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Muscarinic acetylcholine receptors, or mAChRs, are
acetylcholine receptor
An acetylcholine receptor (abbreviated AChR) is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter.
Classification
Like other transmembrane receptors, acetylcholine receptors are classified according ...
s that form
G protein-coupled receptor complexes in the
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
s of certain
neurons
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
and other
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
s. They play several roles, including acting as the main end-receptor stimulated by
acetylcholine
Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Part ...
released from
postganglionic fibers
In the autonomic nervous system, fibers from the ganglion to the effector organ are called postganglionic fibers.
Neurotransmitters
The neurotransmitters of postganglionic fibers differ:
* In the parasympathetic division, neurons are ''choli ...
in the
parasympathetic nervous system
The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...
.
Muscarinic receptors are so named because they are more sensitive to
muscarine
Muscarine, L-(+)-muscarine, or muscarin is a natural product found in certain mushrooms, particularly in ''Inocybe'' and ''Clitocybe'' species, such as the deadly '' C. dealbata''. Mushrooms in the genera ''Entoloma'' and ''Mycena'' have al ...
than to
nicotine
Nicotine is a natural product, naturally produced alkaloid in the nightshade family of plants (most predominantly in tobacco and ''Duboisia hopwoodii'') and is widely used recreational drug use, recreationally as a stimulant and anxiolytic. As ...
.
Their counterparts are
nicotinic acetylcholine receptor
Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral ne ...
s (nAChRs), receptor ion channels that are also important in the
autonomic nervous system
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...
. Many drugs and other substances (for example
pilocarpine
Pilocarpine is a medication used to reduce pressure inside the eye and treat dry mouth. As eye drops it is used to manage angle closure glaucoma until surgery can be performed, ocular hypertension, primary open angle glaucoma, and to bring abou ...
and
scopolamine
Scopolamine, also known as hyoscine, or Devil's Breath, is a natural or synthetically produced tropane alkaloid and anticholinergic drug that is formally used as a medication for treating motion sickness and postoperative nausea and vomiting ...
) manipulate these two distinct receptors by acting as selective
agonist
An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the ago ...
s or
antagonist
An antagonist is a character in a story who is presented as the chief foe of the protagonist.
Etymology
The English word antagonist comes from the Greek ἀνταγωνιστής – ''antagonistēs'', "opponent, competitor, villain, enemy, riv ...
s.
Function
Acetylcholine (ACh) is a
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.
Neuro ...
found in the
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
,
neuromuscular junctions
A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber.
It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction.
Muscles require innervation to ...
and the
autonomic ganglia
An autonomic ganglion is a cluster of nerve cell bodies (a ganglion) in the autonomic nervous system
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that s ...
.
Muscarinic receptors are used in the following roles:
Recovery receptors
ACh is always used as the
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.
Neuro ...
within the
autonomic ganglion
An autonomic ganglion is a cluster of nerve cell bodies (a ganglion) in the autonomic nervous system
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that s ...
. Nicotinic receptors on the postganglionic neuron are responsible for the initial fast depolarization (Fast
EPSP) of that neuron. As a consequence of this, nicotinic receptors are often cited as ''the receptor on the postganglionic neurons at the ganglion''. However, the subsequent hyperpolarization (
IPSP) and slow depolarization (Slow EPSP) that represent the recovery of the postganglionic neuron from stimulation are actually mediated by ''muscarinic'' receptors, types M
2 and M
1 respectively (discussed below).
Peripheral autonomic fibers (sympathetic and parasympathetic fibers) are categorized anatomically as either preganglionic or
postganglionic fiber
In the autonomic nervous system, fibers from the ganglion to the effector organ are called postganglionic fibers.
Neurotransmitters
The neurotransmitters of postganglionic fibers differ:
* In the parasympathetic division, neurons are ''choli ...
s, then further generalized as either adrenergic fibers, releasing noradrenaline, or cholinergic fibers, both releasing acetylcholine and expressing acetylcholine receptors. Both preganglionic sympathetic fibers and preganglionic parasympathetic fibers are cholinergic. Most postganglionic sympathetic fibers are adrenergic: their neurotransmitter is norepinephrine except postganglionic sympathetic fibers to the sweat glands, piloerectile muscles of the body hairs, and the skeletal muscle arterioles do not use adrenaline/noradrenaline.
The
adrenal medulla
The adrenal medulla ( la, medulla glandulae suprarenalis) is part of the adrenal gland. It is located at the center of the gland, being surrounded by the adrenal cortex. It is the innermost part of the adrenal gland, consisting of chromaffin cel ...
is considered a sympathetic ganglion and, like other sympathetic ganglia, is supplied by cholinergic preganglionic sympathetic fibers: acetylcholine is the neurotransmitter utilized at this synapse. The
chromaffin cells
Chromaffin cells, also called pheochromocytes (or phaeochromocytes), are neuroendocrine cells found mostly in the medulla of the adrenal glands in mammals. These cells serve a variety of functions such as serving as a response to stress, monitori ...
of the adrenal medulla act as "modified neurons", releasing adrenaline and noradrenaline into the bloodstream as hormones instead of as neurotransmitters. The other postganglionic fibers of the peripheral autonomic system belong to the parasympathetic division; all are cholinergic fibers, and use acetylcholine as the neurotransmitter.
Postganglionic neurons
Another role for these receptors is at the junction of the innervated tissues and the postganglionic neurons in the parasympathetic division of the autonomic nervous system. Here acetylcholine is again used as a neurotransmitter, and ''muscarinic'' receptors form the principal receptors on the innervated tissue.
Innervated tissue
Very few parts of the sympathetic system use cholinergic receptors. In sweat glands the receptors are of the ''muscarinic'' type. The sympathetic nervous system also has some preganglionic nerves terminating at the
chromaffin cell
Chromaffin cells, also called pheochromocytes (or phaeochromocytes), are neuroendocrine cells found mostly in the medulla of the adrenal glands in mammals. These cells serve a variety of functions such as serving as a response to stress, monitor ...
s in the
adrenal medulla
The adrenal medulla ( la, medulla glandulae suprarenalis) is part of the adrenal gland. It is located at the center of the gland, being surrounded by the adrenal cortex. It is the innermost part of the adrenal gland, consisting of chromaffin cel ...
, which secrete
epinephrine
Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands and ...
and
norepinephrine
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad'', ...
into the bloodstream. Some believe that chromaffin cells are modified postganglionic CNS fibers. In the adrenal medulla, acetylcholine is used as a neurotransmitter, and the receptor is of the ''nicotinic'' type.
The
somatic nervous system
The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.
The somatic nervous system consists of sensory nerves carryi ...
uses a
nicotinic
Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral ner ...
receptor to acetylcholine at the neuromuscular junction.
Higher central nervous system
Muscarinic acetylcholine receptors are also present and distributed throughout the local nervous system, in post-synaptic and pre-synaptic positions. There is also some evidence for
postsynaptic
Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous sys ...
receptors on sympathetic neurons allowing the parasympathetic nervous system to inhibit sympathetic effects.
Presynaptic membrane of the neuromuscular junction
It is known that muscarinic acetylcholine receptors also appear on the pre-synaptic membrane of somatic neurons in the neuro-muscular junction, where they are involved in the regulation of acetylcholine release.
Form of muscarinic receptors
Muscarinic acetylcholine receptors belong to a class of
metabotropic receptor
A metabotropic receptor, also referred to by the broader term G-protein-coupled receptor, is a type of membrane receptor that initiates a number of metabolic steps to modulate cell activity. The nervous system utilizes two types of receptors: met ...
s that use
G protein
G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their act ...
s as their signaling mechanism. In such receptors, the signaling molecule (the
ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
) binds to a monomeric
receptor
Receptor may refer to:
* Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse
*Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ...
that has
seven transmembrane region
G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
s; in this case, the ligand is ACh. This receptor is bound to intracellular proteins, known as G proteins, which begin the information cascade within the cell.
By contrast, nicotinic receptors form pentameric complexes and use a
ligand-gated ion channel
Ligand-gated ion channels (LICs, LGIC), also commonly referred to as ionotropic receptors, are a group of transmembrane ion-channel proteins which open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in res ...
mechanism for signaling. In this case, binding of the ligands with the receptor causes an
ion channel
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of io ...
to open, permitting either one or more specific types of ions (e.g., K
+, Na
+, Ca
2+) to diffuse into or out of the cell.
Receptor isoforms
Classification
By the use of selective radioactively labeled agonist and antagonist substances, five subtypes of muscarinic receptors have been determined, named M
1–M
5 (using an upper case M and subscript number).
M1,
M3,
M5 receptors are coupled with
Gq proteins, while
M2 and
M4 receptors are coupled with G
i/o proteins.
There are other classification systems. For example, the drug
pirenzepine
Pirenzepine (Gastrozepin), an M1 selective antagonist, is used in the treatment of peptic ulcers, as it reduces gastric acid secretion and reduces muscle spasm. It is in a class of drugs known as muscarinic receptor antagonists - acetylcholine ...
is a muscarinic antagonist (decreases the effect of ACh), which is much more potent at M
1 receptors than it is at other subtypes. The acceptance of the various subtypes proceeded in numerical order, therefore, earlier sources may recognize only M
1 and M
2 subtypes, while later studies recognize M
3, M
4and most recently M
5 subtypes.
Genetic differences
Meanwhile,
geneticist
A geneticist is a biologist or physician who studies genetics, the science of genes, heredity, and variation of organisms. A geneticist can be employed as a scientist or a lecturer. Geneticists may perform general research on genetic processe ...
s and
molecular biologist
Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...
s have characterised five genes that appear to encode muscarinic receptors, named m1-m5 (lowercase m; no subscript number). They code for pharmacologic types M
1-M
5. The receptors m1 and m2 were determined based upon partial sequencing of M
1 and M
2 receptor proteins. The others were found by searching for homology, using
bioinformatic
Bioinformatics () is an interdisciplinary field that develops methods and software tools for understanding biological data, in particular when the data sets are large and complex. As an interdisciplinary field of science, bioinformatics combine ...
techniques.
Difference in G proteins
G proteins contain an alpha-subunit that is critical to the functioning of receptors. These subunits can take a number of forms. There are four broad classes of form of G-protein: G
s, G
i, G
q, and G
12/13.
Muscarinic receptors vary in the G protein to which they are bound, with some correlation according to receptor type. G proteins are also classified according to their susceptibility to
cholera toxin
Cholera toxin (also known as choleragen and sometimes abbreviated to CTX, Ctx or CT) is AB5 multimeric protein complex secreted by the bacterium ''Vibrio cholerae''. CTX is responsible for the massive, watery diarrhea characteristic of cholera ...
(CTX) and
pertussis toxin
Pertussis toxin (PT) is a protein-based AB5-type exotoxin produced by the bacterium ''Bordetella pertussis'', which causes whooping cough. PT is involved in the colonization of the respiratory tract and the establishment of infection. Res ...
(PTX, whooping cough). G
s and some subtypes of G
i (G
αt and G
αg) are susceptible to CTX. Only G
i is susceptible to PTX, with the exception of one subtype of G
i (G
αz) which is immune. Also, only when bound with an agonist, those G proteins normally sensitive to PTX also become susceptible to CTX.
The various G-protein subunits act differently upon secondary messengers, upregulating Phospholipases, downregulating cAMP, and so on.
Because of the strong correlations to muscarinic receptor type, CTX and PTX are useful experimental tools in investigating these receptors.
M1 receptor
This receptor is found mediating slow
EPSP at the ganglion in the postganglionic nerve, is common in
exocrine gland
Exocrine glands are glands that secrete substances on to an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucous. Exocrine glands are one of two ...
s and in the CNS.
It is predominantly found bound to G proteins of class
Gq,
which use upregulation of
phospholipase
A phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. Acids trigger the release of bound calcium from cellular stores and the consequent increase in free cytosolic Ca2+, an essential step in ...
C and, therefore,
inositol trisphosphate
Inositol trisphosphate or inositol 1,4,5-trisphosphate abbreviated InsP3 or Ins3P or IP3 is an inositol phosphate signaling molecule. It is made by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid that is located in the p ...
and intracellular calcium as a signaling pathway. A receptor so bound would not be susceptible to CTX or PTX. However, G
i (causing a downstream decrease in
cAMP
Camp may refer to:
Outdoor accommodation and recreation
* Campsite or campground, a recreational outdoor sleeping and eating site
* a temporary settlement for nomads
* Camp, a term used in New England, Northern Ontario and New Brunswick to descri ...
) and G
s (causing an increase in cAMP) have also been shown to be involved in interactions in certain tissues, and so would be susceptible to PTX and CTX, respectively.
M2 receptor
The M
2 muscarinic receptors are located in the heart and lungs. In the heart, they act to slow the
heart rate
Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...
down below the normal baseline
sinus rhythm
A sinus rhythm is any cardiac rhythm in which depolarisation of the cardiac muscle begins at the sinus node. It is characterised by the presence of correctly oriented P waves on the electrocardiogram (ECG). Sinus rhythm is necessary, but not su ...
, by slowing the speed of
depolarization
In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is esse ...
. In humans, under resting conditions, vagal activity dominates over sympathetic activity. Hence, inhibition of M
2 receptors (e.g. by atropine) will cause a raise in heart rate. They also moderately reduce contractile forces of the
atrial
The atrium ( la, ātrium, , entry hall) is one of two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular valves.
There are two atr ...
cardiac muscle
Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle th ...
, and reduce conduction velocity of the
atrioventricular node
The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
(AV node). It also serves to slightly decrease the contractile forces of the
ventricular muscle.
M
2 muscarinic receptors act via a
Gi type receptor, which causes a decrease in cAMP in the cell, inhibition of voltage-gated Ca
2+ channels, and increasing efflux of K
+, in general, leading to inhibitory-type effects.
M3 receptor
The M
3 muscarinic receptors are located at many places in the body. They are located in the smooth muscles of the blood vessels, as well as in the lungs. Because the M
3 receptor is G
q-coupled and mediates an increase in intracellular calcium, it typically causes contraction of smooth muscle, such as that observed during
bronchoconstriction
Bronchoconstriction is the constriction of the airways in the lungs due to the tightening of surrounding smooth muscle, with consequent coughing, wheezing, and shortness of breath.
Causes
The condition has a number of causes, the most common bei ...
and
bladder voiding.
However, with respect to vasculature, activation of M
3 on vascular endothelial cells causes increased synthesis of
nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
,
which diffuses to adjacent vascular smooth muscle cells and causes their relaxation, thereby explaining the paradoxical effect of
parasympathomimetic
A parasympathomimetic drug, sometimes called a cholinomimetic drug or cholinergic receptor stimulating agent, is a substance that stimulates the parasympathetic nervous system (PSNS). These chemicals are also called cholinergic drugs because acet ...
s on vascular tone and bronchiolar tone. Indeed, direct stimulation of vascular smooth muscle, M
3 mediates vasoconstriction in diseases wherein the vascular endothelium is disrupted.
The M
3 receptors are also located in many glands, which help to stimulate secretion in, for example, the salivary glands, as well as other glands of the body.
Like the M
1 muscarinic receptor, M
3 receptors are G proteins of class
Gq that upregulate
phospholipase C
Phospholipase C (PLC) is a class of membrane-associated enzymes that cleave phospholipids just before the phosphate group (see figure). It is most commonly taken to be synonymous with the human forms of this enzyme, which play an important role ...
and, therefore,
inositol trisphosphate
Inositol trisphosphate or inositol 1,4,5-trisphosphate abbreviated InsP3 or Ins3P or IP3 is an inositol phosphate signaling molecule. It is made by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), a phospholipid that is located in the p ...
and intracellular calcium as a signaling pathway.
M4 receptor
M
4 receptors are found in the CNS.
M
4 receptors work via
Gi receptors to decrease cAMP in the cell and, thus, produce generally inhibitory effects. Possible
bronchospasm
Bronchospasm or a bronchial spasm is a sudden constriction of the muscles in the walls of the bronchioles. It is caused by the release (degranulation) of substances from mast cells or basophils under the influence of anaphylatoxins. It causes di ...
may result if stimulated by
muscarinic agonist
A muscarinic agonist is an agent that activates the activity of the muscarinic acetylcholine receptor. The muscarinic receptor has different subtypes, labelled M1-M5, allowing for further differentiation.
Clinical significance
M1
M1-type musca ...
s
M5 receptor
Location of M
5 receptors is not well known.
Like the M
1 and M
3 muscarinic receptor, M
5 receptors are coupled with G proteins of class
Gq that upregulate phospholipase C and, therefore, inositol trisphosphate and intracellular calcium as a signaling pathway.
Pharmacological application
Ligands
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electro ...
targeting the mAChR that are currently approved for clinical use include non-selective antagonists for the treatment of
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
,
atropine
Atropine is a tropane alkaloid and anticholinergic medication used to treat certain types of nerve agent and pesticide poisonings as well as some types of slow heart rate, and to decrease saliva production during surgery. It is typically given i ...
(to dilate the
pupil
The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...
),
scopolamine
Scopolamine, also known as hyoscine, or Devil's Breath, is a natural or synthetically produced tropane alkaloid and anticholinergic drug that is formally used as a medication for treating motion sickness and postoperative nausea and vomiting ...
(used to prevent
motion sickness
Motion sickness occurs due to a difference between actual and expected motion. Symptoms commonly include nausea, vomiting, cold sweat, headache, dizziness, tiredness, loss of appetite, and increased salivation. Complications may rarely include d ...
), and
ipratropium
Ipratropium bromide, sold under the trade name Atrovent among others, is a type of anticholinergic (SAMA: short acting muscarinic antagonist) medication which opens up the medium and large airways in the lungs. It is used to treat the symptoms ...
(used in the treatment of
COPD
Chronic obstructive pulmonary disease (COPD) is a type of progressive lung disease characterized by long-term respiratory symptoms and airflow limitation. The main symptoms include shortness of breath and a cough, which may or may not produce mu ...
).
See also
*
Muscarinic agonist
A muscarinic agonist is an agent that activates the activity of the muscarinic acetylcholine receptor. The muscarinic receptor has different subtypes, labelled M1-M5, allowing for further differentiation.
Clinical significance
M1
M1-type musca ...
*
Muscarinic antagonist
A muscarinic receptor antagonist (MRA) is a type of anticholinergic agent that blocks the activity of the muscarinic acetylcholine receptor. The muscarinic receptor is a protein involved in the transmission of signals through certain parts of the ...
*
Nicotinic acetylcholine receptor
Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral ne ...
*
Nicotinic agonist
A nicotinic agonist is a drug that mimics the action of acetylcholine (ACh) at nicotinic acetylcholine receptors (nAChRs). The nAChR is named for its affinity for nicotine.
Examples include nicotine (by definition), acetylcholine (the endogenous a ...
*
Nicotinic antagonist
A nicotinic antagonist is a type of anticholinergic drug that inhibits the action of acetylcholine (ACh) at nicotinic acetylcholine receptors. These compounds are mainly used for peripheral muscle paralysis in surgery, the classical agent of this ...
*
Vagal escape
The Sympathetic nervous system and Parasympathetic nervous system can offset each other. One of the most classical example is called Vagal Escape. Vagal escape is characterized by a reduction in blood pressure due to muscarinic
Muscarinic ace ...
References
External links
Acetylcholine Receptors (Muscarinic)*
{{DEFAULTSORT:Muscarinic Acetylcholine Receptor
Neurochemistry
Neurophysiology
Acetylcholine receptors