Classify sympathetic receptors.Enumerate thier site of distribution and functions.Give one test to check the function of SNS and PNS
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Sympathetic (Adrenergic) Receptors — Classification, Distribution & Function
1. Classification of Sympathetic (Adrenergic) Receptors
Sympathetic postganglionic neurons release norepinephrine (NE) and epinephrine (Epi), which act on adrenergic receptors — all of which are G-protein coupled receptors (GPCRs). Two major classes exist, each with subtypes:
| Class | Subtypes |
|---|---|
| α (Alpha) | α₁, α₂ |
| β (Beta) | β₁, β₂, β₃ |
| Dopaminergic | D₁, D₂ (sometimes listed separately) |
— Medical Physiology (Boron & Boulpaep), p. 512; Costanzo Physiology 7th Ed., p. 69
2. Distribution and Functions
α₁ Receptors
| Details | |
|---|---|
| G-protein / 2nd messenger | Gq → Phospholipase C → IP₃ + DAG → ↑ intracellular Ca²⁺ |
| Agonist potency | NE ≥ Epi |
| Selective antagonist | Phentolamine, prazosin |
Site of distribution & function:
| Organ / Tissue | Effect |
|---|---|
| Vascular smooth muscle (skin, renal, splanchnic) | Vasoconstriction → ↑ peripheral resistance |
| GI tract sphincters | Contraction (sphincter closure) |
| Urinary bladder sphincter | Contraction (urinary retention) |
| Radial muscle of iris | Mydriasis (pupil dilation) |
| Pilomotor muscles | Piloerection ("goosebumps") |
α₂ Receptors
| Details | |
|---|---|
| G-protein / 2nd messenger | Gi → ↓ adenylyl cyclase → ↓ cAMP |
| Agonist potency | NE ≥ Epi |
| Selective agonist / antagonist | Clonidine / Yohimbine |
Site of distribution & function:
| Organ / Tissue | Effect |
|---|---|
| Presynaptic adrenergic nerve terminals (autoreceptors) | Negative feedback — inhibit further NE release (key regulatory role) |
| GI tract wall | Reduced motility |
| Presynaptic cholinergic terminals (heteroreceptors) | Inhibit ACh release |
| CNS (locus coeruleus) | Sedation, ↓ sympathetic outflow |
The adrenal medulla lacks α₂ receptors and is therefore not subject to feedback inhibition — it can deplete catecholamines during prolonged stress. — Costanzo Physiology 7th Ed., p. 70
β₁ Receptors
| Details | |
|---|---|
| G-protein / 2nd messenger | Gs → ↑ adenylyl cyclase → ↑ cAMP → protein kinase A |
| Agonist potency | Epi > NE |
| Selective antagonist | Metoprolol, atenolol |
Site of distribution & function:
| Organ / Tissue | Effect |
|---|---|
| Heart (SA node) | ↑ Heart rate (positive chronotropy) |
| Heart (ventricular muscle) | ↑ Contractility (positive inotropy) |
| Kidney (juxtaglomerular cells) | ↑ Renin secretion |
| Salivary glands | ↑ Secretion |
| Adipose tissue | Lipolysis |
β₂ Receptors
| Details | |
|---|---|
| G-protein / 2nd messenger | Gs → ↑ adenylyl cyclase → ↑ cAMP |
| Agonist potency | Epi >> NE |
| Selective agonist / antagonist | Terbutaline / Butoxamine |
Site of distribution & function:
| Organ / Tissue | Effect |
|---|---|
| Bronchioles | Bronchodilation (clinical use in asthma) |
| Vascular smooth muscle (skeletal muscle) | Vasodilation |
| GI tract wall | Relaxation (↓ motility) |
| Urinary bladder wall | Relaxation |
| Uterus | Relaxation (tocolysis) |
| Liver | Glycogenolysis → ↑ blood glucose |
At physiologic concentrations, β receptors are more sensitive to catecholamines than α₁ receptors. High local NE concentrations (near synapses) are needed to activate α₁. — Costanzo Physiology 7th Ed., p. 70
β₃ Receptors
| Details | |
|---|---|
| G-protein / 2nd messenger | Gs → ↑ cAMP |
| Selective antagonist | SR59230A |
| Organ / Tissue | Effect |
|---|---|
| Adipose tissue (brown fat) | Lipolysis and thermogenesis |
| Bladder detrusor | Relaxation |
Summary Table (Costanzo / Medical Physiology)
| Receptor | Location | Signal | Key Effect |
|---|---|---|---|
| α₁ | Vascular SM, iris, GI sphincters, bladder neck | IP₃ / ↑Ca²⁺ | Vasoconstriction, mydriasis |
| α₂ | Presynaptic terminals, GI wall | ↓ cAMP | Inhibit NE release (feedback) |
| β₁ | Heart, kidney, adipose | ↑ cAMP | ↑ HR, ↑ contractility, renin release |
| β₂ | Bronchioles, skeletal muscle vessels, GI wall | ↑ cAMP | Bronchodilation, vasodilation |
| β₃ | Adipose (brown fat), bladder | ↑ cAMP | Lipolysis, thermogenesis |
3. Tests of SNS and PNS Function
Test for SNS — Cold Pressor Test
Procedure: The subject immerses one hand in ice-cold water for 1–5 minutes.
Normal response:
- Systolic BP rises by 15–20 mmHg
- Diastolic BP rises by 10–15 mmHg
Mechanism: Cold activates cutaneous pain fibers → CNS → sympathetic efferent outflow → vasoconstriction → ↑ BP.
Interpretation: A reduced or absent response indicates a lesion in the sympathetic efferent arc. If combined with a normal Valsalva response, this localizes the lesion to the baroreceptors or their afferent nerves (seen in diabetic neuropathy and tabes dorsalis).
— Adams & Victor's Principles of Neurology, 12th Ed., p. 551
Test for PNS — Valsalva Maneuver (Heart Rate Response)
Procedure: The subject forcibly exhales against a closed glottis for ~10–15 seconds (strain phase), then releases.
Normal response:
- Strain phase: ↑ intrathoracic pressure → ↓ venous return → ↓ cardiac output → ↓ BP → baroreceptors trigger ↑ sympathetic + ↓ parasympathetic → tachycardia
- Release phase: Venous return rebounds → ↑ BP → baroreceptors trigger ↑ vagal (PNS) tone → reflex bradycardia (overshoot)
Valsalva Ratio = Maximum HR during strain ÷ Minimum HR within 30 s after release
- Normal: ≥ 1.4
- Tests vagal (parasympathetic) efferent limb via the cardiac vagus nerve
— Adams & Victor's Principles of Neurology, 12th Ed., p. 551; Costanzo Physiology 7th Ed., p. 172
Sources:
- Costanzo Physiology, 7th Edition — pp. 69–70
- Medical Physiology (Boron & Boulpaep) — p. 512
- Adams & Victor's Principles of Neurology, 12th Ed. — p. 551
- Guyton & Hall Textbook of Medical Physiology — Ch. 18
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