Autonomic Nervous System Receptors (Debunked from Barash 9th edition)
General Overview
• Receptors: Protein macromolecules located on the plasma membrane that mediate biological
responses upon activation by agonists.
o Agonist: A substance that interacts with a receptor to evoke a biological response (e.g.,
ACh, NE, EPI, DA, ATP in the ANS).
o Antagonist: A substance that inhibits a receptor's response to an agonist.
• Receptor Density: Thousands of receptors are present on a single cell, with ∼25,000 single cells
innervated by one neuron.
Cholinergic Receptors
• Neurotransmitter: Acetylcholine (ACh).
• Classes: Based on location and response to agonists/antagonists.
o Mediate impulses in:
▪ Peripheral Nervous System (PNS).
▪ Ganglia of the Sympathetic Nervous System (SNS).
▪ Neuroeffector junctions in skeletal muscle.
Subtypes of Cholinergic Receptors:
1. Muscarinic Receptors:
o Location:
▪ Postganglionic PNS junctions of cardiac/smooth muscle.
▪ Presynaptic membranes of sympathetic nerve terminals in the myocardium,
coronary vessels, and peripheral vasculature.
o Activation Effects:
▪ Bradycardia, bronchoconstriction, miosis, salivation, gastrointestinal
hypermotility, increased gastric acid secretion.
o Blockade: Atropine blocks muscarinic receptors without affecting nicotinic receptors,
potentially increasing SNS activity by augmenting NE release.
o Presynaptic Function:
▪ Inhibits NE release via adrenergic muscarinic receptors.
▪ Atropine can disrupt this inhibition, leading to sympathomimetic effects.
2. Nicotinic Receptors:
o Location:
▪ Synaptic junctions of SNS and PNS ganglia.
▪ Neuroeffector junction of skeletal muscle.
o Activation Effects:
▪ Low doses stimulate ANS ganglia.
▪ High doses block ganglia, leading to neuromuscular weakness and hypotension.
▪ In the SNS: Stimulation causes hypertension and tachycardia by releasing EPI
and NE.
,Adrenergic Receptors
• Neurotransmitters: Epinephrine (EPI), Norepinephrine (NE), Dopamine (DA).
• Classification: Based on responsiveness to EPI or NE.
Subtypes of Adrenergic Receptors:
1. Alpha (α) Receptors:
o Subdivisions:
▪ α1 (α1A, α1B, α1D): Postjunctional, involved in vasoconstriction and smooth
muscle contraction.
▪ α2 (α2A, α2B, α2C): Prejunctional, inhibit NE release.
2. Beta (β) Receptors:
o Subdivisions:
▪ β1: Increases cardiac contractility and heart rate.
▪ β2: Mediates bronchodilation and vasodilation.
▪ β3: Involved in lipolysis.
3. Dopamine (DA) Receptors:
o Subdivisions: DA1, DA2.
o Location: CNS, renal, mesenteric, and coronary vessels.
o Function: Controversial in peripheral systems but important in CNS. DA can stimulate α
and β receptors dose-dependently.
Key Concepts
1. Sympathomimetic Effects:
o Agonists and antagonists targeting adrenergic receptors influence sympathetic activity,
including heart rate, vasodilation, or vasoconstriction.
2. Pre- and Postsynaptic Adrenergic Receptors:
o Prejunctional (α2): Regulate neurotransmitter release.
o Postjunctional (α1, β1, β2): Mediate physiological responses in effector tissues.
3. Cholinergic and Adrenergic Interplay:
o Close associations between SNS and PNS terminals allow interactions, such as ACh
inhibiting NE release at adrenergic muscarinic receptors.
Summary of Clinical Importance
• Cholinergic Drugs:
o Muscarinic blockers (e.g., atropine) impact both the SNS and PNS.
o Neuromuscular blocking agents mimic effects at nicotinic receptors.
• Adrenergic Drugs:
, o Differentiation in effects arises from variations in α or β receptor activity.
o DA antagonists (e.g., haloperidol) modify dopamine receptor activity.
• Receptor Distribution:
o Varies by organ, affecting the intensity and type of physiological response.
Receptors Overview
• Definition: Receptors are protein macromolecules on the plasma membrane. They act as
target sites for agonists to evoke a biological response.
• Agonist: Substances like ACh, NE, EPI, DA, and ATP that interact with receptors to
evoke responses.
• Antagonist: Substances that block the evocation of responses at receptor sites by
agonists.
• Significance:
o One neuron can innervate up to ~25,000 single cells.
o Several thousand receptors can be present in a single cell.
Cholinergic Receptors
• Mediated by acetylcholine (ACh), they are classified into:
o Muscarinic receptors
o Nicotinic receptors
Muscarinic Receptors
Receptor Location Agonist Antagonist Effects
Postganglionic PNS Bradycardia, decreased
(cardiac & smooth inotropism, bronchoconstriction,
Muscarine,
Muscarinic muscle), presynaptic Atropine miosis, salivation, GI
ACh
sympathetic nerve hypermotility, gastric acid
terminals secretion
Sympathetic nerve Inhibits NE release (similar to
Adrenergic terminals (e.g., α2 stimulation); blockade
ACh Atropine
Muscarinic myocardium, coronary increases NE release,
vessels) augmenting SNS activity
Nicotinic Receptors
General Overview
• Receptors: Protein macromolecules located on the plasma membrane that mediate biological
responses upon activation by agonists.
o Agonist: A substance that interacts with a receptor to evoke a biological response (e.g.,
ACh, NE, EPI, DA, ATP in the ANS).
o Antagonist: A substance that inhibits a receptor's response to an agonist.
• Receptor Density: Thousands of receptors are present on a single cell, with ∼25,000 single cells
innervated by one neuron.
Cholinergic Receptors
• Neurotransmitter: Acetylcholine (ACh).
• Classes: Based on location and response to agonists/antagonists.
o Mediate impulses in:
▪ Peripheral Nervous System (PNS).
▪ Ganglia of the Sympathetic Nervous System (SNS).
▪ Neuroeffector junctions in skeletal muscle.
Subtypes of Cholinergic Receptors:
1. Muscarinic Receptors:
o Location:
▪ Postganglionic PNS junctions of cardiac/smooth muscle.
▪ Presynaptic membranes of sympathetic nerve terminals in the myocardium,
coronary vessels, and peripheral vasculature.
o Activation Effects:
▪ Bradycardia, bronchoconstriction, miosis, salivation, gastrointestinal
hypermotility, increased gastric acid secretion.
o Blockade: Atropine blocks muscarinic receptors without affecting nicotinic receptors,
potentially increasing SNS activity by augmenting NE release.
o Presynaptic Function:
▪ Inhibits NE release via adrenergic muscarinic receptors.
▪ Atropine can disrupt this inhibition, leading to sympathomimetic effects.
2. Nicotinic Receptors:
o Location:
▪ Synaptic junctions of SNS and PNS ganglia.
▪ Neuroeffector junction of skeletal muscle.
o Activation Effects:
▪ Low doses stimulate ANS ganglia.
▪ High doses block ganglia, leading to neuromuscular weakness and hypotension.
▪ In the SNS: Stimulation causes hypertension and tachycardia by releasing EPI
and NE.
,Adrenergic Receptors
• Neurotransmitters: Epinephrine (EPI), Norepinephrine (NE), Dopamine (DA).
• Classification: Based on responsiveness to EPI or NE.
Subtypes of Adrenergic Receptors:
1. Alpha (α) Receptors:
o Subdivisions:
▪ α1 (α1A, α1B, α1D): Postjunctional, involved in vasoconstriction and smooth
muscle contraction.
▪ α2 (α2A, α2B, α2C): Prejunctional, inhibit NE release.
2. Beta (β) Receptors:
o Subdivisions:
▪ β1: Increases cardiac contractility and heart rate.
▪ β2: Mediates bronchodilation and vasodilation.
▪ β3: Involved in lipolysis.
3. Dopamine (DA) Receptors:
o Subdivisions: DA1, DA2.
o Location: CNS, renal, mesenteric, and coronary vessels.
o Function: Controversial in peripheral systems but important in CNS. DA can stimulate α
and β receptors dose-dependently.
Key Concepts
1. Sympathomimetic Effects:
o Agonists and antagonists targeting adrenergic receptors influence sympathetic activity,
including heart rate, vasodilation, or vasoconstriction.
2. Pre- and Postsynaptic Adrenergic Receptors:
o Prejunctional (α2): Regulate neurotransmitter release.
o Postjunctional (α1, β1, β2): Mediate physiological responses in effector tissues.
3. Cholinergic and Adrenergic Interplay:
o Close associations between SNS and PNS terminals allow interactions, such as ACh
inhibiting NE release at adrenergic muscarinic receptors.
Summary of Clinical Importance
• Cholinergic Drugs:
o Muscarinic blockers (e.g., atropine) impact both the SNS and PNS.
o Neuromuscular blocking agents mimic effects at nicotinic receptors.
• Adrenergic Drugs:
, o Differentiation in effects arises from variations in α or β receptor activity.
o DA antagonists (e.g., haloperidol) modify dopamine receptor activity.
• Receptor Distribution:
o Varies by organ, affecting the intensity and type of physiological response.
Receptors Overview
• Definition: Receptors are protein macromolecules on the plasma membrane. They act as
target sites for agonists to evoke a biological response.
• Agonist: Substances like ACh, NE, EPI, DA, and ATP that interact with receptors to
evoke responses.
• Antagonist: Substances that block the evocation of responses at receptor sites by
agonists.
• Significance:
o One neuron can innervate up to ~25,000 single cells.
o Several thousand receptors can be present in a single cell.
Cholinergic Receptors
• Mediated by acetylcholine (ACh), they are classified into:
o Muscarinic receptors
o Nicotinic receptors
Muscarinic Receptors
Receptor Location Agonist Antagonist Effects
Postganglionic PNS Bradycardia, decreased
(cardiac & smooth inotropism, bronchoconstriction,
Muscarine,
Muscarinic muscle), presynaptic Atropine miosis, salivation, GI
ACh
sympathetic nerve hypermotility, gastric acid
terminals secretion
Sympathetic nerve Inhibits NE release (similar to
Adrenergic terminals (e.g., α2 stimulation); blockade
ACh Atropine
Muscarinic myocardium, coronary increases NE release,
vessels) augmenting SNS activity
Nicotinic Receptors
