MPP block 2 lecture 3

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Questions and Answers

What is the role of IP3 in GPCR signaling?

It inhibits the action of Rho-kinase in smooth muscle contraction.

It directly phosphorylates myosin light chains.

It leads to increased intracellular calcium by binding to IP3 gated channels. (correct)

It acts as a secondary messenger to activate PKC.

Which mechanism directly allows for smooth muscle contraction to sustain with minimal energy expenditure?

The latch mechanism of myosin heads onto actin filaments. (correct)

Phosphorylation of calmodulin leading to increased MLCK activity.

Cross-bridge cycling between actin and phosphorylated myosin.

Increased ATP hydrolysis by myosin heads.

What effect does the calcium-calmodulin complex have after its formation?

It re-stimulates GPCR activation leading to more calcium influx.

It activates MLCK and inhibits calcium inhibitors like calponin. (correct)

It directly phosphorylates myosin light chains, bypassing MLCK.

It inhibits MLCK activity, reducing muscle contraction.

What initiates the process of muscle contraction in smooth muscle cells?

Influx of extracellular calcium through calcium channels. Signup and view all the answers

Which component is NOT involved in the activation of MLCK?

Myosin light chain phosphatase. Signup and view all the answers

What is the primary function of DAG in the signaling cascade?

To activate PKC which then leads to other signaling events. Signup and view all the answers

What triggers the smooth muscle relaxation process?

Withdrawal of the initial stimulus like a neurotransmitter. Signup and view all the answers

What is a common point of calcium signaling differentiation between smooth and skeletal muscle?

In smooth muscle, calcium binds to calmodulin, not troponin. Signup and view all the answers

How does Rho-kinase affect muscle contraction?

By inhibiting MLCP, thus increasing myosin phosphorylation. Signup and view all the answers

Which protein plays a critical role in the inhibition of actin-bound myosin ATPase during contraction?

Calponin. Signup and view all the answers

What initiates contraction in smooth muscle cells?

Phosphorylation of myosin through calmodulin-MLCK Signup and view all the answers

Which channel type is primarily responsible for calcium entry in smooth muscle cells?

Voltage-dependent calcium channels Signup and view all the answers

How does calcium-induced calcium release (CICR) occur in smooth muscle cells?

Via intracellular stores in the sarcoplasmic reticulum Signup and view all the answers

What differentiates contraction in skeletal muscle from contraction in smooth muscle?

Myosin-actin interactions organized into sarcomeres in skeletal muscle Signup and view all the answers

What role do varicosities play in smooth muscle contraction?

They act as sites for neurotransmitter release along an axon. Signup and view all the answers

Which of the following factors does NOT stimulate smooth muscle contraction?

Inhibition of MLCK activity Signup and view all the answers

Smooth muscle relaxation primarily involves the dephosphorylation of which protein?

Myosin light chain Signup and view all the answers

What is the primary messenger that mediates the action of hormones in smooth muscle?

Inositol trisphosphate (IP3) Signup and view all the answers

Which component is less developed in smooth muscle compared to skeletal muscle?

Sarcoplasmic reticulum Signup and view all the answers

Which of the following correctly describes the function of calcium in smooth muscle?

Activates enzyme systems leading to phosphorylation of myosin Signup and view all the answers

What is the primary consequence of a decrease in intracellular Ca²⁺ levels in smooth muscle?

Detachment of myosin-actin cross-bridges Signup and view all the answers

Which mechanism primarily contributes to the calcium influx necessary for smooth muscle contraction?

Opening of voltage-gated L-type calcium channels Signup and view all the answers

How do natural remedies such as peppermint oil help in alleviating symptoms of intestinal disorders?

By blocking Ca²⁺ channels in smooth muscle Signup and view all the answers

In contrast to skeletal muscle, smooth muscle excitation-contraction coupling primarily relies on which of the following?

Multiple competing signals converging on Ca²⁺ Signup and view all the answers

What role does inositol trisphosphate (IP3) play in smooth muscle contraction?

It triggers calcium release from the sarcoplasmic reticulum Signup and view all the answers

Which of the following best describes the function of myosin phosphatase (MLCP) in smooth muscle relaxation?

It dephosphorylates myosin, reducing its affinity for actin Signup and view all the answers

What is the effect of calcium-induced calcium release (CICR) in smooth muscle?

It amplifies the initial calcium signal for contraction Signup and view all the answers

How does smooth muscle differ from striated muscle in terms of contraction mechanisms?

Smooth muscle can shorten more than striated muscle fibers due to its unique structure. Signup and view all the answers

What role does Myosin Light Chain Kinase (MLCK) play in smooth muscle contraction?

It phosphorylates myosin, enabling interaction with actin filaments. Signup and view all the answers

Smooth muscle relaxation occurs when calcium levels decrease. What is the immediate result of this decrease?

Dephosphorylation of the light chain of myosin Signup and view all the answers

Which structure in smooth muscle cells is involved in regulating calcium signaling and enhancing contraction?

Caveolae Signup and view all the answers

Which factor has a minor role in calcium influx during smooth muscle contraction compared to voltage-gated channels?

Receptor-operated calcium channels Signup and view all the answers

What unique feature of the sidepolar model allows smooth muscle myocytes to function effectively during contraction?

It allows two actin filaments to be pulled simultaneously in opposite directions. Signup and view all the answers

In smooth muscle, which protein is absent that distinguishes it from striated muscle regarding contraction modulation?

Troponin Signup and view all the answers

What effect does calcium binding to calmodulin have in smooth muscle cells?

It activates MLCK, leading to myosin phosphorylation. Signup and view all the answers

What is the primary structural characteristic of smooth muscle cells that contributes to their ability to contract in multiple directions?

Dense bodies anchored randomly throughout the cytoplasm Signup and view all the answers

Which signaling pathway is primarily responsible for the activation of smooth muscle cells?

Multiple chemical signals activating signaling cascades Signup and view all the answers

What is the primary function of dense plaques in smooth muscle cells?

To anchor actin filaments and transmit force during contraction Signup and view all the answers

How does the organization of actin and myosin in smooth muscle contribute to its plasticity during changes in organ volume?

The lack of striations allows smooth muscle to adjust its tension dynamically. Signup and view all the answers

Study Notes

Intracellular Calcium Dynamics

Decrease in intracellular Ca²⁺ levels leads to calcium unbinding from calmodulin.
Myosin light chain phosphatase (MLCP) dephosphorylates myosin, lowering its affinity for actin.
Myosin-actin cross-bridges detach, resulting in reduced tension and muscle relaxation.

Irritable Bowel Syndrome (IBS)

IBS is a gastrointestinal disorder marked by intestinal cramping, increased flatulence, and altered bowel habits.
Treatment options are limited and may include antispasmodic medications.
Natural remedies like peppermint oil can help by blocking Ca²⁺ channels in smooth muscle, promoting muscle relaxation.

Excitation-Contraction Coupling in Smooth vs. Skeletal Muscle

In smooth muscle, excitation-contraction coupling is influenced by multiple competing signals converging on calcium, unlike the actin-mediated coupling in skeletal muscle.
Three main calcium sources for smooth muscle contraction:
- Ca²⁺ influx across the sarcolemma.
- Calcium-induced calcium release (CICR) from the sarcoplasmic reticulum (SR).
- Inositol trisphosphate (IP3)-mediated Ca²⁺ release from the SR.

Calcium Influx Mechanisms

Voltage-Gated Calcium Channels (VGCCs): Depolarization opens L-type channels, allowing extracellular Ca²⁺ to enter.
Receptor-Operated Calcium Channels (ROCs): Aid in depolarization and Ca²⁺ flux, but have a minor influence in smooth muscle contraction.

Calcium-Induced Calcium Release (CICR)

Initial extracellular Ca²⁺ influx activates ryanodine receptors (RyR) on the SR, releasing more calcium into the cytoplasm and enhancing contraction.

IP3-Mediated Calcium Release

G-protein-coupled receptor (GPCR) activation generates inositol trisphosphate (IP3), which binds to its receptors on the SR, releasing calcium.
Multiple signaling pathways involve phospholipase C (PLC) and Rho-kinase, ultimately influencing MLCP inhibition.

Smooth Muscle Contraction Process

Influx of Ca²⁺ triggers the formation of Ca²⁺-calmodulin complex.
This complex activates myosin light chain kinase (MLCK), which phosphorylates myosin light chains.
Phosphorylated myosin interacts with actin, forming cross-bridges that lead to muscle contraction via power strokes.

Muscle Relaxation

Occurs through stimulus removal, calcium pump activation (primarily SERCA), and removal of Ca²⁺ from the cytoplasm.

Structural Characteristics of Smooth Muscle

Myocytes: Spindle-shaped, non-striated with a single nucleus, smaller than skeletal fibers.
Cytoskeleton: Actin and myosin filaments arranged randomly, allowing multidirectional contraction; dense bodies anchor actin similarly to Z-discs in striated muscle.
Calcium Signaling: Caveolae help concentrate signaling molecules and regulate calcium influx.

Differences in Myocyte Functionality

Skeletal Muscle: Uses organized sarcomeres; contraction regulated by Ca²⁺ binding to troponin for rapid contractions.
Smooth Muscle: Lacks sarcomeres; contraction regulated through phosphorylation via calmodulin and MLCK, allowing sustained contractions.

Neuromuscular Junction in Smooth Muscle

Varicosities along the axon form the sites for neurotransmitter release, controlling contraction via the autonomic nervous system (ANS).

Adaptability of Smooth Muscle

Capable of maintaining contractility as organ luminal volume changes due to inherent plasticity and structural features like minisarcomeres and dense plaques.

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Description

This quiz covers essential aspects of muscle physiology, focusing on intracellular calcium dynamics, excitation-contraction coupling, and the impact of calcium on muscle relaxation. Topics such as irritable bowel syndrome and its relation to calcium signaling are also explored. Test your understanding of these physiological processes and their implications for muscle function.