓
HYPOTENSION

---

The Critical Paradox - Endothelium Intact vs Absent

---

Additional Mechanisms Contributing to Hypotension

1. ↓ Heart rate (M2 on SA node) - bradycardia → ↓ cardiac output
2. ↓ AV node conduction (M2) - negative dromotropy
3. ↓ Cardiac contractility (M2) - negative inotropy → ↓ cardiac output

• The fall in BP then triggers reflex sympathetic discharge (baroreceptor reflex), which can partially oppose the bradycardia - making the net heart rate effect complex.

---

Pilocarpine Exception (Exam Favourite)

1. Brief hypotension (M receptor - NO pathway as above)
2. Followed by paradoxical hypertension - because it also activates M1 receptors in sympathetic ganglia, causing sympathetic ganglion firing → NE release → vasoconstriction