What Is The Function Of Muscle Cells – Smooth muscle (so named because the cells lack striations) is present in the walls of hollow organs such as the bladder, uterus, stomach, intestines, and the arteries and veins of the circulatory system. , and tracts of the respiratory, urinary, and reproductive systems ([link]ab). The eye also contains smooth muscle, where it acts to reshape the iris and reshape the lens; and in the skin where the hair stands on end due to cold temperatures or fear.
Smooth muscle tissue is found around the organs of the digestive, respiratory, reproductive tract and iris of the eye. LM × 1600. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)
What Is The Function Of Muscle Cells
Smooth muscle fibers are spindle-shaped (wide in the middle and tapered at both ends, somewhat like a football) and have a single nucleus; They range from about 30 to 200
Muscle Tissue Types
M (thousands of times smaller than skeletal muscle fibers), and they form their own connective tissue, the endomysium. Although they lack striations and sarcomeres, smooth muscle fibers contain actin and myosin contractile proteins and thick and thin filaments. These thin filaments are anchored by a dense body. The dense body is analogous to the Z-disc of skeletal and cardiac muscle fibers and is attached to the sarcolemma. Calcium ions are supplied from the extracellular fluid through the SR in the fibers and through indentations of the membrane called calveoli.
Because smooth muscle cells lack troponin, cross-bridge formation is not controlled by the troponin-tropomyosin complex but instead by the regulatory protein calmodulin. In smooth muscle fibers, extracellular Ca
The calmodulin complex then activates an enzyme called myosin (light chain) kinase, which activates the myosin heads by phosphorylating (converting ATP to ADP and P).
Attachment to the head). The heads can then attach to actin-binding sites and pull thin filaments. Thin filaments are also anchored to dense bodies; Structures embedded in the inner membrane of the sarcolemma (at adherens junctions) to which cord-like intermediate filaments are also attached. When the thin filaments move behind the thick filaments, they pull on the dense bodies, structures attached to the sarcolemma, which then pull on the central filament networks in the sarcoplasm. This arrangement causes the entire muscle fiber to contract in such a way that the ends are pulled toward the center, causing the center to bulge in a corkscrew motion ([link]).
Energy Metabolism Design Of The Striated Muscle Cell
Dense bodies and intermediate filaments are networked by sarcoplasm, which causes the muscle fiber to contract.
Ion, the diameter of smooth muscle fibers is much smaller than that of skeletal muscle cells. T-tubules are not required to reach the interior of the cell and therefore do not need to transmit action potentials deep into the fiber. Smooth muscle fibers have a limited calcium-storing SR but the sarcolemma (as in cardiac muscle fibers) has calcium channels that open during action potentials along the sarcolemma. Influx of exogenous Ca
Ions back into the SR and out of the cell. However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone. This remaining calcium keeps the muscles slightly constricted, which is important in certain passages and around blood vessels.
Because most smooth muscles must work for long periods of time without rest, their energy output is relatively low, but can continue to contract without expending large amounts of energy. Some smooth muscles can also maintain contractility as Ca
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Is removed and myosin kinase becomes inactive/dephosphorylated. This can occur as a subset of cross-bridges between the myosin heads and actin, called latch-bridges, where the thick and thin filaments are elongated and linked together without the need for ATP. This allows the smooth muscle to maintain muscle “tone” that expends very little energy to the arteries and other visceral organs.
Smooth muscles are not under voluntary control; Thus, it is called an involuntary muscle. Triggers for smooth muscle contraction include hormones, nerve stimulation via the ANS, and local factors. In some places, such as the walls of visceral organs, stretching a muscle can cause it to contract (the tension-relaxation response).
Axons of neurons in the ANS do not form highly organized NMJs with smooth muscle as seen between motor neurons and skeletal muscle fibers. Instead, as axons course through smooth muscle there are a series of neurotransmitter-filled vesicles called varicosities, motor units ([link]). Varicosity releases neurotransmitters into the synaptic cleft. Also, visceral muscle in the walls of hollow organs (except the heart) contains pacemaker cells. Pacemaker cells can spontaneously initiate action potentials and contractions in muscles.
Axon-like swellings from autonomic neurons, called varicosities or “boutons,” form motor units through smooth muscle.
Muscle Tissue: Histology
Smooth muscle is organized in two ways: single-unit smooth muscle, which is more common; and as multiunit smooth muscle. The two body types have different locations and different characteristics. A single-unit muscle has its muscle fibers connected by gap junctions so that the muscle contracts as a unit. This type of smooth muscle is found in the walls of all visceral organs except the heart (which contains cardiac muscle) and is therefore commonly called visceral muscle. Because muscle fibers are not limited by the extent of organization and stretch of sarcomeres, visceral smooth muscle has a stress-relaxation response. This means that the muscle of the hollow organ is stretched when it fills, the mechanical stress of stretching triggers the contraction, but is immediately followed by rest so that the organ does not empty prematurely. This is important for hollow organs, such as the stomach or bladder, which expand continuously as they fill. The smooth muscle surrounding these organs can also maintain muscle tone when the organ is emptied and contracted, a feature that prevents “flabbiness” in an empty organ. In general, visceral smooth muscles produce slow, steady contractions that move food-like substances from the digestive system through the body.
Multiunit smooth muscle cells rarely have gap junctions, and thus are not electrically coupled. As a result, the contraction does not spread from one cell to another, but is instead confined to the originally excited cell. Stimulation for multiunit smooth muscle comes from autonomic nerves or hormones but not from stretch. This type of tissue is found around large blood vessels, in the respiratory tract, and in the eyes.
Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size. Unlike other muscles, smooth muscle can also divide to produce more cells, which is called hyperplasia. This can be clearly seen in the uterus during puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers and greatly increases the size of the myometrium.
Smooth muscle is found around various organs and pathways throughout the body. Smooth muscle cells have one nucleus and are spindle-shaped. Smooth muscle cells can undergo hyperplasia, dividing mitotically to form new cells. Smooth cells are nonstriated, but their sarcoplasm is filled with actin and myosin, as well as dense bodies in the sarcolemma to anchor the thin fibers and a network of intermediate filaments involved in pulling the sarcolemma toward the center of the fiber, shortening it in the process. ca
Cardiac Muscle And Electrical Activity
When ions leave the SR and enter through open voltage-gated calcium channels, they initiate contraction. Smooth muscle contraction begins when Ca
Binds to intracellular calmodulin, which then activates an enzyme called myosin kinase that phosphorylates myosin heads so they can form cross-bridges with actin and then pull thin filaments. Smooth muscle can be stimulated by pacemaker cells, by the autonomic nervous system, by hormones, spontaneously, or by stretch. Some smooth muscle fibers contain latch-bridges, cross-bridges that cycle slowly without the need for ATP; These muscles can maintain low-level contractions for long periods of time. Single-unit smooth muscle tissue contains gap junctions to synchronize membrane depolarization and contraction so that the muscle contracts as a unit. Single-unit smooth muscle in the walls of the viscera, called visceral muscles, has a stress-relaxation response that allows the muscles to stretch, contract, and relax as the organ expands. Multiunit smooth muscle cells do not have gap junctions and contractions do not propagate from one cell to another.
Smooth muscle contracts over a wide range of resting lengths because the actin and myosin filaments in smooth muscle are not as rigidly organized as in skeletal and cardiac muscle.
Single-unit smooth muscle is found in the walls of hollow organs; Multiunit smooth muscle is found in the airways of the lungs and large arteries. Single-unit smooth muscle cells contract synchronously, are connected by gap junctions, and exhibit spontaneous action potentials. Multiunit smooth cells lack gap junctions and their contractions are not synchronous.
Muscle Tissue And Motion
Sarcoplasmic structure that connects the sarcolemma and shortens the muscle as the thin filaments slide past the thick fibers
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