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Questions and Answers
What role does calcium play in smooth muscle contraction?
Which neurotransmitter is known to cause constriction of smooth muscle via M-cholinergic receptors?
What distinguishes tonic muscle contraction from phasic muscle contraction?
Which of the following statements about G-protein coupled receptors (GPCR) is correct?
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How does myosin contribute to the contraction of smooth muscle?
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What type of muscle is characterized by discreet motor units and is always activated to produce tonic contractions?
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Which feature differentiates single-unit smooth muscle from multi-unit smooth muscle?
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What is the primary function of calmodulin in smooth muscle contraction?
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Which type of smooth muscle is known for its phasic contractions and is commonly found in the gastrointestinal tract?
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What is the role of myosin light chain phosphatase in smooth muscle physiology?
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Which of the following statements about smooth muscle contraction is true?
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What type of smooth muscle shows rhythmic contractions and is influenced by action potentials?
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Which of the following hormones or neurotransmitters commonly affects smooth muscle contraction?
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What role do Interstitial Cells of Cajal (ICC) play in smooth muscle contractions?
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What is the primary mechanism by which slow waves propagate through smooth muscle cells?
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During the depolarization phase of the smooth muscle action potential, which type of channels primarily opens?
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Which intracellular protein does calcium bind to initiate smooth muscle contraction?
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What is the effect of the opening of Kir channels during smooth muscle action potential?
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In the context of excitation-contraction coupling, what does MLCK stand for?
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What is the typical amplitude and duration of the slow waves generated by ICC in smooth muscle?
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What triggers the contraction of smooth muscle cells ultimately?
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Study Notes
Smooth Muscle Structure
- Smooth muscle (SM) is found in various tissues, including blood vessels, airways, alimentary tract, bladder, and uterus.
- Cells are not striated and controlled by autonomic nerves, hormones, and local mediators.
- SM contractions are slower than striated muscle and can be maintained for extended periods.
- Some SM cells are phasic and show rhythmic contractions.
- Responsible for smooth muscle contractions are the thick filaments (Myosin) and thin filaments (Actin), which are attached to dense bodies.
Multi-Unit Muscle
- Discreet motor units like skeletal muscle, and each unit must be activated independently,
- This type of muscle always produces smooth, tonic contractions and is not rhythmic.
- Found in the eye, large blood vessels, and airways.
Single-Unit Muscle
- Cells function as a single unit with gap junctions between cells.
- Found in the bladder, small blood vessels, and the gastrointestinal tract.
- The cells can show slow waves or basal electrical rhythms and exhibit tonic contractions in tissues like the bladder and small blood vessels.
- Display phasic (rhythmic) contractions in the gut and uterus.
Pacemaker Activity and Slow Waves
- SM can exhibit rhythmic contractions as a result of "pacemaker" activity or "slow waves".
- The slow waves are generated by interstitial cells of Cajal (ICC) in tissues of the gut.
- These slow waves are not action potentials, but rather sub-threshold depolarizations that can trigger action potentials, allowing the cells to contract.
- The slow waves are transmitted from ICC to neighboring muscle (SM) cells via gap junctions, resulting in synchronized contractions.
Action Potentials in Smooth Muscle
- Action potentials in SM are triggered by the opening of voltage-gated calcium channels (L-type channels).
- These calcium channels open when the membrane potential is depolarized, allowing calcium to enter the cell.
- The entry of calcium causes further depolarization, which leads to a positive feedback loop.
- Repolarization occurs when potassium channels open, causing potassium to leave the cell.
- The resting potential is maintained due to the activity of potassium channels (Kir).
Excitation-Contraction Coupling in Smooth Muscle
- Calcium plays a key role in smooth muscle contraction.
- In response to an action potential, intracellular calcium ([Ca2+]i) increases.
- Calcium binds to calmodulin, which activates myosin light chain kinase (MLCK).
- MLCK phosphorylates myosin, which allows it to bind to actin and initiate contraction.
- Dephosphorylation of myosin by myosin light chain phosphatase (MLCP) terminates contraction.
Neural Control of Smooth Muscle
- Neural control of smooth muscle can be influenced by the neurotransmitters acetylcholine (ACh) and noradrenaline (NA).
- ACh typically causes smooth muscle contraction by acting on muscarinic (M) receptors.
- In the gut, ACh can also induce relaxation via the release of nitric oxide.
- Noradrenaline can cause either relaxation or contraction, depending on the type of receptor it interacts with.
- Typically, noradrenaline causes contraction via alpha (α) adrenergic receptors and relaxation via beta (β) adrenergic receptors.
Smooth Muscle Contraction
- Calcium influx can be triggered by various mechanisms, including:
- Action potentials (e.g., gut)
- Receptor binding (e.g., blood vessels and adrenaline)
- The source of calcium for contraction can vary depending on the type of smooth muscle.
- Some smooth muscles rely primarily on intracellular calcium stores (e.g., airways, blood vessels).
- Others rely mainly on extracellular calcium (e.g., gut).
- Contraction occurs due to the interaction of Actin and Myosin.
Summary
- Three main types of muscle exist: smooth muscle, skeletal muscle, and cardiac muscle.
- Smooth muscle contraction is controlled by the binding of calcium to calmodulin, which activates MLCK (Myosin light chain kinase).
- MLCK phosphorylates Myosin, allowing it to bind to Actin and initiate contraction.
- Tonic muscle produces constant contraction, like in blood vessels.
- Phasic muscle shows rhythmic contractions, like in the gut.
- Slow waves are generated in the gut by ICC.
- Gut muscle contraction is triggered by opening L-type calcium channels, allowing direct calcium entry.
- ACh usually causes contraction but can also relax via the release of nitric oxide.
- Noradrenaline relaxes via b-receptors and stimulates via a-receptors.
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Description
Explore the fascinating structure and types of smooth muscle in this quiz. Learn about smooth muscle's unique characteristics, including the differences between multi-unit and single-unit muscle types. This quiz covers various functions and examples of smooth muscle found in the body.
