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Questions and Answers
What role does α-Actinin play in muscle structure?
What role does α-Actinin play in muscle structure?
Which protein prevents excessive stretching of the sarcomere?
Which protein prevents excessive stretching of the sarcomere?
What is the primary consequence of dysfunctional dystrophin?
What is the primary consequence of dysfunctional dystrophin?
What is the optimal sarcomere length for achieving maximum muscle performance?
What is the optimal sarcomere length for achieving maximum muscle performance?
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Where does calcium regulation primarily occur in skeletal muscle contraction?
Where does calcium regulation primarily occur in skeletal muscle contraction?
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What is the function of the T-tubule system in muscle fibers?
What is the function of the T-tubule system in muscle fibers?
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Which structure is associated with anchoring the contractile machinery to the muscle cell membrane?
Which structure is associated with anchoring the contractile machinery to the muscle cell membrane?
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What happens to muscle fibers during Duchenne muscular dystrophy?
What happens to muscle fibers during Duchenne muscular dystrophy?
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What component contributes to cellular respiration in muscle cells?
What component contributes to cellular respiration in muscle cells?
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Which of the following describes the extracellular function of calcium in muscle contraction?
Which of the following describes the extracellular function of calcium in muscle contraction?
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What is the primary function of the head region of myosin?
What is the primary function of the head region of myosin?
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How many light chains associate with each head region of myosin?
How many light chains associate with each head region of myosin?
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What role does troponin play in skeletal muscle contraction?
What role does troponin play in skeletal muscle contraction?
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What defines the sarcomere in skeletal muscle?
What defines the sarcomere in skeletal muscle?
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Which statement regarding the relationship of thick to thin filaments is true?
Which statement regarding the relationship of thick to thin filaments is true?
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What structural feature of myosin allows it to pull during contraction?
What structural feature of myosin allows it to pull during contraction?
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Which protein inhibits actin/myosin interaction in skeletal muscle?
Which protein inhibits actin/myosin interaction in skeletal muscle?
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What is the function of the Z disk in a sarcomere?
What is the function of the Z disk in a sarcomere?
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Which type of protein can be classified as structural within the sarcomere?
Which type of protein can be classified as structural within the sarcomere?
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What is the main driving ion for action potentials in skeletal muscle?
What is the main driving ion for action potentials in skeletal muscle?
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What is primarily responsible for driving K+ ions out of the cell?
What is primarily responsible for driving K+ ions out of the cell?
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Which ion is found at higher concentrations inside skeletal muscle cells compared to outside?
Which ion is found at higher concentrations inside skeletal muscle cells compared to outside?
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What condition is characterized by autoantibodies against nicotinic acetylcholine receptors?
What condition is characterized by autoantibodies against nicotinic acetylcholine receptors?
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What facilitates the excitation-contraction coupling in skeletal muscle?
What facilitates the excitation-contraction coupling in skeletal muscle?
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How does the action potential propagate in skeletal muscle fibers?
How does the action potential propagate in skeletal muscle fibers?
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Which treatment is commonly used in managing myasthenia gravis?
Which treatment is commonly used in managing myasthenia gravis?
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What is the role of L-type calcium channels in skeletal muscle action potentials?
What is the role of L-type calcium channels in skeletal muscle action potentials?
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What occurs during excitation-contraction coupling immediately after an action potential enters the T-tubules?
What occurs during excitation-contraction coupling immediately after an action potential enters the T-tubules?
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In what way does calcium affect muscle contraction?
In what way does calcium affect muscle contraction?
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What is the typical result of a neuromuscular junction failure?
What is the typical result of a neuromuscular junction failure?
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What is the cellular effect of acetylcholine release at the neuromuscular junction?
What is the cellular effect of acetylcholine release at the neuromuscular junction?
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What is the consequence of antibodies affecting nicotinic acetylcholine receptors in myasthenia gravis?
What is the consequence of antibodies affecting nicotinic acetylcholine receptors in myasthenia gravis?
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What is the primary function of the sarcoplasmic reticulum in muscle contraction?
What is the primary function of the sarcoplasmic reticulum in muscle contraction?
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Study Notes
Skeletal Muscle Structure
- Myosin: Composed of two heavy chains, a head region containing ATPase activity and an actin-binding site, two light chains (regulatory and essential), a neck region allowing the head to pivot, and a tail region that forms a coiled-coil structure.
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Actin: Part of the thin filament and interacts with myosin. Key proteins associated with actin include:
- Troponin: Ca++ sensitive protein, uncovers myosin binding site with increased Ca++ levels.
- TnC: Ca++ sensing
- TnI: inhibits actin/myosin interaction
- TnT: tethers troponin to tropomyosin
- Troponin: Ca++ sensitive protein, uncovers myosin binding site with increased Ca++ levels.
- Thick filaments: Formed by two bundles of myosin, joined tail-to-tail.
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Sarcomere: The functional unit of skeletal muscle, defined between two Z disks.
- Z disk: Anchors thin filaments.
- Overlap: Between thick and thin filaments results in banding patterns.
- Crossbridge: Region of overlap between thick and thin filaments.
Sarcomere Proteins
- α-Actinin: Binds thin filaments to the Z discs.
- Titin: A large protein that attaches to the Z disk at one end and the thick filament at the other. It acts as a spring limiting sarcomere stretch and centers the thick filaments.
- Dystrophin: A large protein associated with Z disks that anchors the contractile array to the cytoskeleton and surface membrane, aligning Z disks in adjacent myofibrils and muscle fibers.
Clinical Connection: Muscular Dystrophy
- Duchenne muscular dystrophy: Caused by a recessive X-linked mutation in the dystrophin gene.
- Loss of dystrophin function: Prevents cytoskeleton and contractile machinery from attaching to the sarcolemma, resulting in muscle necrosis and wasting.
Cross-Bridge Cycling and Contraction
- Cross-bridge cycling: A series of events involving myosin heads attaching to actin filaments, pulling the thin filaments, releasing from actin, and returning to their original position.
- Calcium regulations: Calcium binds to troponin, causing a conformational change and exposing the myosin binding sites on actin, facilitating cross-bridge cycling and muscle contraction.
- Sliding filaments: The thin filaments slide past the thick filaments, shortening the sarcomere.
Skeletal Muscle Mechanics
- Preload: Stretching a muscle to optimize actin and myosin interaction.
- Optimal sarcomere length: The resting skeletal muscle length maximizes the potential for crossbridge formation.
- Overstretching: Can separate thick and thin filaments, preventing contraction.
- Peak muscle performance: Occurs when crossbridge formation is maximized.
Organizational Hierarchy of Skeletal Muscle
- Skeletal muscle: Composed of muscle fibers.
- Muscle fibers: Composed of myofibrils.
- Myofibrils: Composed of sarcomeres.
Sarcoplasm
- Sarcoplasm: The cytoplasm of the muscle cells.
- Components: Rich in Mg2+, phosphates, glycogen granules, myoglobin, and mitochondria.
Membrane Systems of the Sarcomere
- T-Tubules: Carry action potentials from the cell surface to structures involved in Ca++ release.
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Sarcoplasmic reticulum: Specialized organelle responsible for Ca++ storage and release.
- SERCA pumps: Maintain the Ca++ gradient within the SR.
- Calsequestrin: Binds Ca++ and helps prevent Ca++ leakage from the SR.
- Triads: Junctions between T-tubules and the SR, critical for excitation-contraction coupling.
Neuromuscular Junction (NMJ)
- Synapse: Junction between motor neurons and skeletal muscle fibers.
- Neurotransmitter: Acetylcholine (ACh) released from the presynaptic terminal.
- Postsynaptic membrane: Contains ACh receptors.
Skeletal Muscle Action Potential
- Depolarization: Caused by the influx of Na+ ions.
- Repolarization: Caused by the efflux of K+ ions.
- Refractory period: A period of time after an action potential when another action potential cannot be generated.
Ion Chemical Gradients
- Potassium (K+): Is higher inside the cell than outside.
- Sodium (Na+), Chloride (Cl-), Calcium (Ca++): Are higher outside the cell than inside.
Membrane Potential Changes
- Resting membrane potential: Approximately -70 mV (negative inside).
- Depolarization: Caused by Na+ influx, membrane potential becomes less negative.
- Repolarization: Caused by K+ efflux, membrane potential returns to resting.
Action Potential Characteristics
- All-or-none principle: Action potentials occur in their full strength or not at all.
- Conduction: Action potentials propagate along the membrane without decreasing in amplitude.
Tissue-Specific Action Potentials
- Skeletal muscle: Action potentials have a fast rise to peak and a relatively long duration.
Clinical Connection: Myasthenia Gravis
- Autoimmune condition: Antibodies against ACh receptors.
- Effect on NMJ: Interferes with normal signaling at the NMJ.
- Symptoms: Muscle weakness.
- Treatment: Anticholinesterase inhibitors.
Excitation-Contraction Coupling
- Electrical to mechanical transduction: Action potentials trigger muscle contraction.
- T-tubule function: Carry action potentials to triads.
- Triad function: Trigger Ca++ release from the SR.
Calcium Delivery to Muscle Fibers
- Depolarization of T-tubules: Leads to Ca++ release from the SR.
Excitation-Contraction Coupling Mechanics
- Action potential: Propagates down the T-tubules
- T-tubule depolarization: Triggers Ca++ release from the SR.
- Ca++ binds to troponin: Exposing myosin binding sites on actin.
- Cross-bridge cycling: Causes muscle contraction.
- Ca++ removal: Muscle relaxation.
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Description
Test your knowledge on the structure of skeletal muscle, including the roles of myosin, actin, and the sarcomere. This quiz covers key proteins, their functions, and the organization of muscle filaments. Dive into the details of muscle morphology and understand how these components work together.
