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Questions and Answers
Which of the following statements about thick filaments is true?
Which of the following statements about thick filaments is true?
Troponin is one of the proteins found in thin filaments.
Troponin is one of the proteins found in thin filaments.
True
What are the primary components of a thick filament?
What are the primary components of a thick filament?
Many myosin molecules with heads protruding from opposite ends.
Each thin filament consists of two strands of ______ subunits twisted into a helix.
Each thin filament consists of two strands of ______ subunits twisted into a helix.
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Match the following components with their respective functions or characteristics:
Match the following components with their respective functions or characteristics:
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What is the primary structure that generates force during muscle contraction?
What is the primary structure that generates force during muscle contraction?
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Thin filaments slide past thick filaments during muscle contraction.
Thin filaments slide past thick filaments during muscle contraction.
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Name the two types of filaments involved in the sliding filament model.
Name the two types of filaments involved in the sliding filament model.
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The _____ disc marks the boundary of a sarcomere.
The _____ disc marks the boundary of a sarcomere.
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Match the following parts of a muscle fiber with their functions:
Match the following parts of a muscle fiber with their functions:
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What must occur for muscle fiber shortening to happen?
What must occur for muscle fiber shortening to happen?
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In a relaxed muscle state, thin and thick filaments completely overlap.
In a relaxed muscle state, thin and thick filaments completely overlap.
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During contraction, actin and myosin _____ more.
During contraction, actin and myosin _____ more.
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What initiates the sliding filament mechanism of muscle contraction?
What initiates the sliding filament mechanism of muscle contraction?
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During muscle contraction, the A bands shorten.
During muscle contraction, the A bands shorten.
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What is the role of intracellular Ca2+ levels during muscle contraction?
What is the role of intracellular Ca2+ levels during muscle contraction?
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The action potential is propagated along the ______ after it is generated.
The action potential is propagated along the ______ after it is generated.
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Match the following components with their functions in muscle contraction:
Match the following components with their functions in muscle contraction:
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What happens to the Z discs during muscle contraction?
What happens to the Z discs during muscle contraction?
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Excitation-contraction coupling does not involve ion permeability changes.
Excitation-contraction coupling does not involve ion permeability changes.
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How do cross bridges contribute to muscle contraction?
How do cross bridges contribute to muscle contraction?
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What ion primarily enters the cell during the depolarization phase of action potential generation?
What ion primarily enters the cell during the depolarization phase of action potential generation?
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During the action potential, voltage-gated K+ channels open before Na+ channels.
During the action potential, voltage-gated K+ channels open before Na+ channels.
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What generates the end plate potential at the neuromuscular junction?
What generates the end plate potential at the neuromuscular junction?
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The spread of local depolarization current along the sarcolemma opens __________ channels.
The spread of local depolarization current along the sarcolemma opens __________ channels.
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Match the following channels with their states during an action potential:
Match the following channels with their states during an action potential:
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What follows the opening of voltage-gated sodium channels in action potential generation?
What follows the opening of voltage-gated sodium channels in action potential generation?
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The local depolarization wave is responsible for starting new action potentials in adjacent areas of the sarcolemma.
The local depolarization wave is responsible for starting new action potentials in adjacent areas of the sarcolemma.
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What is the role of acetylcholine (ACh) in muscle cell activation?
What is the role of acetylcholine (ACh) in muscle cell activation?
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What causes the detachment of the cross bridge in the cross bridge cycle?
What causes the detachment of the cross bridge in the cross bridge cycle?
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The myosin head pivots and bends to pull the actin filament toward the M line.
The myosin head pivots and bends to pull the actin filament toward the M line.
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What happens to the myosin head during the 'cocking' phase of the cross bridge cycle?
What happens to the myosin head during the 'cocking' phase of the cross bridge cycle?
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In the absence of ATP, myosin heads will not detach, causing __________.
In the absence of ATP, myosin heads will not detach, causing __________.
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Match the following steps of the cross bridge cycle with their descriptions:
Match the following steps of the cross bridge cycle with their descriptions:
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Which molecule is hydrolyzed to provide energy for the myosin head to move to a high-energy state?
Which molecule is hydrolyzed to provide energy for the myosin head to move to a high-energy state?
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What triggers the release of Ca2+ from the sarcoplasmic reticulum?
What triggers the release of Ca2+ from the sarcoplasmic reticulum?
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Troponin prevents myosin from binding to actin until calcium binds to it.
Troponin prevents myosin from binding to actin until calcium binds to it.
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ADP and Pi are released during the cocking phase of the cross bridge cycle.
ADP and Pi are released during the cocking phase of the cross bridge cycle.
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What is the role of calcium ions (Ca2+) in the cross bridge cycle?
What is the role of calcium ions (Ca2+) in the cross bridge cycle?
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What happens when calcium binds to troponin?
What happens when calcium binds to troponin?
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The process where myosin binds to actin to form ___________ is known as contraction.
The process where myosin binds to actin to form ___________ is known as contraction.
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Match the components in muscle contraction with their roles:
Match the components in muscle contraction with their roles:
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What is the outcome of myosin binding to exposed active sites on actin?
What is the outcome of myosin binding to exposed active sites on actin?
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The contraction process occurs before E-C coupling is complete.
The contraction process occurs before E-C coupling is complete.
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What does the term E-C coupling refer to?
What does the term E-C coupling refer to?
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Study Notes
Muscular System Overview
- Muscles comprise nearly half of the body's mass.
- Muscles transform chemical energy (ATP) into directed mechanical energy, creating force.
- Muscles are classified into three types: skeletal, cardiac, and smooth.
- Prefixes like "myo," "mys," and "sarco" are frequently used in muscle-related terminology.
Muscle Tissue Types
Skeletal Muscle
- Skeletal muscles are attached to bones and skin.
- Their cells are elongated, called muscle fibers, and are striated (striped).
- Skeletal muscle contractions are voluntary (consciously controlled).
- These muscles contract quickly but tire easily.
- They require nervous system stimulation for contraction.
Cardiac Muscle
- Found only in the heart, forming the bulk of its walls.
- Cardiac muscle cells are striated.
- These muscles can contract without nervous system stimulation.
- Contraction of cardiac muscle is involuntary.
Smooth Muscle
- Found in the walls of hollow organs (e.g., stomach, urinary bladder, airways).
- Smooth muscle cells lack striations.
- Smooth muscle contractions are involuntary.
- Smooth muscle can contract without nervous system stimulation.
Muscle Tissue Comparisons
- A table comparing characteristics of skeletal, cardiac, and smooth muscle is provided.
- Key distinctions include body location, cell shape/appearance, myofibrils/sarcomeres, and presence of T tubules and/or gap junctions.
- Different types also have various contraction regulation mechanisms.
Special Characteristics of Muscle Tissue
- Excitability (responsiveness): The ability to receive and respond to stimuli.
- Contractility: The ability to shorten forcibly when stimulated.
- Extensibility: The ability to be stretched.
- Elasticity: The ability to recoil to resting length.
Muscle Functions
- Movement: Movement of bones or fluids (e.g., blood).
- Maintaining posture and body position.
- Stabilizing joints.
- Heat generation: Primarily skeletal muscles.
- Additional functions: Protecting organs, forming valves, controlling pupil and lumen size, and causing "goosebumps."
Skeletal Muscle Structure and Function
- Each skeletal muscle is served by one nerve, one artery, and one or more veins.
- The connective tissue sheaths (epimysium, perimysium, and endomysium) surround and support muscle fibers.
- Muscles attach in at least two places:
- Insertion (movable bone)
- Origin (immovable or less movable bone)
- Some attachments are direct, others indirect (tendons or aponeuroses).
Skeletal Muscle Fiber Structure
- Skeletal muscle fibers are long, cylindrical, multinucleate cells with peripheral nuclei.
- Sarcolemma: The plasma membrane of the muscle fiber.
- Sarcoplasm: The cytoplasm of the muscle fiber containing glycosomes and myoglobin (oxygen-binding protein).
- Myofibrils: Contractile organelles within the muscle fiber, containing sarcomeres (the functional units of contraction).
- Sarcoplasmic reticulum: Specialized smooth endoplasmic reticulum that regulates calcium ion levels within the muscle fiber.
- T tubules: Tubular infoldings of the sarcolemma that penetrate through the muscle fiber, bringing the action potential from the surface membrane into the interior of the cell.
Myofibrils and Sarcomeres
- Myofibrils are densely packed, rod-like elements that compose roughly 80% of the muscle cell volume.
- They are composed of repeating subunits called sarcomeres.
- Sarcomeres exhibit striations (alternating light and dark bands).
- The structure of thick and thin filaments within sarcomeres accounts for the banding patterns.
Sliding Filament Model of Contraction
- In a relaxed muscle, thin and thick filaments overlap only at the ends of the A band.
- During contraction, thin filaments slide past thick filaments, causing the sarcomere to shorten.
- Myosin heads bind to actin, forming cross bridges, pulling the thin filaments toward the center of the sarcomere.
- ATP is crucial for cross-bridge detachment and myosin head recocking.
Physiology of Skeletal Muscle Fibers
- Excitation: A nervous system signal is required before contraction can occur.
- Excitation-contraction coupling: The events that connect the nerve signal to the muscle contraction.
- AP propagated along the sarcolemma and down into T tubules, which eventually causes SR to release calcium ions.
Neuromuscular Junction (NMJ)
- A specialized area where a motor neuron synaptic terminal meets a muscle fiber.
- Synaptic vesicles contain acetylcholine, a neurotransmitter involved in signal transmission across the synapse.
Events at the Neuromuscular Junction
- Nerve impulse arrives at the axon terminal.
- ACh is released into the synaptic cleft.
- ACh diffuses across the cleft and binds to receptors on the sarcolemma.
- This binding triggers a local electrical event (end-plate potential).
- The end-plate potential triggers an action potential in the muscle fiber.
Destruction of Acetylcholine
- Acetylcholinesterase breaks down ACh in the neuromuscular junction.
- This termination of ACh activity prevents continuous muscle fiber contraction.
Channels in Muscle Contraction
- Voltage-gated channels are central to the process.
- ACh binds to receptors, opening ligand-gated channels allowing Na+ and K+ flow.
- Voltage-gated Na+ channels open leading to depolarization (action potential).
- Voltage-sensitive proteins in T tubules help trigger Ca2+ release.
Role of Calcium in Contraction
- At low Ca2+ levels, tropomyosin blocks active sites on actin, preventing myosin from binding.
- At higher Ca2+ levels, Ca2+ binds to troponin, moving tropomyosin away, exposing active sites, and allowing myosin binding.
Cross Bridge Cycle
- The cycle of cross-bridge formation, working stroke, and detachment continually pulls thin filaments inward.
- Requires energy from ATP.
- Crucial to muscle contraction.
Muscle Mechanics
- Isometric contractions: Muscle tension increases but does not exceed the load, no shortening occurs.
- Isotonic contractions: Muscle shortens because muscle tension exceeds the load.
- Force and duration of contraction vary: Based on stimulus frequency and intensity.
- Motor units: A motor neuron and all the muscle fibers it supplies. Smaller motor units enable fine control
Homeostatic Imbalances
- Myasthenia gravis: Autoimmune disease where antibodies block ACh receptors leading to progressive muscle weakness.
- Rigor mortis: When death occurs, muscle fibers run out of ATP causing cross-bridge detachment failure, resulting in muscle stiffening.
Muscular Dystrophies
- Duchenne muscular dystrophy (DMD): Inherited sex-linked disorder, characterized by a deficiency or absence of dystrophin that supports the sarcolemma.
Polio
- Polio is a viral infection that destroys motor neurons.
Muscle Action Types
- Agonists: Main movers in joint actions
- Antagonists: Muscles that oppose or reverse agonist actions
- Synergists: Aid agonists in a movement
- Fixators: Stabilize bones involved in the movement.
Muscle Names
- Location: Reflecting the muscles' location (e.g., frontalis, pectoralis).
- Size: Indicating muscle size (e.g., maximus, minimus).
- Shape: Based on muscle shape (e.g., deltoid, trapezius).
- Direction of fibers: Muscle fibers' orientation (e.g., rectus, oblique).
- Number of origins: Based on the number of attachments to the bone (e.g., bicep, tricep).
- Attachments: The location and attachment points on bones.
- Movement actions: The type of movement the muscle performs (e.g., flexors, extensors).
Muscle Fiber Types
- Speed of contraction (slow/fast twitch): Classified based on the speed of myosin ATPase activity.
- Metabolic pathways: Classified based on how they generate ATP (aerobic vs. anaerobic).
- Oxidative fibers: Use aerobic pathways to generate ATP, suited for endurance activities.
- Glycolytic fibers: Use anaerobic glycolysis to generate ATP, more suited for short, powerful bursts of activity.
Exercise Types
- Isotonic: Muscles change in length during contraction (e.g., lifting weights).
- Isometric: Muscle tension increases but no change in length (e.g., holding a heavy object).
- Anaerobic: Respiration without oxygen, can result in lactic acid buildup.
- Aerobic: Respiration using oxygen, a longer term energy source.
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Test your knowledge on thick filaments in muscle physiology with this quiz. Explore the essential components of thick filaments, their functions, and the sliding filament model. Stay sharp on terms like sarcomere and muscle contraction mechanisms.
