Dominic Stone
Vascular smooth muscle is a type of muscle that composes most of the walls of blood vessels. It is innervated primarily by the sympathetic nervous system through adrenergic receptors. The main function of vascular smooth muscle tone is to regulate the caliber of blood vessels in the body by contracting or relaxing to change both the volume of blood vessels and the local blood pressure. This mechanism is responsible for redistributing blood within the body to areas where it is needed. Smooth muscle also has a role in regulating the function of various hollow organ systems, including the vasculature, airways, gastrointestinal tract, uterus, and reproductive tract. In the case of vascular injury, smooth muscle cells can undergo phenotypic switching, contributing to vascular disease.
| Characteristics | Values |
|---|---|
| Definition | A type of smooth muscle that makes up most of the walls of blood vessels |
| Function | Regulate the calibre of the blood vessels in the body by contracting or relaxing to change both the volume of blood vessels and the local blood pressure |
| Composition | Vascular smooth muscle cells (VSMCs) |
| Innervation | Sympathetic nervous system through adrenergic receptors (adrenoceptors) |
| Types of adrenergic receptors | Alpha-1, alpha-2, and beta-2 |
| Main endogenous agonist | Norepinephrine (NE) |
| Effect of alpha-1 receptor activation | Vasoconstriction (contraction of vascular smooth muscle cells decreasing vessel diameter) |
| Occurrence of alpha-1 receptor activation | In response to shock or low blood pressure |
| Role in organ systems | Regulating the function of hollow organ systems including the vasculature, airways, gastrointestinal tract, uterus, reproductive tract, bladder, and urethra |
| Fundamental roles of smooth muscle | Alter the shape of an organ and withstand the force of an internal load presented to that organ |
| Mechanism of action | Mechanical coupling, enabling powerful and coordinated contractions at a low energy cost |
| Cell shape | Fusiform, approximately 200 microns long and 5 microns in diameter, with a large central nucleus |
| Role in disease | Excessive proliferation contributes to vascular inflammation, plaque formation, atherosclerosis, restenosis, and pulmonary hypertension |
| Role in neovascularization | Transplantation of vascular smooth muscle cells into the heart has been shown to promote neovascularization and improve cardiac function |
| Role in vascular remodeling | SMC plasticity is important in the response of the vascular wall to injury |
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What You'll Learn
Vascular smooth muscle cells are the most abundant cell in vessels
Vascular smooth muscle is the type of smooth muscle that makes up most of the walls of blood vessels. Vascular smooth muscle cells (VSMCs) are the most abundant cell type in vessels, playing crucial structural and physiological roles in the cardiovascular system. They are involved in all the physiological functions and pathological changes that occur in the vascular wall.
VSMCs have two fundamental roles: firstly, to alter the shape of an organ, and secondly, to withstand the force of an internal load presented to that organ. Smooth muscle in the gastrointestinal tract, for example, must undergo intermittent but coordinated phasic contractions to propel food through the alimentary canal. In contrast, smooth muscle in the vasculature is more often in various states of tonic contraction, dynamically regulated to relax or contract in response to specific neuro-hormonal and haemodynamic signals.
The main function of vascular smooth muscle tone is to regulate the calibre of blood vessels in the body. Vascular smooth muscle contracts or relaxes to change both the volume of blood vessels and local blood pressure, redistributing blood within the body to areas where it is needed. Arteries have a greater deal of smooth muscle within their walls than veins, resulting in a greater wall thickness. This is because they have to carry pumped blood away from the heart to all the organs and tissues that need oxygenated blood.
Excessive vasoconstriction leads to high blood pressure, while excessive vasodilation leads to low blood pressure. Agonists of alpha-2 receptors in the vascular smooth muscle lead to vasoconstriction, while agonism of beta-2 receptors causes vasodilation and low blood pressure. Excessive proliferation of vascular smooth muscle cells contributes to the progression of pathological conditions, such as vascular inflammation, plaque formation, atherosclerosis, restenosis, and pulmonary hypertension.
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SMCs are important regulators of vascular remodelling
Vascular smooth muscle is a type of muscle that makes up most of the walls of blood vessels. SMCs, or smooth muscle cells, are a major component of the vascular wall and play a crucial role in vascular remodelling. They exhibit remarkable phenotypic plasticity, which refers to their ability to switch between different phenotypes or states. This plasticity is an important feature in the response of the vascular wall to vascular injury or disease.
Phenotypic modulation of SMCs leads to highly proliferative and migratory cells, which are the major components of remodelled arteries. Coactivator β-catenin promotes SMC proliferation and neointimal hyperplasia after vascular injury. On the other hand, TCF7L2 negatively regulates vascular remodelling by suppressing SMC proliferation. Endothelial and SMC MEF2C are essential for angiogenesis and SMC differentiation, respectively, while endothelial GATA-6 regulates SMC proliferation in neointimal hyperplasia via paracrine PDGF-B signalling.
Transcription factors and their coactivators are critical for regulating SMC phenotypic modulation and vascular remodelling. Olfm2, a regulatory protein, modifies SMC phenotype and promotes vascular remodelling by interacting with SRF and inhibiting myocardin binding. Proangiogenic factor Aggf1 interacts with myocardin to stabilise the SRF-myocardin complex, inhibiting arterial remodelling. Additionally, ZFP148, a transcription factor regulating macrophage polarization, inhibits SMC marker expression. FOXO3a induces SMC apoptosis, attenuating vascular remodelling through MMP13 expression, while BMAL1 regulates arterial wall remodelling and degeneration by repressing Timp4 transcription and activating MMPs in SMCs.
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SMCs maintain the normal matrix of the vascular wall
Vascular smooth muscle is a type of muscle that composes most of the walls of blood vessels. It is innervated primarily by the sympathetic nervous system through adrenergic receptors. The three types of adrenergic receptors are alpha-1, alpha-2, and beta-2. The main endogenous agonist of these cell receptors is norepinephrine (NE).
SMCs (smooth muscle cells) maintain the normal matrix of the vascular wall. SMCs are the major cell type of the media, with a few present in the normal human intima. SMCs are surrounded by vascular wall interlaminar matrix protein fibers or filaments, which are composed of vascular cells (SMCs, ECs, and fibroblasts). SMCs are quiescent in the media, and upon injury, they undergo phenotypic transformation, exhibiting phenotype plasticity as they become proliferating, secreting, and migrating cells with a capacity to become myofibroblasts and participate in repair. These secretory cells lose most of their muscle markers.
Phenotypic transformation is regulated by several factors, including platelet-derived growth factor (PDGF), TGF-β, and oxidized low-density lipoprotein (oxyLDL). SMC plasticity is an important feature in the response of the vascular wall to vascular injury. SMCs are important regulators of vascular remodeling with the secretion of matrix metalloproteinases (MMPs), MT1-MMP, and tissue inhibitors of metalloproteinases (TIMPs). SMCs may also undergo transformation to exhibit osteogenic and chondrogenic phenotypes in response to vascular injury or disease.
Smooth muscle has two fundamental roles: to alter the shape of an organ and to withstand the force of an internal load presented to that organ. Smooth muscle has developed mechanisms of mechanical coupling, which enable the development of powerful and coordinated contractions at a relatively low energy cost. Smooth muscle in the vasculature is often in various states of tonic contraction but can be dynamically regulated to relax or contract in response to specific neuro-hormonal and haemodynamic signals.
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Vascular smooth muscle tone regulates blood vessel calibre
Vascular smooth muscle (VSM) is a type of smooth muscle that composes most of the walls of blood vessels. It is innervated primarily by the sympathetic nervous system through adrenergic receptors (adrenoceptors).
The main function of vascular smooth muscle tone is to regulate the calibre of blood vessels in the body. This is achieved through the contraction and relaxation of VSM, which changes the volume of blood vessels and local blood pressure. This mechanism is responsible for redistributing blood within the body to areas where it is needed, such as areas with temporarily enhanced oxygen consumption.
The contractile state of VSM is regulated by hormones, vasoactive peptides, and ROS. Endothelial cells secrete vasoactive agents and ROS that modulate vessel diameter by influencing VSM cell function. The sympathetic nervous system also plays a role in regulating VSM tone through the release of norepinephrine (NE), which acts as a tonic vasoconstrictor. NE binds to alpha-1 receptors, causing vasoconstriction, and beta-2 receptors, causing vasodilation.
VSM also has a basal vascular tone or partial constriction, known as myogenic tone, which helps maintain resistance artery and arteriolar diameter at an intermediate level of constriction. This allows changes in vessel calibre to occur in response to vasodilator or vasoconstrictor stimuli. The maintenance of this basal vascular tone is important in regulating blood flow to tissues, as small changes in vessel calibre can produce very large alterations in blood flow.
Overall, the vascular smooth muscle tone plays a crucial role in regulating blood vessel calibre, blood pressure, and blood distribution throughout the body.
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Smooth muscle has two fundamental roles
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. Smooth muscle is found throughout the body and serves a variety of functions. Smooth muscle is found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. It is also found in the walls of blood vessels and lymph vessels (excluding blood and lymph capillaries), where it is known as vascular smooth muscle.
Vascular smooth muscle is the type of smooth muscle that makes up most of the walls of blood vessels. It contracts or relaxes to change both the volume of blood vessels and local blood pressure, which helps redistribute blood within the body to areas where it is needed. For example, smooth muscle in the gastrointestinal tract must undergo intermittent but coordinated phasic contractions to propel the bolus of food through the alimentary canal.
- To alter the shape of an organ: Smooth muscle can undergo powerful and coordinated contractions at a relatively low energy cost, which helps alter the shape of an organ. For example, in the gastrointestinal tract, smooth muscle contractions help propel food through the alimentary canal.
- To withstand the force of an internal load presented to that organ: Smooth muscle can withstand the force of an internal load presented to an organ. For example, in the urinary bladder, smooth muscle helps maintain contractile tone, which is necessary for preserving bladder function.
Smooth muscle also plays a vital role in regulating the function of various hollow organ systems, including the vasculature, airways, gastrointestinal tract, uterus, reproductive tract, bladder, urethra, and several other systems. It is essential for maintaining blood pressure and flow, opening and closing airways, and regulating tissue oxygenation.
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Frequently asked questions
Vascularity refers to the density of blood vessels in a given area of the body. Vascular smooth muscle is the type of muscle that makes up most of the walls of blood vessels.
Vascular smooth muscles are one of the main cell types of human blood vessels and play crucial structural and physiological roles in the cardiovascular system. They are responsible for regulating blood vessel calibre by contracting or relaxing to change blood volume and pressure.
The primary function of vascular smooth muscles is to regulate blood vessel calibre, ensuring adequate blood flow to areas with increased oxygen demand. They also play a role in organ shape alteration and withstanding internal loads.
Excessive proliferation and phenotype switching of vascular smooth muscle cells contribute to vascular diseases such as vascular inflammation, atherosclerosis, and hypertension.
Vascular smooth muscles are involved in regulating the function of various hollow organ systems, including the vasculature, airways, gastrointestinal tract, reproductive tract, and bladder. They help alter organ shape and respond to internal loads.
