Logan Steele
Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, where it binds to and activates two muscarinic receptor subtypes, M2 and M3, thus causing smooth muscle excitation and contraction. Acetylcholine is released from short postganglionic nerves that supply parasympathetic innervation to smooth muscles of visceral organs, such as the airways, the urinary bladder, and the myometrium. The neurotransmitter stimulates muscarinic receptors, leading to the activation of cationic channels and the generation of mICAT, which depolarizes the cells and provides the critical trigger for stimulation of voltage-dependent Ca2+ channels. The ensuing influx of Ca2+ is essential for contraction and intestinal motility.
| Characteristics | Values |
|---|---|
| Main neurotransmitter of | Parasympathetic nervous system |
| Receptors activated | Muscarinic cholinergic receptors (M2 and M3) |
| Receptor subtypes | M1, M2, M3, M4, M5 |
| Contraction mechanism | Excitation-contraction coupling |
| Contraction initiation | Opening of ion channels selective for monovalent cations (Na+, K+) |
| Other ions | Influx of Ca2+ |
| Other receptors | Nicotinic receptors |
| Effect on K+ current | Activates and then suppresses spontaneous K+ current transients |
| Role in asthma | Hyperresponsiveness due to excessive contraction of airway smooth muscle |
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What You'll Learn
- Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles
- Muscarinic receptors play a key role in the parasympathetic nervous control of various peripheral tissues
- Muscarinic receptor activation evokes calcium release from stores, raising internal calcium concentration
- Acetylcholine activates and then suppresses spontaneous K+ current transients
- Muscarinic receptor subtypes M2 and M3 act in synergy to open TRPC4 channels, initiating membrane depolarization
Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles
Acetylcholine (ACh) is a neurotransmitter that acts as a chemical messenger between neurons, allowing them to communicate with each other and with other specialized cells. It is most commonly associated with the neuromuscular junction, where motor neurons located in the ventral spinal cord synapse with muscles in the body to activate them.
In the context of visceral smooth muscles, acetylcholine is the primary excitatory neurotransmitter. It binds to and activates two subtypes of muscarinic receptors, M2 and M3, which are present in most visceral smooth muscle tissues. This activation causes smooth muscle excitation and contraction. Specifically, acetylcholine-induced membrane depolarization is the central event of excitation-contraction coupling, leading to calcium (Ca2+) entry via L-type Ca2+ channels and subsequent smooth muscle contraction.
The process of acetylcholine-induced smooth muscle contraction is complex and involves multiple signaling pathways. One key pathway is through the activation of muscarinic receptors, which leads to the generation of a non-selective cation current called the muscarinic-activated cation current (mICat). This current, primarily carried by Na+, causes depolarization of the cell, resulting in the activation of voltage-dependent Ca2+ channels. The increase in intracellular calcium concentration initiates contraction.
Another pathway involves the activation of Gαo protein (an M2 effect) and phospholipase C (an M3 effect). Phospholipase C hydrolyzes phosphatidylinositol bisphosphate into two second messengers, inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 binds to receptors in the smooth endoplasmic reticulum, releasing calcium into the cell's cytosol and further increasing the intracellular calcium concentration. DAG activates protein kinase C, which can phosphorylate a range of downstream effector molecules with tissue-specific functions.
Acetylcholine plays a crucial role in regulating various physiological functions, including cardiac contractions, blood pressure, intestinal peristalsis, and glandular secretion. Its effects can be terminated by anticholinergics, which interfere with acetylcholine's action on tissues, and by acetylcholinesterase, which cleaves acetylcholine into choline and acetate after it unbinds from its receptor.
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Muscarinic receptors play a key role in the parasympathetic nervous control of various peripheral tissues
Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles. It is the main neurotransmitter of the parasympathetic nervous system, and it plays a key role in the parasympathetic nervous control of various peripheral tissues. Acetylcholine is released from short postganglionic nerves that supply parasympathetic innervation to smooth muscles of visceral organs, such as the airways, the urinary bladder, and the myometrium.
In airway smooth muscle, muscarinic receptors are also present and mediate contraction and bronchoconstriction. The release of acetylcholine from the parasympathetic nerves is controlled by muscarinic autoreceptors located on the nerves. The hyperresponsiveness characteristic of asthma may result from excessive acetylcholine release and increased contraction of airway smooth muscle due to M2 receptor dysfunction.
Pharmacologic studies have suggested that both M2 and M3 muscarinic receptors are involved in gastrointestinal smooth muscle contraction in response to acetylcholine and other muscarinic agonists. The activation of muscarinic receptors evokes calcium release, which raises the internal free calcium concentration and causes the opening of calcium-activated potassium channels. This leads to an increase in action potential discharge and contraction in the whole muscle.
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Muscarinic receptor activation evokes calcium release from stores, raising internal calcium concentration
Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, where it binds to and activates two muscarinic receptor subtypes, M2 and M3. Muscarinic receptor activation evokes calcium release from stores, raising the internal calcium concentration. This process causes the opening of calcium-activated potassium channels, suppressing the voltage-dependent inward calcium current. The rise in internal calcium also causes the opening of channels that lead to membrane depolarization, increasing the action potential discharge and contraction in the whole muscle.
The activation of muscarinic receptors by acetylcholine initiates a series of events that result in smooth muscle contraction. Muscarinic receptors play a key role in the parasympathetic nervous control of various peripheral tissues, including smooth muscles. In smooth muscle tissues such as the gastrointestinal tract, airways, and urinary system, muscarinic receptors are predominantly composed of the M2 subtype (about 80%) and the M3 subtype (about 20%). The activation of these receptors by acetylcholine triggers the release of calcium from intracellular stores, leading to an increase in the internal calcium concentration.
The increase in intracellular calcium has multiple effects on the smooth muscle cell. Firstly, it causes the opening of calcium-activated potassium channels, which results in the suppression of the voltage-dependent inward calcium current. This suppression ensures that the cell does not become overloaded with calcium. Secondly, the rise in internal calcium concentration triggers the opening of additional channels, leading to membrane depolarization. This depolarization further enhances the entry of calcium into the cell, creating a positive feedback loop that amplifies the contractile response.
The opening of these channels increases the action potential discharge, leading to a greater contraction in the entire muscle. This process is essential for maintaining the proper function of smooth muscles in various physiological systems, including the gastrointestinal tract and airways. The interplay between the M2 and M3 muscarinic receptor subtypes is crucial for regulating smooth muscle contraction. The activation of these receptors by acetylcholine triggers a cascade of events that ultimately result in the contraction of smooth muscle cells.
The release of acetylcholine from the parasympathetic nerves is controlled by muscarinic autoreceptors located on the nerves themselves. This regulatory mechanism ensures that the appropriate amount of acetylcholine is released to achieve the desired level of smooth muscle contraction. In the airways, for example, excessive contraction of the airway smooth muscle can lead to hyperresponsiveness, which is characteristic of asthma. Understanding the role of muscarinic receptor activation and calcium release in smooth muscle contraction provides valuable insights into the development of therapeutic interventions for various physiological disorders.
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Acetylcholine activates and then suppresses spontaneous K+ current transients
Acetylcholine, the major excitatory neurotransmitter to the smooth muscle of the mammalian intestine, is known to depolarize smooth muscle cells with an apparent increase in membrane conductance. The neurotransmitter acetylcholine stimulates muscarinic receptors, M2 and M3, leading to the activation of cationic channels and the generation of mICAT, which depolarizes the cells, providing the critical trigger for the stimulation of voltage-dependent Ca2+ channels. The ionic mechanisms that are triggered by muscarinic receptor activation and underlie this response are poorly understood due to technical problems associated with the electrophysiological study of smooth muscle.
The muscarinic action of acetylcholine in certain neurons has been shown to involve the switching off of a resting K+ current (M-current). A similar mechanism has been identified in the smooth muscle of the amphibian stomach. The activation of muscarinic receptors evokes calcium release from stores, which raises the internal free calcium concentration and causes the opening of calcium-activated potassium channels. The rise in internal calcium suppresses the voltage-dependent inward calcium current.
In single smooth muscle cells of rabbit jejunum, it was found that muscarinic receptor activation switches on a non-selective, voltage-sensitive inward current. Acetylcholine activates and then suppresses spontaneous K+ current transients, which are probably triggered by rises in intracellular Ca2+ in these cells. Acetylcholine activates single sodium channels in smooth muscle cells. Acetylcholine modulation of the short-circuit current across the rabbit lens has also been observed.
The activation of mICat requires co-stimulation of both M2 and M3 receptor subtypes. Under physiological ionic conditions, this current, primarily carried by Na+, causes depolarization of the cell, resulting in the activation of voltage-dependent Ca2+ channels. The net effect of these events is an increase in cytosolic [Ca2+] that initiates contraction.
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Muscarinic receptor subtypes M2 and M3 act in synergy to open TRPC4 channels, initiating membrane depolarization
Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, such as the gastrointestinal tract, airways, and the urinary system. It is released from postganglionic nerves that supply parasympathetic innervation to smooth muscles.
Muscarinic receptors play a key role in the parasympathetic nervous control of various peripheral tissues, including smooth muscles. Acetylcholine stimulates these muscarinic receptors, leading to the activation of cationic channels and the generation of mICAT, which causes cell depolarization.
The M2 and M3 muscarinic receptor subtypes act synergistically to open TRPC4 channels, initiating membrane depolarization. This process is essential for smooth muscle contraction. The activation of mICAT requires the costimulation of both M2 and M3 receptor subtypes. The ensuing influx of Ca2+ is critical for contraction and intestinal motility.
In single cells, muscarinic receptor activation by acetylcholine evokes calcium release, raising the internal free calcium concentration. This increase in intracellular Ca2+ suppresses the voltage-dependent inward calcium current and activates calcium-activated potassium channels. The rise in internal calcium also contributes to the opening of channels that cause membrane depolarization, increasing action potential discharge and contraction in the whole muscle.
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Frequently asked questions
Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles. It binds to and activates two muscarinic receptor subtypes, M2 and M3, causing smooth muscle excitation and contraction.
Muscarinic receptors are a type of acetylcholine receptor. They play a key role in the parasympathetic nervous control of various peripheral tissues including smooth muscles.
Muscarinic receptors have been divided into five subtypes: M1, M2, M3, M4, and M5.
M2 receptors regulate the release of acetylcholine. In airway smooth muscle, M2 receptor dysfunction can lead to excessive acetylcholine release and increased contraction, contributing to conditions like asthma.
M3 receptors are involved in the excitation-contraction coupling in gastrointestinal smooth muscle cells. Activation of M3 receptors leads to the generation of a non-selective cation current, causing depolarization and subsequent contraction.
