Dominic Stone
Smooth muscles are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. They are also present in the tracts of the respiratory, urinary, and reproductive systems, as well as in the eyes and skin. Smooth muscles are typically classified as either single-unit or multi-unit, but these terms are oversimplifications as smooth muscles are generally influenced by a combination of neural elements, cell-to-cell communication, and locally produced activators/inhibitors. Smooth muscles are activated by a combination of smooth muscle cells, interstitial cells of Cajal, and platelet-derived growth factor receptor alpha, which work together as a functional syncytium. Smooth muscles are unique in that they can be stimulated by multiple factors, including local humoral factors, circulating hormones, and mechanical stimulation, and they do not require action potentials to contract. Smooth muscles are innervated by the autonomic nervous system, which includes the sympathetic nervous system, and they respond to neurotransmitters such as acetylcholine and norepinephrine. So, while smooth muscles as a whole are influenced by the sympathetic nervous system, it is not accurate to describe any single smooth muscle as sympathetic.
| Characteristics | Values |
|---|---|
| Definition | Smooth muscles are non-striated muscles that are capable of maintaining tone for extended periods and often contract involuntarily. |
| Location | Smooth muscles are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. They are also found in the walls of blood vessels, lymph vessels, and the tracts of the respiratory, urinary, and reproductive systems. |
| Types | Single-unit and multi-unit smooth muscle. Single-unit smooth muscle is also known as visceral smooth muscle, while multi-unit smooth muscle is primarily under neural control. |
| Control | Smooth muscles are controlled by the autonomic nervous system, hormones, and intrinsic factors in the organ. |
| Sympathetic Stimulation | Smooth muscles receive sympathetic stimulation from spinal levels T1 to L2 of the spine, which contributes to the sympathetic trunk that supplies autonomic nervous supply to organs and tissues throughout the body. |
| Function | Smooth muscles serve a variety of functions depending on their location, including digestion, nutrient collection, toxin removal, electrolyte balance, and regulation of blood pressure and tissue oxygenation. |
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What You'll Learn
- Smooth muscles are controlled by the autonomic nervous system, hormones, and intrinsic factors
- Smooth muscles are found in the walls of hollow organs, including the stomach and intestines
- Smooth muscles are slower to contract than skeletal muscles
- Smooth muscles are non-striated and appear homogenous under a microscope
- Smooth muscles are activated by multiple factors, including hormones and mechanical stimulation
Smooth muscles are controlled by the autonomic nervous system, hormones, and intrinsic factors
Smooth muscles are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. They are also present in the tracts of the respiratory, urinary, and reproductive systems, as well as in the skin and eyes. Smooth muscles are unique in that they can be contracted and controlled involuntarily, without conscious thought. This is achieved through the autonomic nervous system, which controls the automatic functions of the body necessary for survival. The autonomic nervous system includes the sympathetic and parasympathetic nervous systems, which work together to regulate critical body functions such as digestion, respiratory rate, urination, heart rate, and blood pressure.
The autonomic nervous system controls smooth muscle contraction through the release of neurotransmitters and hormones, ultimately leading to a calcium release in smooth muscle tissue. Smooth muscle contraction is dependent on this calcium influx, which is regulated by calcium channels. These calcium channels open slowly, leading to a slow repolarization process. Sodium channels may also be involved in the activation of calcium channels by increasing the rate of depolarization. Smooth muscle cells can also form a spontaneous pacemaker current, further influencing their contraction.
In addition to the autonomic nervous system, smooth muscles are also regulated by hormones and intrinsic factors. For example, smooth muscles in the gastrointestinal tract are activated by a combination of smooth muscle cells, interstitial cells of Cajal, and platelet-derived growth factor receptor alpha. Smooth muscle cells are spindle-shaped with a single nucleus, and they contain the proteins myosin and actin, which enable contraction. Myosin, in particular, plays a crucial role in smooth muscle contraction, with different isoforms and light chains contributing to this process.
Smooth muscles respond to various stimuli, including norepinephrine and epinephrine, by producing vasoconstriction or vasodilation. This response depends on the distribution of adrenergic receptors, the amount of catecholamines released, and the sensitivities of the receptors. For instance, arterial smooth muscle responds to carbon dioxide by producing vasodilation and to oxygen by producing vasoconstriction. Smooth muscles are also influenced by local humoral factors, mechanical stimulation, and cell-to-cell communication, resulting in a coordinated response.
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Smooth muscles are found in the walls of hollow organs, including the stomach and intestines
Smooth muscles are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. They are also present in the walls of blood and lymph vessels, except for the capillaries. Smooth muscles are responsible for various functions in the body, including digestion, nutrient collection, and toxin elimination. They play a crucial role in regulating blood pressure and tissue oxygenation. Smooth muscles differ from skeletal and cardiac muscles in terms of structure, function, and regulation of contraction. They exhibit greater elasticity and functionality within a larger length-tension curve. This ability to stretch and contract is essential in organs like the intestines and urinary bladder.
Smooth muscles are classified into two types: single-unit and multi-unit smooth muscles. Single-unit smooth muscles, also known as visceral smooth muscles, are electrically connected and contract uniformly. They are found in the walls of internal organs and blood vessels. On the other hand, multi-unit smooth muscles are independent and require individual innervation, allowing for more precise muscle control. They are found in the iris and hair erector muscles. Smooth muscle cells are spindle-shaped and smaller than skeletal muscle cells. They contain proteins like actin and myosin, which are responsible for muscle contraction.
The gastrointestinal tract's smooth muscles are activated by a combination of smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and platelet-derived growth factor receptor alpha (PDGFRα). These components work together as an SIP functional syncytium. Smooth muscles in the gastrointestinal tract respond to norepinephrine and epinephrine by producing vasoconstriction. This response is mediated through alpha-1 adrenergic receptors. Additionally, smooth muscles in the skin cause hair to stand erect in response to cold temperatures or fear.
The function of smooth muscles extends beyond individual organs to regulate entire organ systems. They play a crucial role in the propulsion of food in the gastrointestinal tract and the regulation of blood flow and pressure in the cardiovascular system. Smooth muscles are derived from both mesoderm and neural crest cells, contributing to various tissues throughout the body. Their unique properties, such as elasticity and contractility, make them essential for maintaining the body's basic functions.
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Smooth muscles are slower to contract than skeletal muscles
Smooth muscles are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. They are also present in the tracts of the respiratory, urinary, and reproductive systems, as well as in the skin and eyes. Smooth muscles differ from skeletal muscles in terms of structure, function, and regulation of contraction. Smooth muscles are involuntary, meaning their functions are not directly controlled, while skeletal muscles are under voluntary control.
Smooth muscle action potentials are unique as the membrane potential initiates or modulates contraction. This graded membrane response can be stimulated by local humoral factors, circulating hormones, or mechanical stimulation. Action potentials in smooth muscle cells are slower than those in skeletal muscle cells and can last up to fifty times longer. This is because calcium channels in smooth muscle cells open more slowly than in skeletal muscles, leading to slow repolarization as potassium channels are also slower to react.
The calcium release mechanism also differs between smooth and skeletal muscles. In smooth muscles, calcium release from the sarcoplasmic reticulum does not couple to the ryanodine receptor. Instead, calcium binds to a calmodulin protein, activating myosin light chain kinase (MLCK), which then phosphorylates a regulatory light chain on myosin. This process increases myosin ATPase activity, promoting interaction between the myosin head and actin, leading to cross-bridge cycling and muscle contraction.
Additionally, smooth muscles do not contain sarcomeres, the repeating arrays found in striated muscles like skeletal muscles. As a result, smooth muscles lack the troponin complex required for skeletal muscle contraction, further contributing to their slower contraction compared to skeletal muscles. Smooth muscles demonstrate greater elasticity and function within a larger length-tension curve, allowing them to stretch while maintaining contractility, which is essential in organs like the intestines and urinary bladder.
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Smooth muscles are non-striated and appear homogenous under a microscope
Smooth muscles are non-striated, meaning they do not exhibit the same striped appearance as skeletal or cardiac muscle tissue when observed under a microscope. Instead, smooth muscles appear homogenous, with small, spindle-shaped (fusiform) cells that are challenging to distinguish due to their indistinct cell membranes. Smooth muscles are found in various parts of the body, including the walls of hollow organs such as the stomach, intestines, bladder, and uterus, as well as in blood vessels and lymph vessels (excluding capillaries). They play a crucial role in digestion, nutrient collection, toxin elimination, and the regulation of blood pressure and tissue oxygenation.
The unique structure of smooth muscles contributes to their functionality. Smooth muscle cells are activated by a combination of smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and platelet-derived growth factor receptor alpha (PDGFrα) that work together as an electrically coupled functional syncytium. Each smooth muscle cell has a spindle shape, with a wide middle and tapering ends, and contains a single nucleus. The proteins myosin and actin, which are essential for contraction, make up a significant portion of the cytoplasm.
Myosin, a key component in smooth muscle contraction, has two heavy chains (MHC) that form the head and tail domains. These heavy chains coil around each other, giving myosin II two heads. The gene MYH11 codes for these heavy chains and has splice variants resulting in four distinct isoforms. Additionally, smooth muscles may contain MHC that is not involved in contraction and can arise from multiple genes.
Smooth muscle action potentials are distinct from those of skeletal muscles. The graded membrane response in smooth muscles can be stimulated by various factors, including local humoral factors, circulating hormones, and mechanical stimulation. Calcium channels in smooth muscle cells open more slowly than in skeletal muscles, leading to slower repolarization as potassium channels also exhibit a slower response. Sodium channels may also be present on the smooth muscle membrane, influencing the activation of calcium channels and increasing the rate of depolarization.
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Smooth muscles are activated by multiple factors, including hormones and mechanical stimulation
Smooth muscles are present throughout the body, including in the stomach, intestines, bladder, and uterus. They are also found in the walls of blood vessels, the respiratory and lymph tracts, and in the eyes. Smooth muscles are unique in that they can be contracted and controlled involuntarily, without conscious thought from the individual. This is achieved through the nervous system, which uses hormones, neurotransmitters, and other receptors to control smooth muscle activity.
Smooth muscle action potentials are also unique in that membrane potential acts to initiate or modulate contraction. As such, graded membrane responses can be stimulated by multiple factors, including local humoral factors, circulating hormones, and mechanical stimulation such as the stretching of cells. Smooth muscle contraction depends on calcium influx, which is increased within the cell through two processes. Firstly, depolarization, hormones, or neurotransmitters cause calcium to enter the cell through L-type channels in the membrane. This stimulates the release of calcium from the sarcoplasmic reticulum through ryanodine receptors and IP3, a process known as calcium-induced calcium release.
Hormones play a significant role in smooth muscle activation, particularly in the uterus. For example, uterine smooth muscle atrophy after menopause indicates that this muscle is under hormonal control. Additionally, hormones can stimulate uterine contraction. Smooth muscles are also activated by receptors that increase the concentration of Ca2+ ions, which are essential for smooth muscle contraction.
Smooth muscles are activated by a combination of factors, including smooth muscle cells, interstitial cells of Cajal, and platelet-derived growth factor receptor alpha. These factors work together as an electrically coupled functional syncytium. Smooth muscle cells can also proliferate and migrate upon the activation of chemical and mechanical factors, which is crucial for tissue homeostasis and development.
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Frequently asked questions
Unitary muscles, also known as single-unit smooth muscles, are made up of cells that are coupled by gap junctions, allowing ions and action potentials to pass between adjacent cells. This allows the entire sheet of smooth muscle to function as a unit.
Unitary muscles can be influenced by sympathetic stimulation or the adrenal medulla, which causes vasoconstriction. However, they are also influenced by the parasympathetic nervous system and enteric nervous system.
Unitary muscles include the smooth muscles of the uterus (myometrium), gastrointestinal tract, and the iris of the eye.
Unitary muscles generate their own low level of rhythmic contraction, which may also be stimulated by stretch. This is transmitted from cell to cell via the gap junctions.
