Calcium ions are supplied by the SR in the fibers and by sequestration from the extracellular fluid through membrane indentations called calveoli. Because smooth muscle cells do not contain troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex but instead by the regulatory protein calmodulin. The heads can then attach to actin-binding sites and pull on the thin filaments.
The thin filaments also are anchored to the dense bodies; the structures invested in the inner membrane of the sarcolemma at adherens junctions that also have cord-like intermediate filaments attached to them. When the thin filaments slide past the thick filaments, they pull on the dense bodies, structures tethered to the sarcolemma, which then pull on the intermediate filaments networks throughout the sarcoplasm.
This arrangement causes the entire muscle fiber to contract in a manner whereby the ends are pulled toward the center, causing the midsection to bulge in a corkscrew motion Figure.
T-tubules are not required to reach the interior of the cell and therefore not necessary to transmit an action potential deep into the fiber. Smooth muscle fibers have a limited calcium-storing SR but have calcium channels in the sarcolemma similar to cardiac muscle fibers that open during the action potential along the sarcolemma. However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone.
This remaining calcium keeps the muscle slightly contracted, which is important in certain tracts and around blood vessels. Because most smooth muscles must function for long periods without rest, their power output is relatively low, but contractions can continue without using large amounts of energy. This can happen as a subset of cross-bridges between myosin heads and actin, called latch-bridges , keep the thick and thin filaments linked together for a prolonged period, and without the need for ATP.
Smooth muscle is not under voluntary control; thus, it is called involuntary muscle. The triggers for smooth muscle contraction include hormones, neural stimulation by the ANS, and local factors.
In certain locations, such as the walls of visceral organs, stretching the muscle can trigger its contraction the stress-relaxation response. Axons of neurons in the ANS do not form the highly organized NMJs with smooth muscle, as seen between motor neurons and skeletal muscle fibers. Instead, there is a series of neurotransmitter-filled bulges called varicosities as an axon courses through smooth muscle, loosely forming motor units Figure.
A varicosity releases neurotransmitters into the synaptic cleft. Also, visceral muscle in the walls of the hollow organs except the heart contains pacesetter cells. A pacesetter cell can spontaneously trigger action potentials and contractions in the muscle. Smooth muscle is organized in two ways: as single-unit smooth muscle, which is much more common; and as multiunit smooth muscle.
The two types have different locations in the body and have different characteristics. Single-unit muscle has its muscle fibers joined by gap junctions so that the muscle contracts as a single unit. Join our Forum: Smooth muscle vs dense regular connective tissue. Our community might be able to help! Try to answer the quiz below and see what you have learned so far about smooth muscles.
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Biology Definition: Question: What is smooth muscle? Answer: Smooth muscle is an involuntary, non-striated type of vertebrate muscle capable of slow rhythmic involuntary contractions. Smooth muscle, also called an involuntary muscle , displays no cross stripes when examined under a microscope.
It is made up of spindle-shaped narrow cells with a single centrally-located nucleus. Smooth muscles contract involuntarily and slowly. A great part of internal organs and the majority of the area of the digestive tract is lined with smooth muscles. The muscular system includes all the muscles of the animal body. There are three types of muscles : skeletal muscles , smooth muscles, and cardiac muscles.
Both the skeletal muscles and the cardiac muscles have striations when viewed under the microscope. In contrast, the smooth muscle lacks striations. This is because of the uniform distribution of myosin filaments in the smooth muscle cell. Apart from the lack of striations, the smooth muscle differs from the other two by the cell shape. The smooth muscle cells are typically spindle-shaped and the nuclei are centrally located.
They are also capable of contracting to a much smaller fraction of their resting length. They are responsible for the rhythmic involuntary movements of these organs. Their contraction is relatively slower than that of skeletal muscles. Nevertheless, the smooth muscle tissues remain contracted for longer periods than the skeletal muscle tissues. Question: Where is a smooth muscle found in the body?
Answer: They are located in different parts of the body. They are also found in the wall of the lungs and the reproductive system of both genders. In blood vessels, they help in the maintenance and control of blood pressure and also help in the flow of oxygen.
Smooth muscles are Which of the following does not have a smooth muscle? In the past, this was thought to be related to hemodynamics and underlying vessel structure. However, there is increasing evidence that smooth muscle cell embryonic lineage may play a role in determining the location and presentation of the disease. Vascular smooth muscle cells sometimes referred to as mural cells, are important for vascular development and stability.
Mural cells wrap around larger vessels and are heavily relied upon in the regulation of blood flow, endothelial network growth, and vessel stability. However, little is know about the effect of their developmental origins or the signaling process that leads to vessel development. The development of vascular smooth muscle cells is an important target for vascular tissue engineering and therapeutic revascularization.
Due to smooth muscles' widespread presence throughout the body, blood supply and lymphatic contributions vary by region. Almost every artery in the body supplies blood to smooth muscle whether that is in the form of endothelial smooth muscle located directly in the artery or smooth muscle within an organ system such as arteries of the gastrointestinal tract.
It becomes more important to recognize how smooth muscles impact blood supply themselves. For example, within the cardiovascular system, smooth muscle helps to regulate blood flow by controlling the diameter of the vessel.
As previously discussed vascular pathologies of smooth muscle can have devasting effects on the body and lead to significant pathology. Atherosclerosis once thought to be only a function of hemodynamics and vessel structure has more recently been shown to be linked as well to smooth muscle development. Asthma occurs when smooth muscle constriction leads to obstruction of the airway.
Recent studies have shown that the smooth muscle layer may be increased in thickness before the onset of asthma even occurs, from which a genetic link may be derived. Similar to the blood supply, the innervation of smooth muscle varies widely by location and function. Vascular smooth muscle is primarily innervated by the sympathetic nervous system. Alpha-1 and alpha-2 receptors function to cause vasoconstriction by contracting vascular smooth muscle cells leading to systemic hypertension.
Beta-2 receptors also respond to sympathetic stimulation but produce a vasodilatory effect and which will lead to systemic hypotension.
However, parasympathetic stimulation also plays an important role in the contraction of smooth muscle cells. Studies performed as early as demonstrated the effect of parasympathetic innervation on the gastrointestinal tract. Each of these contributions finds its way into the sympathetic trunk which functions to route autonomic nervous supply to organs and tissue throughout the body.
The parasympathetic nervous system functions in three parts, the cranial nerves, vagus nerve, and pelvic splanchnic nerves. Each nerve in the parasympathetic system regulates a specific portion of the body, the vagus, for instance, innervates the gastrointestinal tract from the esophagus to the proximal portion of the large intestines, while also sending out branches to the heart, larynx, trachea, bronchi, liver, and pancreas.
The sympathetic and parasympathetic nervous systems are collectively referred to as the autonomic nervous system. The complex nature of the autonomic nervous system allows for tight unconscious control of digestions, respiratory rate, urination, heart rate, blood pressure, and many other critical body functions.
Ultimately innervation from the autonomic nervous system leads to a calcium release in smooth muscle tissue. Smooth muscle contraction is dependent on calcium influx.
Calcium is increased within the smooth muscle cell through two different processes. First depolarization, hormones, or neurotransmitters cause calcium to enter the cell through L-type channels located in the caveolae of the membrane. Intracellular calcium then stimulates the release of calcium from the sarcoplasmic reticulum SR by way of ryanodine receptors and IP3, this process is referred to as calcium-induced calcium release.
Once calcium has entered the cell it is free to bind calmodulin, which transforms into activated calmodulin. Once phosphorylation has occurred a conformational change takes place in the myosin head, this increases myosin ATPase activity which promotes interaction between the myosin head and actin. Cross-bridge cycling then occurs, and tension is generated. The tension generated is relative to the amount of calcium concentration within the cell.
ATPase activity is much lower in smooth muscle than it is in skeletal muscle. This factor leads to the much slower cycling speed of smooth muscle. However, the longer period of contraction leads to a potentially greater force of contraction in smooth muscle.
Smooth muscle contraction is enhanced even further through the use of connexins. Connexins allow for intercellular communication by allowing calcium and other molecules to flow to neighboring smooth muscle cells.
This action allows for rapid communication between cells and a smooth contraction pattern. Dephosphorylation of myosin light chains terminates smooth muscle contraction. Unlike skeletal muscle smooth muscle is phosphorylated during its activation. This creates a potential difficulty in that simply reducing calcium levels won't produce muscle relaxation.
Myosin light chain phosphatase MLCP is, instead is responsible for dephosphorylation of the myosin light chains ultimately leading to smooth muscle relaxation. Smooth muscle consists of two types single-unit and multi-unit. Single-unit smooth muscle consists of multiple cells connected through connexins that can be stimulated in a synchronous pattern from only one synaptic input. Connexins allow for cell-to-cell communication between groups of single-unit smooth muscle cells.
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