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Questions and Answers
What is the primary function of M2-receptors?
What is the primary function of M2-receptors?
M3-receptors are responsible for causing contraction of skeletal muscle.
M3-receptors are responsible for causing contraction of skeletal muscle.
False
Where are NM-cholinoceptors primarily located?
Where are NM-cholinoceptors primarily located?
Neuromuscular junction
Cholinergic drugs such as __________ can affect the secretion of gastric parietal cells.
Cholinergic drugs such as __________ can affect the secretion of gastric parietal cells.
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Match the following cholinergic drugs with their classifications:
Match the following cholinergic drugs with their classifications:
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What effect does acetylcholine have on blood vessels?
What effect does acetylcholine have on blood vessels?
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Nicotinic receptors are involved in the contraction of skeletal muscle.
Nicotinic receptors are involved in the contraction of skeletal muscle.
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Which factor primarily mediates vasodilation in blood vessels when acetylcholine acts?
Which factor primarily mediates vasodilation in blood vessels when acetylcholine acts?
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What is the primary neurotransmitter of the parasympathetic system?
What is the primary neurotransmitter of the parasympathetic system?
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The release of acetylcholine occurs only in the autonomic nervous system.
The release of acetylcholine occurs only in the autonomic nervous system.
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Name one drug that inhibits the release of acetylcholine.
Name one drug that inhibits the release of acetylcholine.
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A specific acetylcholineesterase hydrolyses acetylcholine very ________.
A specific acetylcholineesterase hydrolyses acetylcholine very ________.
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Match the following cholinergic receptors with their effects:
Match the following cholinergic receptors with their effects:
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Which of the following locations contains nonspecific butyrylcholinesterase?
Which of the following locations contains nonspecific butyrylcholinesterase?
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Cholinergic receptors are only found in the central nervous system.
Cholinergic receptors are only found in the central nervous system.
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What blocks the transport of Ach into the synaptic vesicle?
What blocks the transport of Ach into the synaptic vesicle?
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Which medication primarily targets post-operative gastric distension and has no nicotinic actions?
Which medication primarily targets post-operative gastric distension and has no nicotinic actions?
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Carbachol has both muscarinic and nicotinic actions and is used for the treatment of glaucoma.
Carbachol has both muscarinic and nicotinic actions and is used for the treatment of glaucoma.
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What is the primary therapeutic use of Methacholine?
What is the primary therapeutic use of Methacholine?
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Bethanechol is used for urinary retention and __________.
Bethanechol is used for urinary retention and __________.
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Match the following drugs with their specific actions or uses:
Match the following drugs with their specific actions or uses:
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Which of the following adverse effects is NOT typically associated with cholinergic drugs?
Which of the following adverse effects is NOT typically associated with cholinergic drugs?
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Acetylcholine is less susceptible to hydrolysis by acetylcholinesterase compared to Methacholine.
Acetylcholine is less susceptible to hydrolysis by acetylcholinesterase compared to Methacholine.
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What is the mechanism of action of acetylcholine at the neuromuscular junction?
What is the mechanism of action of acetylcholine at the neuromuscular junction?
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Study Notes
Cholinergic System
- Acetylcholine (ACh) is a neurotransmitter of the parasympathetic system.
- Nerves synthesizing, storing, and releasing ACh are called "cholinergic."
Sites of ACh Release
- Ganglia: All pre-ganglionic fibers of the autonomic nervous system (ANS), both sympathetic and parasympathetic.
- Postganglionic parasympathetic nerve endings
- Sweat glands: Sympathetic postganglionic nerve endings supplying sweat glands
- Skeletal muscles: Somatic nerve endings supplying skeletal muscles
- Adrenal medulla
- Central nervous system (CNS): brain & spinal cord
Acetylcholine (ACh)
- Major neurotransmitter of autonomic and somatic sites
- Hemicholinium blocks choline uptake in axoplasm
- Transport of ACh into synaptic vesicles is blocked by vesamicol
- Botulinum toxin inhibits ACh release, relieving spasms in spastic disorders and facial wrinkles
Termination of ACh Action
- Specific acetylcholinesterase hydrolyzes the ester bond of choline very rapidly at cholinergic sites, RBCs, and gray matter.
- Nonspecific butyrylcholinesterase (pseudo) hydrolyzes ACh slowly in plasma, liver, and intestines, and white matter.
Cholinergic Receptor
- Muscarine alkaloids mimic parasympathetic nerve discharge but not in autonomic ganglia.
- Nicotinic alkaloids stimulate autonomic ganglia, neuromuscular junctions, but not other autonomic effector cells.
Classification of M-Cholinoceptors
- Classified based on molecular structure, signal transduction, and ligand affinity.
- M1-receptors are found on nerve cells (e.g., ganglia), facilitating impulse transmission from preganglionic axons to ganglion cells and in gastric parietal cells.
- M2-receptors mediate ACh effects on the heart, slowing diastolic depolarization in sinoatrial pacemaker cells and decreasing heart rate by opening K+ channels.
- M3-receptors are in glandular epithelia, increasing secretory activity, and smooth muscles. In smooth vessels, they trigger vasodilation indirectly through the release of nitric oxide (NO). In the CNS, these receptors have diverse functions, including regulation of cortical excitability, memory, learning, pain processing, and brain stem motor control.
Characteristics of Nicotinic Receptors
- NM-cholinoceptors: Found at the neuromuscular junction, causing depolarization of the muscle end plate and skeletal muscle contraction.
- NN-cholinoceptors: Located in autonomic ganglia, depolarizing the postganglionic membrane (in the adrenal medulla, causing catecholamine release).
Cholinergic Drugs
- Choline esters: Acetylcholine, Bethanechol, Carbachol, Methacholine.
- Alkaloids: Muscarine, Arecoline, Pilocarpine.
Acetylcholine (Prototype)
- Orally destroyed; intravenously administered, quickly destroyed by pseudocholinesterase.
- Nonspecific action.
- Rarely used therapeutically.
Physiology of ACh & Action in Vitro Preparations
- Eye (M3): Sphincter muscle of the iris (miosis) and ciliary muscle contraction for near vision.
- Heart (M2): SA node hyperpolarization, slowing of the rate of impulse generation; decreased AV node conduction velocity; small decrease in ventricular contractile strength.
- Blood vessels (M3): Vasodilation (mediated through endothelium-derived relaxing factor, EDRF, which is nitric oxide).
- Smooth muscle (M3): Tone and peristalsis in the gastrointestinal (GI) tract; sphincter relaxation; evacuation of bowels.
- Bronchial smooth muscle: Bronchoconstriction
Other Effects
- Detrusor muscle: Contract, allowing bladder emptying
- Glands: (e.g. sweating, saliva, lacrimation)
Skeletal Muscle (NM)
- ACh at the muscle end plate causes contraction of muscle fiber
- High doses can lead to muscle twitching and fasciculations
ACh Substitutes
- Effective orally; more selective in action.
- Methacholine, Carbachol, Bethanechol are less susceptible to acetylcholinesterase, and resistant to pseudocholinesterase.
- Bethanechol primarily affects M3 receptors.
Carbachol
- Ester of carbamic acid
- Muscarinic and nicotinic actions
- Prolonged duration of action (DOA)
- Therapeutic use: glaucoma
Methacholine
- Has a methyl group
- Primarily muscarinic actions
- Longer DOA than acetylcholine
- Therapeutic use: Relief of paroxysmal atrial tachycardia (given as a subcutaneous injection)
Bethanechol Chloride
- Structure related to acetylcholine; acetate replaced by carbamate and choline is methylated.
- Muscarinic effects prevalent
- Prolonged duration of action
- Therapeutic uses: post-operative gastric distension, paralytic ileus and bladder atonia
Adverse Effects
- Extension of pharmacological action
- Flushing, salivation, sweating, bradycardia, bronchospasm, hypotension
- Postoperative paralytic ileus, urinary retention, & abdominal distension
- Bethanechol generally preferred due to a wider safety margin.
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Description
Explore the key aspects of the cholinergic system, including the synthesis, release, and action of acetylcholine (ACh), the major neurotransmitter involved in both the autonomic and somatic nervous systems. This quiz covers the sites of ACh release, its functions, and mechanisms of action. Test your understanding of cholinergic signaling and its significance in neuromuscular transmission.
