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Questions and Answers
What is the structural characteristic of thick filaments in smooth muscle?
What is the structural characteristic of thick filaments in smooth muscle?
- Thick filaments contain coils of actin filaments.
- Thick filaments are exclusively anchored at the cell membrane.
- Thick filaments are parallel assemblies of 3–5 filaments. (correct)
- Thick filaments are organized into a single long chain.
How do myosin heads function in smooth muscle contraction?
How do myosin heads function in smooth muscle contraction?
- Myosin heads only bind to thick filaments.
- Myosin heads are activated by Ca2+-CaM-dependent phosphorylation. (correct)
- Myosin heads bind only to actin filaments when ATP is present.
- Myosin heads can rotate without any ATP consumption.
Which model best describes the organization of contractile units in smooth muscle?
Which model best describes the organization of contractile units in smooth muscle?
- They consist of parallel arrays with varying lengths of thick and thin filaments. (correct)
- They form discrete sacromeres with precise alignment.
- They are organized in a linear fashion similar to skeletal muscle.
- They are composed of branching networks of myofilaments.
What role does calcium play in muscle contraction?
What role does calcium play in muscle contraction?
What regulates thin filament activity in smooth muscle?
What regulates thin filament activity in smooth muscle?
What is a unique feature of thick filaments in smooth muscle compared to those in striated muscle?
What is a unique feature of thick filaments in smooth muscle compared to those in striated muscle?
How do myosin heads in smooth muscle differ in function from those in skeletal muscle?
How do myosin heads in smooth muscle differ in function from those in skeletal muscle?
What are 'minisarcomeres' in the context of smooth muscle?
What are 'minisarcomeres' in the context of smooth muscle?
What role does calcium play in the contraction of smooth muscle?
What role does calcium play in the contraction of smooth muscle?
Which protein inhibits myosin ATPase in smooth muscle under normal conditions?
Which protein inhibits myosin ATPase in smooth muscle under normal conditions?
What is a primary regulatory mechanism for smooth muscle contraction?
What is a primary regulatory mechanism for smooth muscle contraction?
Which statement accurately describes the structural organization of smooth muscle?
Which statement accurately describes the structural organization of smooth muscle?
What characterizes the 'sidepolar' model of smooth muscle contraction?
What characterizes the 'sidepolar' model of smooth muscle contraction?
What occurs when calcium levels are high in smooth muscle cells?
What occurs when calcium levels are high in smooth muscle cells?
Which protein is involved in the regulation of muscle contraction by modulating ATPase activity?
Which protein is involved in the regulation of muscle contraction by modulating ATPase activity?
Which characteristic distinguishes smooth muscle contractile units from those of skeletal muscle?
Which characteristic distinguishes smooth muscle contractile units from those of skeletal muscle?
What is the role of calponin in smooth muscle contraction?
What is the role of calponin in smooth muscle contraction?
How does smooth muscle respond to various neurotransmitters and hormones?
How does smooth muscle respond to various neurotransmitters and hormones?
Which of the following accurately compares smooth muscle with skeletal muscle?
Which of the following accurately compares smooth muscle with skeletal muscle?
What mechanism is primarily responsible for the relaxation of smooth muscle?
What mechanism is primarily responsible for the relaxation of smooth muscle?
Which factor directly influences the phosphorylation state of MLC20 in smooth muscle?
Which factor directly influences the phosphorylation state of MLC20 in smooth muscle?
What role does myosin phosphatase serve in smooth muscle contraction regulation?
What role does myosin phosphatase serve in smooth muscle contraction regulation?
What is the primary difference in force regulation between skeletal and smooth muscle?
What is the primary difference in force regulation between skeletal and smooth muscle?
Which of the following natural remedies is mentioned as a treatment option for irritable bowel syndrome?
Which of the following natural remedies is mentioned as a treatment option for irritable bowel syndrome?
How does the decrease in sarcoplasmic Ca2+ concentration affect smooth muscle contraction?
How does the decrease in sarcoplasmic Ca2+ concentration affect smooth muscle contraction?
What does the term 'contractile unit' refer to in the context of smooth muscle?
What does the term 'contractile unit' refer to in the context of smooth muscle?
Which protein is known to promote the phosphorylation of MLC20 in smooth muscle?
Which protein is known to promote the phosphorylation of MLC20 in smooth muscle?
Study Notes
Relaxing Smooth Muscle
- Myosin phosphatase is a permanently active enzyme which removes phosphate groups from MLC20
- When calcium levels in the sarcoplasm fall and MLCK is deactivated, myosin phosphatase quickly strips MLC20 of phosphate groups and the muscle relaxes
Clinical Connection: Irritable Bowel Syndrome (IBS)
- IBS is a gastrointestinal disorder associated with intestinal cramping, increased flatulence, and altered bowel habits
- There is no known cause or cure for IBS
- Treatment options include antispasmodics
- Natural remedies such as peppermint oil can help by blocking calcium channels in smooth muscle, reducing contractility and relaxing the muscle
Regulation of Smooth Muscle
- Skeletal muscle force regulation depends on changes in intracellular calcium
- Smooth muscle force is regulated by changes in MLC20 phosphorylation
- MLC20 phosphorylation is dependent on MLCK and myosin phosphatase, meaning there are multiple control points
Contractile Units of Smooth Muscle
- Smooth muscle contractile units are densely packed actin and myosin filaments called "minisarcomeres"
- Smooth muscle does not contain Z-discs therefore there are no striations
- Thick filaments contain two heavy chains which contain ATPase activity and an actin-binding site
- Thick filaments also contain two pairs of light chains: essential and regulatory
“Sidepolar” Model of Contractile Units
- The myosin head groups within a thick filament are arranged sideways, allowing two actin filaments to be pulled simultaneously in different directions
- This arrangement allows for smooth muscle cells to shorten to a greater degree than striated muscle fibers
Thin Filaments
- Smooth muscle thin filaments do not contain troponin
- They are regulated by two proteins, caldesmon and calponin
- Caldesmon inhibits myosin ATPase activity but this inhibition is relieved by high calcium-calmodulin concentrations or by phosphorylation via calcium-calmodulin dependent protein kinase
- Calponin is an actin-associated myosin ATPase inhibitor
Organization of Smooth Muscle
- Smooth muscle arrays are tethered to the sarcolemma by dense plaques
- These plaques link adjacent cells mechanically
Membrane System of Smooth Muscle
- Smooth muscle cells lack T-tubules.
- Caveolae are linear arrays of membranes which perform a similar function to T-tubules
- Caveolae are flask-shaped sarcolemmal pockets which form narrow junctions with the underlying SR
- These pockets are enriched in L-type calcium channels
- The sarcoplasmic reticulum is a tubular network which stores calcium until contraction begins
- The smooth muscle SR contains two types of calcium release channels: those that are calcium activated and those that are activated by a calcium-dependent protein kinase
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Description
This quiz covers the mechanisms of smooth muscle relaxation, particularly focusing on myosin phosphatase and calcium levels. It also explores clinical connections, such as Irritable Bowel Syndrome (IBS), its symptoms, and treatment options including antispasmodics and natural remedies. Test your knowledge on smooth muscle physiology and its clinical implications.
