Dominic Stone
The piloerector muscle, also known as the arrector pili muscle, is responsible for causing hair follicles to stand erect, often in response to cold temperatures or fear. This muscle is innervated by the sympathetic autonomic nervous system and contains both muscarinic and alpha-1 adrenergic receptors. Alpha-1 receptors are G-protein coupled receptors that mediate smooth muscle contraction and are activated by catecholamines such as norepinephrine and epinephrine. Activation of alpha-1 receptors leads to an increase in intracellular calcium, resulting in muscle contraction and vasoconstriction. In the piloerector muscle, activation of these receptors triggers the contraction that causes hair to stand on end.
| Characteristics | Values |
|---|---|
| Receptor type | Muscarinic receptors |
| Receptor class | G protein-coupled receptors (GPCR) |
| Receptor composition | A single polypeptide |
| Polypeptide regions | Seven, each made up of 20-25 amino acids arranged in an α helix |
| Polypeptide regions' hydrophobicity | High |
| Interaction site with G proteins | The fifth internal loop and the carboxyl-terminal tail of the polypeptide receptor |
| Agonist binding site | A circular pocket formed by the upper portions of the seven membrane-spanning regions |
| Neurotransmitter | Acetylcholine (ACh) |
| Neurotransmitter function | Excitatory actions at the neuromuscular junction, at autonomic ganglion, at certain glandular tissues and in the CNS |
| Neurotransmitter at the adrenal medulla | Yes |
| Neurotransmitter at parasympathetic innervated organs | Yes |
| Neurotransmitter at sweat glands | Yes |
| Neurotransmitter at the piloerector muscle of the sympathetic ANS | Yes |
| Neurotransmitter at the neuromuscular junction between the motor nerve and skeletal muscle | Yes |
| Neurotransmitter in the central nervous system | Yes |
| Cholinergic pathways | Few important long-axon identified |
| Notable cholinergic pathway | Cholinergic projection from the nucleus basalis of Meynert (in the basal forebrain) to the forebrain neocortex and associated limbic structures |
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What You'll Learn
- Muscarinic receptors are located on piloerector muscles
- Alpha-1 receptors are Gq-type G-protein coupled receptors
- Alpha-1 receptors are stimulated in instances of hypoperfusion
- Alpha-1 receptor agonists are used to treat shock and heart failure
- Alpha-1 receptor antagonists are used to treat refractory hypertension
Muscarinic receptors are located on piloerector muscles
Muscarinic acetylcholine receptors (mAChRs) are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They are mainly found in the parasympathetic nervous system and play a role in the sympathetic nervous system in the control of sweat glands.
Muscarinic receptors are classified as G protein-coupled receptors (GPCR) and are located at parasympathetic autonomically innervated visceral organs, on the sweat glands and piloerector muscles, and both post-synaptically and pre-synaptically in the CNS.
The biochemical responses to stimulation of muscarinic receptors involve the receptor occupancy causing an altered conformation of an associated GTP-binding protein (G protein). G protein is made up of three subunits: α, β, and γ. In response to the altered conformation of the muscarinic receptor, the α subunit of the G protein releases bound guanosine diphosphate (GDP) and simultaneously binds guanosine triphosphate (GTP). The binding of GTP "activates" the G protein, allowing dissociation of the α subunit from the trimeric complex and for it to interact with effector systems to mediate specific responses.
There are two broad classes of cholinergic receptors: nicotinic and muscarinic. This classification is based on two chemical agents that mimic the effects of ACh at the receptor site: nicotine and muscarine. Nicotinic receptors are located at the NMJ, autonomic ganglia, and sparsely in the CNS. The NMJ nicotinic ACh receptor consists of five polypeptide subunits: two α subunits and one each of β, δ, and γ. The binding surface of the receptor appears to be primarily on the α subunits, near the outer surface of the molecule.
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Alpha-1 receptors are Gq-type G-protein coupled receptors
Upon activation, a heterotrimeric G protein, Gq, activates phospholipase C (PLC), which causes phosphatidylinositol to be transformed into inositol trisphosphate (IP3) and diacylglycerol (DAG). DAG stays near the membrane, while IP3 diffuses into the cytosol and finds the IP3 receptor on the endoplasmic reticulum, triggering calcium release. This triggers further effects, primarily through the activation of the enzyme Protein Kinase C.
The alpha-1 receptor is associated with the Gq subunit. The Gq subunit activates phospholipase C, which hydrolyzes PIP2 to DAG and IP3 in signal transduction pathways. DAG then acts as a second messenger that activates protein kinase C, and IP3 plays a role in the phosphorylation of certain proteins.
Alpha-1 receptors are subdivided into three highly homologous subtypes: α1A-, α1B-, and α1D-adrenergic receptor subtypes. There is no α1C receptor, as it was found to be identical to the previously discovered α1A receptor subtype. Catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) signal through the alpha-1 receptor in the central and peripheral nervous systems.
The activation of alpha-1 receptors has several effects on the body, including increased vascular smooth muscle contraction, increased pupillary dilator muscle contraction, and increased intestinal and bladder sphincter muscle contraction.
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Alpha-1 receptors are stimulated in instances of hypoperfusion
Alpha-1 receptors are a type of adrenergic receptor that binds to catecholamines, including epinephrine and norepinephrine. These receptors are primarily responsible for mediating smooth muscle contraction. When there is a decrease in cardiac output or systemic vasculature resistance, leading to hypoperfusion, alpha-1 receptors become stimulated. This stimulation has several important implications for the body's response to shock, congestion, and heart-related issues.
During instances of hypoperfusion, the stimulation of alpha-1 receptors triggers a cascade of intracellular events. The activation of these receptors leads to the activation of phospholipase C, which, in turn, increases the levels of IP3 and DAG. This increase ultimately results in a rise in intracellular calcium concentrations, causing smooth muscle contraction and glycogenolysis. This process is particularly important in the arterial vasculature of skeletal muscle, where alpha-1 receptors play a role in mediating vasoconstriction.
The stimulation of alpha-1 receptors during hypoperfusion is also relevant in the context of shock. Alpha-1 agonists, such as phenylephrine, are commonly administered intravenously to manage various types of shock, including vasodilatory shock, septic shock, and cardiopulmonary arrest. By stimulating alpha-1 receptors, these agonists can help improve blood flow and circulation during shock episodes.
Additionally, the stimulation of alpha-1 receptors has implications for managing congestion. Phenylephrine, an alpha-1 agonist, can be administered orally to treat upper airway congestion. Stimulating the alpha-1 receptors in this context leads to a decrease in mucus secretion, providing relief from congestion.
Furthermore, the role of alpha-1 receptors in hypoperfusion is relevant to heart-related conditions. Alpha-1 agonists are used in the management of heart failure decompensation. On the other hand, alpha-1 antagonists, also known as alpha-blockers, can be used to lower blood pressure and manage hypertension. These antagonists work by inhibiting the uptake of catecholamines in smooth muscle cells, resulting in vasodilation.
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Alpha-1 receptor agonists are used to treat shock and heart failure
Alpha-1 receptors are G-protein coupled receptors (GPCRs) associated with the Gq heterotrimeric G protein. They are primarily found on vascular smooth muscle and myocardial tissue. When stimulated, they cause vasoconstriction and positive inotropic effects, respectively.
Alpha-1 receptor agonists are a class of medications used to manage various disorders, including shock and heart failure. Shock, in this context, refers to vasodilatory shock, which is caused by a sudden drop in blood pressure that prevents the heart from pumping enough oxygen-rich blood to the body. This can lead to organ failure and death if not treated promptly. Alpha-1 receptor agonists are used to treat this condition by stimulating alpha-1 receptors, which results in vasoconstriction and an increase in blood pressure.
In the context of heart failure, alpha-1 receptor agonists have been proposed as a potential treatment option due to their ability to increase contractility in failing hearts. This is particularly relevant in chronic heart failure, when catecholamine levels are elevated, and beta-adrenergic receptors are down-regulated and dysfunctional. By selectively activating alpha-1 adrenergic receptors in the heart, it may be possible to treat heart failure effectively.
It is important to note that alpha-1 receptor agonists can have adverse effects when taken at toxic doses, leading to increased sympathetic tone, tachycardia, hypertension, and hypotension. Therefore, the administration of these medications is carefully monitored to ensure patient safety.
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Alpha-1 receptor antagonists are used to treat refractory hypertension
Alpha-1 receptors are G-protein coupled receptors (GPCR) that are members of the G protein-coupled receptor superfamily. They are subdivided into three subtypes: alpha-1A, alpha-1B, and alpha-1D. These receptors are found in the central and peripheral nervous systems and play a role in smooth muscle contraction, particularly in the blood vessels of the skin, gastrointestinal system, kidney, and brain. When activated, they increase intracellular calcium concentrations, leading to smooth muscle contraction and glycogenolysis.
Alpha-1 receptor antagonists, also known as alpha-blockers, are a class of drugs that bind to and block these alpha-1 receptors. By inhibiting the uptake of catecholamines in smooth muscle cells, they cause vasodilation and a reduction in blood pressure. This mechanism is particularly useful in treating hypertension, especially in cases of refractory hypertension where other treatments may not be effective.
Alpha-1 receptor antagonists are recommended as adjunctive therapy for hypertension, meaning they are used alongside other treatments rather than as a standalone therapy. This is because, while they can effectively lower blood pressure, they have not been shown to improve long-term survival rates. In fact, long-term use of these antagonists has been associated with an increased risk of heart failure, stroke, and cardiovascular disease.
Despite this, alpha-1 receptor antagonists have distinct advantages over other antihypertensive agents. They have been shown to beneficially affect blood lipids, making them suitable for patients with diabetes or elevated serum lipid levels. Additionally, they are associated with a reasonably low incidence of serious adverse effects and are free of any adverse metabolic effects.
Overall, alpha-1 receptor antagonists play a crucial role in managing refractory hypertension by targeting the alpha-1 receptors involved in smooth muscle contraction and vasoconstriction. While they are not recommended as a first-line therapy, they offer a valuable option for patients who have not responded to other treatments.
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Frequently asked questions
Piloerector muscles, also known as arrector pili muscles, are tiny muscles that attach to hair follicles and cause hair to stand erect when contracted.
Muscarinic receptors, classified as G protein-coupled receptors (GPCR), are located in the piloerector muscles.
The function of the muscarinic receptors in the piloerector muscles is to respond to the neurotransmitter acetylcholine (ACh), which is released by the sympathetic nervous system during times of stress or cold, causing the hair to stand up.
