Acetylcholine (ACh) mediates its effects through two main receptor types: nicotinic and muscarinic. Understanding their location, function, and signaling mechanisms is critical for pharmacology and physiology exams.
Nicotinic ACh Receptors (nAChR)
- Type: Ligand-gated ion channels
- Mechanism: Binding of ACh opens the channel → Na⁺ influx, K⁺ efflux, and occasionally Ca²⁺ influx → depolarization
- Subtypes and Locations:
| Subtype | Location | Function / Notes |
|---|---|---|
| Nn (neuronal) | Autonomic ganglia, adrenal medulla | Mediates synaptic transmission in the autonomic nervous system |
| Nm (muscle) | Neuromuscular junction of skeletal muscle | Muscle contraction |
Muscarinic ACh Receptors (mAChR)
- Type: G-protein–coupled receptors (GPCRs)
- Mechanism: Act via second messengers (e.g., cAMP, IP₃/DAG)
- Subtypes and Locations:
| Subtype | Location | Effect |
|---|---|---|
| M1 | CNS, gastric parietal cells | Cognitive function, gastric acid secretion |
| M2 | Heart (atria, SA/AV nodes) | ↓Heart rate, ↓AV conduction |
| M3 | Smooth muscle, exocrine glands | ↑Contraction/secretion (e.g., bronchoconstriction, salivation) |
| M4 | CNS | Modulates neurotransmission |
| M5 | CNS | Modulates dopamine release |
Special case: Sweat glands (sympathetic cholinergic) → muscarinic receptor-mediated secretion
Key Points
- Nicotinic = ligand-gated ion channel, fast response
- Muscarinic = GPCR, slower response via second messengers
- Subtype location and function are exam favorites
- Sweat glands are an exception: sympathetic, but use muscarinic ACh receptors
Learning Objective
- Differentiate nicotinic vs. muscarinic receptors in terms of structure, mechanism, and location
- Identify the subtypes of ACh receptors and their clinical significance
- Apply this knowledge to pharmacology and pathophysiology scenarios (e.g., cholinergic drugs, autonomic disorders)
You must be logged in to post a comment.