Study Notes: Cholinergic Receptors

1. Overview

• Cholinergic receptors are receptors stimulated by acetylcholine (ACh).

• They are part of the parasympathetic nervous system (rest and digest).

• Main Function: Mediate parasympathetic effects on target organs (slow heart rate, increase digestion, etc.).

2. Learning Outcomes

You should be able to:

1. Identify the subtypes of cholinergic receptors.

2. Describe their locations in the body.

3. Explain the physiological effects when stimulated.

3. Parasympathetic Response (“Rest and Digest”)

Typical organ responses:

• Eyes: Pupil constriction; lens accommodation for near vision; tear production.

• Mouth: Increased saliva for digestion.

• Lungs: Bronchoconstriction (less airflow needed).

• Heart: Decreased heart rate and contractility.

• Pancreas: Secretion of digestive enzymes.

• GI tract: Increased peristalsis and secretion.

• Bladder: Contraction of detrusor muscle → urination.

4. Parasympathetic Pathway Anatomy

• Origin: Craniosacral outflow.

  • Cranial nerves: III (oculomotor), VII (facial), IX (glossopharyngeal), X (vagus).

  • Sacral nerves: S2–S4.

• Two-neuron pathway:

  • Preganglionic neuron: Releases ACh → binds nicotinic receptors.

  • Postganglionic neuron: Also releases ACh → binds muscarinic receptors on target organs.

5. Types of Cholinergic Receptors

A. Nicotinic Receptors

• Activated by: Nicotine.

• Type: Ligand-gated ion channels (allow Na⁺ influx → depolarization).

• Subtypes:

  1. Nn (nicotinic neuronal):

     • Location: Autonomic ganglia (CNS & PNS).

     • Function: Transmit signals from preganglionic → postganglionic neurons.

  2. Nm (nicotinic muscular):

     • Location: Neuromuscular junction (skeletal muscle).

     • Function: Muscle contraction via depolarization and calcium release.

B. Muscarinic Receptors

• Activated by: Muscarine (from mushrooms).

• Type: G-protein-coupled receptors (GPCRs).

• Subtypes: M1 – M5.

  • M1, M2, M3 are clinically important.

  • M4, M5 mainly found in the brain (less pharmacologic focus).

6. Muscarinic Receptor Subtypes

Subtype	Location	Effect when ACh binds	Mechanism
M1	Brain, Stomach	↑ Cognitive function, ↑ Gastric acid & pepsinogen secretion	Stimulatory
M2	Heart (SA node, AV node, atria)	↓ Heart rate, ↓ Contractility	Inhibitory
M3	Smooth muscles, glands, eyes, bladder, uterus	Contraction and secretion	Stimulatory

7. M3 Receptor Effects

A. On Glands:

• Lacrimal glands: ↑ Tears.

• Salivary glands: ↑ Saliva.

• Bronchioles: ↑ Mucus secretion.

• Pancreas: ↑ Digestive enzymes and insulin release (β-cell activation).

• GI tract: ↑ Mucus and other secretions.

B. On Smooth Muscle:

• Eye (iris): Pupil constriction (miosis).

• Ciliary body: Lens accommodation for near vision.

• Bronchioles: Bronchoconstriction.

• GI tract: ↑ Peristalsis.

• Bladder: Detrusor contraction → urination.

• Uterus: Contraction (important in pregnancy/labor).

8. Summary of Effects

• M1: Stimulation (↑ gastric acid, ↑ cognition).

• M2: Inhibition (↓ HR, ↓ atrial contraction).

• M3: Stimulation (↑ glandular secretion, smooth muscle contraction).

9. Agonists & “DUMBBELLS” Mnemonic

Agonizing (stimulating) cholinergic receptors → parasympathetic side effects:

DUMBBELLS:

• D – Diarrhea / Drooling

• U – Urination

• M – Miosis (pupil constriction)

• B – Bradycardia (slow HR)

• B – Bronchoconstriction

• E – Emesis (vomiting)

• L – Lacrimation (tears)

• S – Salivation / Sweating

These effects represent excess ACh activity (e.g., cholinergic toxicity).

10. Key Takeaways

• Cholinergic = ACh-driven → parasympathetic “rest and digest.”

• Nicotinic (Nn, Nm) = ion channels for rapid transmission.

• Muscarinic (M1–M3) = GPCRs for smooth muscle and glandular regulation.

• M1 & M3 = stimulatory, M2 = inhibitory.

• Agonists mimic parasympathetic activation → “DUMBBELLS.”