MPP: BLOCK 2: SMOOTH MUSCLE PHYSIOLOGY

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Questions and Answers

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?

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?

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?

Calcium binds to regulatory proteins and initiates contraction. Signup and view all the answers

What regulates thin filament activity in smooth muscle?

Calcium-CaM-dependent protein kinases. Signup and view all the answers

What is a unique feature of thick filaments in smooth muscle compared to those in striated muscle?

Sided arrangement of myosin head groups Signup and view all the answers

How do myosin heads in smooth muscle differ in function from those in skeletal muscle?

They allow for more extensive shortening of myocytes Signup and view all the answers

What are 'minisarcomeres' in the context of smooth muscle?

Contractile units made of actin and myosin Signup and view all the answers

What role does calcium play in the contraction of smooth muscle?

It binds to calmodulin to initiate contraction Signup and view all the answers

Which protein inhibits myosin ATPase in smooth muscle under normal conditions?

Caldesmon Signup and view all the answers

What is a primary regulatory mechanism for smooth muscle contraction?

Phosphorylation of myosin light chains Signup and view all the answers

Which statement accurately describes the structural organization of smooth muscle?

It features 'minisarcomeres' without Z-discs Signup and view all the answers

What characterizes the 'sidepolar' model of smooth muscle contraction?

Myosin heads pull actin filaments in opposite directions Signup and view all the answers

What occurs when calcium levels are high in smooth muscle cells?

Activation of calmodulin and contraction Signup and view all the answers

Which protein is involved in the regulation of muscle contraction by modulating ATPase activity?

Caldesmon Signup and view all the answers

Which characteristic distinguishes smooth muscle contractile units from those of skeletal muscle?

Ability to shorten beyond limits Signup and view all the answers

What is the role of calponin in smooth muscle contraction?

Inhibits myosin ATPase activity Signup and view all the answers

How does smooth muscle respond to various neurotransmitters and hormones?

It is modulated by hundreds of chemical signals Signup and view all the answers

Which of the following accurately compares smooth muscle with skeletal muscle?

Smooth muscle can maintain contractility during volume changes Signup and view all the answers

What mechanism is primarily responsible for the relaxation of smooth muscle?

Dephosphorylation by myosin phosphatase Signup and view all the answers

Which factor directly influences the phosphorylation state of MLC20 in smooth muscle?

Intracellular calcium levels Signup and view all the answers

What role does myosin phosphatase serve in smooth muscle contraction regulation?

Deactivates MLCK Signup and view all the answers

What is the primary difference in force regulation between skeletal and smooth muscle?

Smooth muscle regulation primarily involves MLC20 phosphorylation. Signup and view all the answers

Which of the following natural remedies is mentioned as a treatment option for irritable bowel syndrome?

Peppermint oil Signup and view all the answers

How does the decrease in sarcoplasmic Ca2+ concentration affect smooth muscle contraction?

It leads to muscle relaxation. Signup and view all the answers

What does the term 'contractile unit' refer to in the context of smooth muscle?

The interaction between actin and myosin Signup and view all the answers

Which protein is known to promote the phosphorylation of MLC20 in smooth muscle?

Myosin light chain kinase (MLCK) Signup and view all the answers

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.