Muscle Physiology: Chapter on Contraction Mechanism
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Questions and Answers

What initiates the process of excitation-contraction coupling in muscle fibers?

  • Generation of an action potential at the myoneural junction (correct)
  • Release of neurotransmitters at the synapse
  • Activation of myofibrils by calcium
  • Spreading of calcium throughout the muscle fiber

    • Which structure is responsible for the rapid release of calcium during excitation-contraction coupling?

  • T-tubules
  • Terminal cisternae of the triads (correct)
  • Mitochondria
  • Endoplasmic reticulum

    • What role does calcium play in the process of muscle contraction?

  • It directly contracts the muscle fibers
  • It initiates the production of ATP
  • It binds to myofibrils and triggers shortening (correct)
  • It inhibits muscle relaxation

    • What happens to calcium concentration around myofibrils when depolarization of the sarcolemma ceases?

    <p>It is actively transported back into the lumen, decreasing concentration</p> Signup and view all the answers

    What is the primary component of the A-bands in myofibrils?

    <p>Myosin filaments</p> Signup and view all the answers

    What is the diameter of myosin filaments mentioned in the content?

    <p>15 nm</p> Signup and view all the answers

    At which junction do the longitudinal sarcotubules terminate?

    <p>A-I junction</p> Signup and view all the answers

    What are myofibrils primarily composed of?

    <p>Myo filaments (myosin and actin)</p> Signup and view all the answers

    What is the role of satellite cells in muscle tissue?

    <p>Participate in muscle repair after injury</p> Signup and view all the answers

    At what age is Gower’s Sign often apparent in individuals with muscle issues?

    <p>Age 5</p> Signup and view all the answers

    Which of the following best describes atrophy in muscle tissue?

    <p>Thinning or decrease in muscle diameter</p> Signup and view all the answers

    What type of cells primarily form myotubes during muscle development?

    <p>Myoblasts</p> Signup and view all the answers

    What is the primary function of mitochondria (sarcosomes) in muscle tissue?

    <p>Energy production for muscle contraction</p> Signup and view all the answers

    Muscle fibers can increase in size due to which process?

    <p>Hypertrophy</p> Signup and view all the answers

    The thickness of muscle fibers can vary considerably within the same muscle. What usually causes this variance?

    <p>Species differences</p> Signup and view all the answers

    What are myofibrils primarily composed of?

    <p>Contractile elements</p> Signup and view all the answers

    What type of muscle tone do smooth muscles in the walls of blood vessels maintain under normal circumstances?

    <p>Partial contraction</p> Signup and view all the answers

    Which hormones are known to cause vasoconstriction in smooth muscles?

    <p>Norepinephrine and angiotensin</p> Signup and view all the answers

    What is the role of postganglionic autonomic nerve fibers in relation to smooth muscle?

    <p>They pass over or penetrate the muscle, synapsing with muscle fibers.</p> Signup and view all the answers

    How do changes in estrogen levels affect uterine smooth muscle?

    <p>They alter actin and myosin content in the muscle.</p> Signup and view all the answers

    What is the effect of bradykinin and prostaglandin on smooth muscle tone?

    <p>They result in loss of muscle tone or vasodilation.</p> Signup and view all the answers

    Which part of the digestive tract contains striated muscle?

    <p>Esophagus</p> Signup and view all the answers

    What is the main function of smooth muscle at the pylorus?

    <p>To regulate the flow of food and prevent acid reflux</p> Signup and view all the answers

    What happens to the length of the A-bands during the contraction and relaxation cycle?

    <p>They remain constant.</p> Signup and view all the answers

    In which part of the digestive system are villi present?

    <p>Jejunum</p> Signup and view all the answers

    What are the segments of myofibrils between successive Z-lines called?

    <p>Sarcomeres</p> Signup and view all the answers

    What happens to smooth muscle cells during anesthesia according to the clinical significance mentioned?

    <p>They become immobile, making respiratory function difficult.</p> Signup and view all the answers

    Which type of motor units contract more slowly due to containing red fibers?

    <p>Red fibers</p> Signup and view all the answers

    Which of the following correctly describes the structure of myofibrils in muscle tissue?

    <p>Myofibrils are linked end-to-end by Z-discs.</p> Signup and view all the answers

    What type of muscle is found in the walls of the respiratory passages from the trachea to the alveolar ducts?

    <p>Smooth muscle</p> Signup and view all the answers

    What type of mitochondria are found in the subsarcolemmal region?

    <p>Spherical subsarcolemmal mitochondria</p> Signup and view all the answers

    Which property of red fibers allows them to be very resistant to fatigue?

    <p>Ability to regenerate ATP through oxidation</p> Signup and view all the answers

    How does smooth muscle in the large intestine primarily function?

    <p>Drives smooth contraction for peristalsis</p> Signup and view all the answers

    What is a consequence of smooth muscle cell migration in atherosclerosis?

    <p>Formation of atherosclerotic plaques</p> Signup and view all the answers

    What structural feature bisects each I-band?

    <p>Z-line</p> Signup and view all the answers

    How are the slender, paired mitochondria positioned in relation to Z-lines?

    <p>Transversely oriented on either side</p> Signup and view all the answers

    Which fibers are identified as the largest among all fiber types?

    <p>White fibers</p> Signup and view all the answers

    Study Notes

    Excitation-Contraction Coupling

    • Initiated at the myoneural junction with the generation of an action potential traveling along the sarcolemma and T-tubules.
    • Action potential activates calcium release from the sarcoplasmic reticulum at the T-tubules and terminal cisternae interface.
    • Calcium binds to myofibrils in the sarcoplasm, initiating muscle shortening.
    • Upon cessation of nerve impulse, calcium is actively transported back into terminal cisternae, resulting in muscle relaxation.

    Myofibrils Structure

    • Myofibrils consist of smaller myofilaments, primarily myosin and actin.
    • Myosin filaments measure 1.5 μm in length and 15 nm in diameter, located in A-bands.
    • Sarcotubules within myofibrils connect with terminal cisternae at A-I junctions, creating triads essential for muscle function.
    • Sarcomeres, defined by segments between Z-lines, contain A-bands and I-bands which define contraction cycles.

    Muscle Development and Repair

    • Myoblasts, the embryonic precursors, fuse to form myotubes, leading to differentiated muscle fibers.
    • Muscle fibers can undergo hypertrophy (increase in size) and atrophy (decrease in size).
    • Satellite cells assist in muscle repair but skeletal muscle has limited regenerative ability in adults.
    • Gower’s Sign is observed in children with muscle weakness, reflecting impaired ability to rise from the ground.

    Mitochondrial Organization

    • Mitochondria cluster around myofibrils to supply ATP necessary for muscle contraction, arranged in longitudinal columns within sarcoplasm.
    • Mitochondria are abundant near Z-lines, positioned to maximize energy availability during muscle activity.

    Muscle Fiber Types

    • White fibers (fast-twitch) are the largest muscle fiber type, adapted for powerful and rapid contractions.
    • Red fibers (slow-twitch) contract more slowly, are resistant to fatigue, and effective for maintaining posture.

    Smooth Muscle Physiology

    • Shortening in smooth muscle occurs through a sliding mechanism similar to that in striated muscle, involving actin and myosin filaments.
    • Innervation is provided by post-ganglionic autonomic nerve fibers that influence contraction via synaptic contacts.
    • Vascular smooth muscle maintains muscle tone, modulated by hormones like norepinephrine and bradykinin, influencing vasoconstriction and vasodilation.

    Clinical Relevance of Smooth Muscle

    • Migration of smooth muscle cells in atherosclerosis contributes to plaque formation, affecting vascular health.
    • During anesthesia, smooth muscles can become immobile, complicating respiration, highlighting the importance of proper intubation techniques.

    Tissues Involving Muscle Types

    • Esophagus has striated muscle; smooth muscle is prominent in the pyloric sphincter, stomach, and intestines.
    • Smooth muscle is also found in respiratory passages, unable to gain structure in alveoli, essential for maintaining airway function.

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    Description

    Explore the dynamics of excitation-contraction coupling, myofibril structure, and muscle development. This quiz covers the intricate processes involved in muscle function, including calcium signaling and sarcomere organization. Test your knowledge on how muscle fibers interact and the roles myoblasts play in muscle repair.

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