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Questions and Answers
What percentage of the body is composed of skeletal muscle?
What percentage of the body is composed of skeletal muscle?
What are the two main types of filaments found in myofibrils?
What are the two main types of filaments found in myofibrils?
What role do titin molecules play in skeletal muscle?
What role do titin molecules play in skeletal muscle?
What structure encloses the skeletal muscle fiber?
What structure encloses the skeletal muscle fiber?
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Which bands in myofibrils contain actin filaments?
Which bands in myofibrils contain actin filaments?
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What is the main function of mitochondria in muscle fibers?
What is the main function of mitochondria in muscle fibers?
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What occurs at the neuromuscular junction to initiate muscle contraction?
What occurs at the neuromuscular junction to initiate muscle contraction?
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What is the function of the sarcoplasmic reticulum in muscle fibers?
What is the function of the sarcoplasmic reticulum in muscle fibers?
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What happens to myosin heads when ATP is not available during muscle contraction?
What happens to myosin heads when ATP is not available during muscle contraction?
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At what sarcomere length does a muscle generate maximum force of contraction?
At what sarcomere length does a muscle generate maximum force of contraction?
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How does the load affect the velocity of muscle contraction?
How does the load affect the velocity of muscle contraction?
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What is the formula for calculating work output during muscle contraction?
What is the formula for calculating work output during muscle contraction?
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Which of the following ions must be pumped to maintain ionic balance during muscle activity?
Which of the following ions must be pumped to maintain ionic balance during muscle activity?
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What happens to active tension in a muscle during excessive stretching beyond 2.2 micrometers?
What happens to active tension in a muscle during excessive stretching beyond 2.2 micrometers?
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Which statement best describes the Fenn effect?
Which statement best describes the Fenn effect?
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Which process requires energy during muscle contraction?
Which process requires energy during muscle contraction?
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What is the duration for which the concentration of ATP in muscle fiber can sustain full contraction?
What is the duration for which the concentration of ATP in muscle fiber can sustain full contraction?
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Which source of energy can sustain muscle contraction even in the absence of oxygen?
Which source of energy can sustain muscle contraction even in the absence of oxygen?
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What is the primary energy source for sustained long-term muscle contraction?
What is the primary energy source for sustained long-term muscle contraction?
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How long can the combined energy from stored ATP and phosphocreatine sustain maximal muscle contraction?
How long can the combined energy from stored ATP and phosphocreatine sustain maximal muscle contraction?
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What happens to the rate of ATP formation by glycolysis compared to when foodstuffs react with oxygen?
What happens to the rate of ATP formation by glycolysis compared to when foodstuffs react with oxygen?
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Which substance is primarily derived from the breakdown of glycogen during glycolysis?
Which substance is primarily derived from the breakdown of glycogen during glycolysis?
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For prolonged maximal muscle activity, which energy source provides the majority of energy?
For prolonged maximal muscle activity, which energy source provides the majority of energy?
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What is the role of phosphocreatine in ATP reconstitution?
What is the role of phosphocreatine in ATP reconstitution?
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What role does acetylcholine play at the muscle fiber membrane?
What role does acetylcholine play at the muscle fiber membrane?
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What is primarily responsible for initiating an action potential in muscle fibers?
What is primarily responsible for initiating an action potential in muscle fibers?
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What happens to Ca ions following a muscle contraction?
What happens to Ca ions following a muscle contraction?
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How is the myosin filament structured?
How is the myosin filament structured?
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What type of enzyme activity is exhibited by the myosin head?
What type of enzyme activity is exhibited by the myosin head?
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What is the structure of actin filaments composed of?
What is the structure of actin filaments composed of?
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What happens to the cross-bridges between actin and myosin during muscle contraction?
What happens to the cross-bridges between actin and myosin during muscle contraction?
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What is the function of the light chains in a myosin molecule?
What is the function of the light chains in a myosin molecule?
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What is the primary role of tropomyosin in muscle contraction?
What is the primary role of tropomyosin in muscle contraction?
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Which subunit of troponin has an affinity for calcium ions?
Which subunit of troponin has an affinity for calcium ions?
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What initial effect does calcium have on the troponin-tropomyosin complex?
What initial effect does calcium have on the troponin-tropomyosin complex?
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What happens when the troponin-tropomyosin complex is present on an actin filament?
What happens when the troponin-tropomyosin complex is present on an actin filament?
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According to the walk-along theory of muscle contraction, what occurs when a myosin head attaches to an active site on actin?
According to the walk-along theory of muscle contraction, what occurs when a myosin head attaches to an active site on actin?
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In the resting state, what is the effect of tropomyosin on muscle contraction?
In the resting state, what is the effect of tropomyosin on muscle contraction?
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What is required for the contraction process to initiate involving the troponin-tropomyosin complex?
What is required for the contraction process to initiate involving the troponin-tropomyosin complex?
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Which component is responsible for the strong initial binding of actin and myosin when troponin-tropomyosin is absent?
Which component is responsible for the strong initial binding of actin and myosin when troponin-tropomyosin is absent?
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Study Notes
Skeletal Muscle Contraction
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Skeletal muscle comprises about 40% of the body's mass, with smooth and cardiac muscle accounting for another 10%.
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Skeletal muscles are composed of numerous muscle fibers, each of which contains smaller subunits.
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Each fiber is typically innervated by a single nerve ending located near its middle.
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Myofibrils are complex organelles within muscle fibers, comprised of myofilaments.
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Sarcomeres are segments of myofibrils; the basic units of muscle contraction.
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Myofilaments, or filaments, are extended macromolecular structures composed of thick and thin filaments.
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The thick filaments are myosin filaments, with globular heads (myosin heads).
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The thin filaments are actin filaments, and associated regulatory proteins (tropomyosin and troponin)
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The sarcolemma is the membrane enclosing a muscle fiber, composed of a plasma membrane and a coat of polysaccharide material containing collagen fibrils.
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Tendon fibers fuse with the sarcolemma at each end of the muscle fiber, forming bundles of tendons that connect muscles to bones.
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Each muscle fiber contains hundreds to thousands of myofibrils.
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Myofibrils are composed of 1500 myosin and 3000 actin filaments.
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Light bands (I-bands) contain only actin filaments, dark bands (A-bands) contain both actin and myosin filaments.
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Small projections from myosin filaments are cross-bridges; interaction between these bridges and actin filaments causes muscle contraction.
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Titin filamentous molecules maintain the side-by-side relationship between myosin and actin filaments.
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Titin molecules hold the myosin and actin filaments in place.
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Sarcoplasm is the intracellular fluid between myofibrils. It contains K, Mg, phosphate, protein enzymes, and mitochondria.
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Mitochondria provide energy in the form of ATP to the contracting myofibrils.
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The sarcoplasmic reticulum regulates Ca storage, release, and reuptake, thereby controlling muscle contraction.
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Action potential travels along a motor nerve to its endings on muscle fibers.
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At each ending, neurotransmitter acetylcholine is released.
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Acetylcholine acts on a muscle fiber membrane to open channels that allow Na+ ions to diffuse into the muscle.
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Local depolarization initiates an action potential along the muscle fiber membrane.
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The action potential depolarizes the muscle membrane; the sarcoplasmic reticulum releases large quantities of Ca2+ ions.
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Ca2+ initiates attractive forces between actin and myosin filaments, causing them to slide past each other (contraction).
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After contraction, Ca2+ is pumped back into the sarcoplasmic reticulum.
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The myosin filament is made up of 200+ individual myosin molecules.
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The protruding arms and heads of myosin molecules are called cross-bridges.
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Myosin head acts as an ATPase enzyme that cleaves ATP and uses the energy to energize contraction.
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A thin filament consists of two strands of actin subunits intertwined with tropomyosin and troponin.
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In the resting state, tropomyosin covers the active sites on the actin filaments, preventing myosin-actin interactions.
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Ca2+ binding to troponin causes a conformational change in tropomyosin, exposing active sites on actin, allowing contraction.
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As soon as actin filaments are activated by Ca2+ ions, myosin heads are attracted to the active sites and initiate contraction (walk-along theory).
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When a myosin head attaches to an active site, intramolecular forces between the head and arm of its cross-bridge change.
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Large amounts of ATP are cleaved to form ADP during contraction; the more work performed by the muscle, the more ATP is cleaved (Fenn effect).
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Cross-bridge formation occurs, energized myosin head attaches to an actin myofilament, forming cross-bridge.
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Energized myosin head pivots and bends, changing to low-energy state.
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As a result, actin filament is pulled towards the M line.
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Cross-bridge detachment occurs after ATP attaches to myosin, which weakens the link between myosin and actin.
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Myosin head detaches and returns to high-energy position (cocking).
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In the absence of ATP, myosin heads will not detach, causing rigor mortis.
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ATP is derived from phosphocreatine, glycolysis, and oxidative metabolism.
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Glycolysis is a rapid source of ATP but is limited by lactate and H+ accumulation.
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Oxidative metabolism is the primary source of energy for sustained contraction.
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Efficiency of muscle contraction is <25%, the remainder is transformed into heat.
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Isometric contractions do not shorten the muscle, while isotonic contractions shorten the muscle in response to a constant tension.
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Isometric system records changes in force of muscle contraction independently of load inertia.
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Whole muscle has connective tissue, sarcomeres in different parts contract differently.
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When muscle is at normal resting length (about 2 micrometers), it contracts with maximal force.
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The maximum tension decreases as the muscle is stretched beyond its normal length.
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Muscle tone results from a low rate of nerve impulses from the spinal cord.
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Nerve impulses are partly controlled by signals from the brain and partly by signals from muscle spindles.
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Prolonged muscle contraction causes muscle fatigue due to depletion of muscle glycogen.
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Nerve signal transmission can diminish with prolonged activity.
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Without blood flow, muscle fatigue occurs rapidly due to loss of nutrients, especially oxygen.
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Body movements result from simultaneous agonist and antagonist muscle contraction, controlled by centers in the brain and spinal cord. Muscle activation ratios determine body positioning.
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Muscle remodeling occurs rapidly (e.g., within a few weeks) in response to function changes.
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Muscle hypertrophy results from increased actin and myosin filament numbers in muscle fibers, causing enlargement.
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Muscle atrophy results from a decrease in muscle mass due to reduced muscle proteins.
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Atrophy occurs rapidly after denervation.
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The muscle fibers are destroyed and replaced by fibrous and fatty tissue during denervation atrophy.
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Daily stretching or appliance use prevents muscle shortening due to atrophy.
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Recovery in diseases like polio involves branching of nerve fibers and formation of macromotor units
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The presence of rigor mortis after death is due to ATP depletion and autolysis of muscle proteins.
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Muscular dystrophy involves progressive weakness and degeneration of muscle fibers due to genetic mutations.
Fast vs. Slow Muscle Fibers
- Every muscle contains a mixture of fast and slow muscle fibers.
- Fast fibers respond quickly, enabling rapid movements, and contain plenty of glycolytic enzymes.
- Slow fibers respond slowly but enable sustained contractions for longer durations; they contain a rich blood supply and high levels of mitochondria.
Muscle Contraction Characteristics
- Skeletal muscles contract by summation of individual twitches - by increasing the frequency of stimulation or the number of motor units activated.
- Muscle force varies based on the number of motor units and their stimulation frequency.
- Frequency summation and multiple fiber summation cause tetanus.
Changes in Muscle Strength at the Onset of Contraction
- A phenomenon called the staircase effect (or Treppe) describes an increase in muscle strength during the first few contractions due to increased Ca2+ levels and sarcoplasmic reticulum.
Other Important Points
- The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.
- At the neuromuscular junction, acetylcholine is released, initiating muscle contraction.
- Acetylcholine is broken down by acetylcholinesterase, ending the signal and allowing relaxation.
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Description
Dive into the fascinating world of skeletal muscle contraction. This quiz covers the anatomy of muscle fibers, myofibrils, and the roles of myofilaments in contraction. Test your knowledge on the key structures and processes involved in muscle function.
