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Questions and Answers
What is the role of troponin in muscle contraction?
What is the role of troponin in muscle contraction?
Which statement accurately describes the structure of a sarcomere?
Which statement accurately describes the structure of a sarcomere?
What initiates the exposure of actin binding sites for myosin during muscle contraction?
What initiates the exposure of actin binding sites for myosin during muscle contraction?
What occurs to the I bands during muscle contraction?
What occurs to the I bands during muscle contraction?
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What is the primary function of myosin in skeletal muscle contraction?
What is the primary function of myosin in skeletal muscle contraction?
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What characterizes isotonic contraction in skeletal muscle?
What characterizes isotonic contraction in skeletal muscle?
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What happens to myosin when ATP is split?
What happens to myosin when ATP is split?
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What role does tropomyosin play during muscle relaxation?
What role does tropomyosin play during muscle relaxation?
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What is the role of acetylcholine at the myoneural junction?
What is the role of acetylcholine at the myoneural junction?
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Which component prevents calcium from diffusing into the cytosol in a resting skeletal muscle?
Which component prevents calcium from diffusing into the cytosol in a resting skeletal muscle?
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What happens to the calcium after muscle contraction has occurred?
What happens to the calcium after muscle contraction has occurred?
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What initiates the muscle action potential at the neuromuscular junction?
What initiates the muscle action potential at the neuromuscular junction?
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Which of the following accurately describes the process during muscle contraction?
Which of the following accurately describes the process during muscle contraction?
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What effect does depolarization of the skeletal muscle membrane have?
What effect does depolarization of the skeletal muscle membrane have?
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Which receptor functions as a calcium channel on the sarcoplasmic reticulum?
Which receptor functions as a calcium channel on the sarcoplasmic reticulum?
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During muscle relaxation, which energy source is utilized for calcium reuptake?
During muscle relaxation, which energy source is utilized for calcium reuptake?
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What is the primary role of DHP in muscle contraction?
What is the primary role of DHP in muscle contraction?
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What effect does cytosolic Ca2+ have on the muscle contraction process?
What effect does cytosolic Ca2+ have on the muscle contraction process?
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What is the primary mechanism underlying muscle relaxation following contraction?
What is the primary mechanism underlying muscle relaxation following contraction?
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How is troponin-C involved in muscle contraction?
How is troponin-C involved in muscle contraction?
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What physiological process occurs when multiple action potentials are generated before muscle relaxation?
What physiological process occurs when multiple action potentials are generated before muscle relaxation?
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What role does SERCA play in muscle physiology?
What role does SERCA play in muscle physiology?
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What mechanism enhances peak force in skeletal muscle during voluntary contraction?
What mechanism enhances peak force in skeletal muscle during voluntary contraction?
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In the neuromuscular junction, what is the role of cholinesterase?
In the neuromuscular junction, what is the role of cholinesterase?
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Study Notes
Skeletal Muscle Physiology
- Skeletal muscle is attached to the skeleton and moves bones at joints when it contracts.
- Skeletal muscles are made of many multinucleated muscle cells, called muscle fibers.
- Each muscle fiber has thousands of contractile filaments called myofilaments.
- Myofilaments are of two types: thin (actin) and thick (myosin).
- Actin filaments have binding sites for myosin.
- Myosin filaments have heads that bind to actin.
Learning Objectives
- Outline the functional structure of skeletal muscle.
- Outline the structure of the myoneural junction.
- Describe excitation-contraction coupling.
- Define a simple muscle twitch.
- Describe isotonic and isometric skeletal muscle contractions.
Gross Anatomy of Skeletal Muscle
- Muscle fibers are composed of myofibrils.
- Myofibrils are arranged in repeating units called sarcomeres.
- Sarcomeres are bounded by Z lines.
- Each sarcomere has a central dark band (A band) and two peripheral light bands (I bands).
- The A band contains thick filaments (myosin).
- The I bands contain thin filaments (actin).
- The H zone is the lighter region in the middle of the A band.
- The bare zone is a region in the H zone devoid of thin filaments.
Proteins of the Thin Filaments
- Actin is the structural protein of the thin filament.
- Actin has attachment sites for myosin.
- Tropomyosin blocks myosin-binding sites on actin.
- Troponin is composed of three subunits: troponin-T (binds to tropomyosin), troponin-I (binds to actin and inhibits contraction), and troponin-C (binds to calcium).
- Under resting conditions, calcium is not bound to troponin, preventing actin-myosin interaction.
- Binding of calcium to troponin-C causes the troponin-tropomyosin complex to move, exposing the myosin-binding site on actin.
Proteins of the Thick Filaments
- Myosin has ATPase activity.
- Splitting ATP puts myosin in a high-energy state, increasing its affinity for actin.
- Myosin binding to actin initiates the power stroke.
- If the force generated by the power stroke is sufficient to move the load, the muscle shortens (isotonic contraction).
- If the force generated is not sufficient to move the load, the muscle stays the same length (isometric contraction).
Neuromuscular Transmission
- The junction between the nerve and muscle is called the myoneural junction.
- The myoneural junction consists of:
- Nerve terminal with acetylcholine granules.
- Space between the nerve terminal and muscle membrane.
- Skeletal muscle membrane.
Events Occurring at the Myoneural Junction
- Release of acetylcholine from motor nerve terminals into the space between the nerve and muscle.
- Acetylcholine crosses the space and binds to specific receptors on the skeletal muscle membrane, increasing membrane permeability to sodium.
- Action potential is generated and spreads along the skeletal muscle membrane.
- Depolarization of the muscle membrane causes calcium release from intracellular stores into the sarcoplasm.
- Calcium causes contractile filaments to slide over each other, leading to muscle shortening.
- Calcium is pumped back into intracellular stores, leading to muscle relaxation.
Regulation of Cytosolic Calcium
- The sarcoplasmic reticulum (SR) has a high concentration of calcium.
- The SR creates a strong electrochemical gradient for calcium to diffuse into the cytosol.
- Key receptors involved in calcium flux: dihydropyridine (DHP) and ryanodine (RyR).
- DHP is a voltage-gated calcium channel.
- RyR is a calcium channel on the SR membrane.
- In the resting state, RyR is blocked by DHP, preventing calcium from diffusing into the cytosol.
Sequence of Skeletal Muscle Contraction
- Skeletal muscle action potential is initiated at the neuromuscular junction.
- The action potential travels down the T-tubule.
- The voltage change causes a conformational shift in DHP, removing its block of RyR.
- Removal of the DHP block allows calcium to diffuse into the cytosol.
- The rise in cytosolic calcium opens more RyR channels (calcium-induced calcium release).
- Calcium binds to troponin-C, initiating cross-bridge cycling.
- Calcium is pumped back into the SR by SERCA.
- The fall in cytosolic calcium causes tropomyosin to cover actin's binding sites for myosin, and the muscle relaxes.
Mechanism of Relaxation
- The effect of acetylcholine is terminated by the enzyme cholinesterase at the neuromuscular junction.
- Calcium is actively pumped back into the SR.
- Myosin-binding sites on actin are covered again.
Summation and Recruitment
- Under normal circumstances, enough calcium is released by a single action potential to completely saturate troponin-C binding sites, which means that all available cross-bridges are activated.
- Force is increased by summation and recruitment.
- Summation occurs when multiple action potentials are generated before the muscle has completely relaxed, adding the force from the previous action potentials.
- Recruitment occurs by activating more motor units, each containing more muscle fibers, by engaging more muscle fibers, to cause stronger force production.
Isometric and Isotonic Contractions
- Isometric contraction: The muscle does not shorten, but the force developed is maximal.
- Isotonic contraction: The muscle shortens, but the force developed is not maximal.
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Description
Test your knowledge on the physiology of skeletal muscle. This quiz covers the structure and function of skeletal muscles, including muscle fibers, myofilaments, and the process of excitation-contraction coupling. Explore key concepts related to muscle contractions and the gross anatomy of skeletal muscle.
