Types of receptors physio and their properties
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I now have rich content. Let me compile the comprehensive answer covering all receptor types in physiology.
Types of Receptors in Physiology and Their Properties
Receptors in physiology are classified from two main angles:
- By stimulus type (sensory/physiological classification)
- By molecular/signal transduction mechanism (biochemical classification)
I. Classification by Stimulus Type (Sensory Receptors)
These transduce environmental stimuli into electrical signals. From Costanzo Physiology 7th Edition:
| Type | Stimulus | Examples | Location |
|---|---|---|---|
| Mechanoreceptors | Pressure / changes in pressure | Pacinian corpuscles (touch), Meissner corpuscles, baroreceptors (carotid sinus), hair cells (cochlea/vestibular) | Skin, subcutaneous tissue, vessel walls, inner ear |
| Photoreceptors | Light | Rods (dim light, grayscale), Cones (color/bright light) | Retina |
| Chemoreceptors | Chemicals | Olfactory receptors, taste buds, carotid/aortic bodies (O₂/CO₂), ventrolateral medulla (CSF pH) | Olfactory mucosa, tongue, arterial walls, brainstem |
| Thermoreceptors | Temperature changes | Cold receptors, warm receptors | Skin |
| Nociceptors | Extremes of pressure, temperature, noxious chemicals | Thermal nociceptors, polymodal nociceptors | Skin, viscera |
II. Classification by Signal Transduction Mechanism (Molecular Receptors)
This is the core physiological and pharmacological classification.
1. Ion Channel-Linked Receptors (Ionotropic Receptors)
- Also called ligand-gated ion channels or transmitter-gated ion channels
- Ligand binding directly opens or closes an ion channel - no second messenger needed
- Fastest onset of all receptor types (milliseconds)
- Key example: Nicotinic acetylcholine receptors (N-AChR)
- Found at the neuromuscular junction (N₁), autonomic ganglia (N₂), and adrenal medulla (N₃)
- Binding of ACh opens Na⁺ and K⁺ channels → membrane depolarization
- This is the mechanism of skeletal muscle contraction
- Other examples: GABA-A receptor (Cl⁻ channel - inhibitory), NMDA/AMPA glutamate receptors (Na⁺/Ca²⁺), glycine receptors
Key property: Direct gating - the receptor IS the channel. No amplification cascade.
2. G Protein-Coupled Receptors (GPCRs) - Metabotropic Receptors
The largest and most pharmacologically important receptor family. From Costanzo Physiology:
- Single polypeptide chain that crosses the cell membrane 7 times (seven-pass transmembrane proteins)
- Ligand binds extracellular domain; intracellular domain couples to a G protein (heterotrimeric: α, β, γ subunits)
- G proteins cycle between inactive (GDP-bound) and active (GTP-bound) states
- Onset: seconds to minutes
Major subtypes and second messengers:
| Receptor | G Protein | Mechanism | Second Messenger | Effect |
|---|---|---|---|---|
| α₁ adrenoceptors | Gq | Activates phospholipase C | IP₃ + DAG → ↑ intracellular Ca²⁺; activates PKC | Smooth muscle contraction (vasoconstriction, GI/bladder sphincter contraction, iris dilation) |
| α₂ adrenoceptors | Gi | Inhibits adenylyl cyclase | ↓ cAMP | Inhibits NE release (presynaptic), ↓ GI motility |
| β₁ adrenoceptors | Gs | Activates adenylyl cyclase | ↑ cAMP → activates PKA | ↑ Heart rate (SA node), ↑ AV conduction, ↑ contractility, renin secretion |
| β₂ adrenoceptors | Gs | Activates adenylyl cyclase | ↑ cAMP | Vasodilation (skeletal muscle), bronchodilation, ↓ GI motility, ↓ bladder wall tone |
| M₁, M₃ muscarinic | Gq | Activates phospholipase C | IP₃ → ↑ Ca²⁺ | Glandular secretion, GI smooth muscle contraction, pupillary constriction |
| M₂ muscarinic | Gi | Inhibits adenylyl cyclase | ↓ cAMP; also opens K⁺ channels | ↓ Heart rate, ↓ AV conduction |
The Gs → cAMP pathway in detail (β₁ example):
- NE binds β₁ receptor
- αs subunit releases GDP, binds GTP → detaches from Gβγ
- αs-GTP activates adenylyl cyclase → ATP → cAMP
- cAMP activates protein kinase A (PKA)
- PKA phosphorylates target proteins → tissue-specific physiologic effect
- Intrinsic GTPase converts GTP back to GDP → signal terminates
The Gq → IP₃/DAG pathway (α₁/M₁/M₃):
- Agonist binds → αq-GTP activates phospholipase C
- PLC cleaves PIP₂ → IP₃ + DAG
- IP₃ releases Ca²⁺ from ER → activates calmodulin and Ca²⁺-dependent kinases
- DAG activates protein kinase C (PKC)
3. Enzyme-Linked Receptors (Receptor Tyrosine Kinases and others)
- Single transmembrane domain
- Ligand binding activates intrinsic enzymatic activity (usually tyrosine kinase)
- Examples: insulin receptor, growth factor receptors (EGF, PDGF), cytokine receptors
- Onset: minutes to hours
- Signal through MAP kinase cascade, PI-3-kinase, JAK-STAT pathways
4. Intracellular / Nuclear Receptors
- Located in cytoplasm or nucleus (not on cell membrane)
- Only activated by lipid-soluble (lipophilic) ligands that can cross the plasma membrane
- Ligand-receptor complex acts as a transcription factor - directly alters gene expression
- Slowest onset (hours) but longest duration of action
- Examples:
- Steroid hormones: cortisol (glucocorticoid receptor), aldosterone, testosterone, estrogen, progesterone
- Thyroid hormones: T₃/T₄
- Vitamin D and retinoic acid
III. Autonomic Receptor Summary
From Costanzo Physiology 7th Edition (Table 2.4):
| Receptor | Target Tissue | Mechanism |
|---|---|---|
| α₁ | Vascular smooth muscle (skin, renal, splanchnic), GI sphincters, bladder sphincter, iris (radial) | Gq → IP₃ → ↑ [Ca²⁺]ᵢ → contraction |
| α₂ | GI wall, presynaptic adrenergic neurons | Gi → ↓ adenylyl cyclase → ↓ cAMP |
| β₁ | Heart, salivary glands, adipose, kidney (JGA) | Gs → ↑ adenylyl cyclase → ↑ cAMP |
| β₂ | Skeletal muscle vasculature, GI wall, bladder wall, bronchioles | Gs → ↑ adenylyl cyclase → ↑ cAMP → relaxation |
| Nicotinic (N) | Skeletal muscle NMJ (N₁), autonomic ganglia (N₂), adrenal medulla (N₃) | Ion channel (Na⁺/K⁺) → depolarization |
| Muscarinic (M₁,M₃) | All PNS effector organs, sweat glands | Gq → IP₃ → ↑ [Ca²⁺]ᵢ |
| Muscarinic (M₂) | Heart SA/AV node | Gi → ↓ cAMP + ↑ K⁺ conductance → slows heart |
IV. Key Properties Comparison
| Property | Ion Channel (Ionotropic) | GPCR (Metabotropic) | Enzyme-Linked | Nuclear |
|---|---|---|---|---|
| Ligand type | Hydrophilic | Hydrophilic | Hydrophilic | Lipophilic |
| Location | Plasma membrane | Plasma membrane | Plasma membrane | Cytoplasm/Nucleus |
| Onset | Milliseconds | Seconds-minutes | Minutes-hours | Hours |
| Duration | Very brief | Moderate | Prolonged | Very prolonged |
| Mechanism | Direct ion flux | Second messenger cascade | Phosphorylation cascade | Gene transcription |
| Amplification | None | High (cascade) | High | High |
| Examples | Nicotinic AChR, GABA-A, NMDA | All adrenoceptors, muscarinic, dopamine | Insulin, GH, growth factors | Steroids, thyroid hormone, Vit D |
Sources:
- Costanzo Physiology, 7th Edition - Ch. 2 (Autonomic Nervous System) & Ch. 3 (Sensory Physiology)
- Katzung's Basic and Clinical Pharmacology, 16th Edition - Ch. 9 (Adrenoceptor Agonists)
- Neuroscience: Exploring the Brain, 5th Edition - Ch. 5-6 (Synaptic Transmission)
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