Skeltal muscle physiology | Anatomy2Medicine
Skeltal muscle physiology

Skeltal muscle physiology

Structure of Skeletal Muscle

    • Skeletal muscle is organized into progressively smaller anatomical units.
    • Muscle fibers are surrounded by a plasma membrane more commonly called the sarcolemma.
    • Muscle fibers are composed of a bundle of fibrous structures called myofibrils,
    • Each myofibril is a linear arrangement of repeating structures called sarcomeres.
    • Sarcomeres are the fundamental contractile unit of skeletal muscle and are char- acterized by their highly ordered appearance under a polarizing light microscope

 

  • Thick filaments – Myosin (MCQ)

 

      • Thick filaments in the A band are composed primarily of the protein myosin. (MCQ)
      • There are two essential light chains and two myosin regulatory light chains.
      • Each heavy chain is associated with a globular head
        • The two globular heads of myosin heavy chains  can hydrolyze ATP to ADP and inorganic phosphate (MCQ)
        • also have the intrinsic ability to interact with actin.

 

  • Thin filaments – : actin, tropomyosin, and troponin(MCQ)

 

      • are composed of three primary proteins: actin, tropomyosin, and troponin.
      • Actin is composed of G-actin and  F-actin

 

  • Each G-actin monomer contains binding sites for myosin, tropomyosin, and troponin I. (MCQ)
  • Tropomyosin is an elongated protein that lies within the two grooves formed by the double stranded F-actin

 

      • Each thin filament contains 40–60 tropomyosin molecules. (MCQ)
      • Troponin is a complex of three separate proteins: (MCQ)
        • Troponin T binds the other two troponin subunits to tropomyosin
        • Troponin C binds Ca2+, the crucial regulatory step in muscle contraction. (MCQ)
        • Troponin I is responsible for the inhibitory conformation of the tropomyosin-troponin complex observed in the absence of Ca2+.(MCQ)

 

  • Tubules, a tubular network, are located at the junctions of A bands and I bands and contain a protein called the dihydropyridine receptor. (MCQ)

 

  • The sarcoplasmic reticulum (SR)
    • site of Ca2+ storage near the transverse tubules (T-tubules). (MCQ)
    • It contains a Ca2+-release channel known as the ryanodine receptor. (MCQ)
  • Several steps are involved in the mechanics of muscle contraction:
  • Action potentials in muscle cell membrane cause depolarization of the T-tubules, which opens Ca2+-release channels in the SR and increases intracellular Ca2+.(MCQ)
  • Ca2+ releases the troponin-tropomyosin inhibitory influence so that the active sites on each G-actin monomer are uncovered. (MCQ)
  • The myosin globular heads that protrude from the thick filament bind with G-actin active sites, thus forming crossbridges. (MCQ)
    • How is power stroke generated
      • Intramolecular forces (stored energy) within the myosin molecules allow myosin to flex in the so-called hinge regions.
      • These areas are the two proteolytic enzyme–sensitive regions in the myosin molecule. The action of flexing of the myosin molecule causes the globular heads (still attached to actin) to tilt toward the center of the sarcomere.
      • This movement, called the power stroke, creates tension that results from shortening of individual sarcomeres.
      • Immediately after the tilt, the crossbridge is broken and the globular heads snap back to the upright position.
      • At this point, a new crossbridge can be formed if ATP and Ca2+ are available in the vicinity of thick and thin filaments. (MCQ)

 

  • In the absence of Ca2+, crossbridge formation is not possible.

 

    • Relaxation occurs when Ca2+ uptake into the SR lowers intracellular Ca2+.(MCQ)
  • Biochemical events that occur during a muscle contraction cycle involve

an active complex and the rigor complex.

 

  • Myosin with ATP bound to it (myosin-ATP complex) has a low affinity for
  • the G-actin active sites.

 

    • When Ca2+ binds to troponin and tropomyosin, tropomyosin rotates out of the way so that the active sites on G-actin are uncovered.
    • Myosin-ATP is simultaneously hydrolyzed to myosin-ADP, which has a high affinity for the G-actin active sites.
    • Consequently, an active complex, or crossbridge, is formed between actin and myosin-ADP.
    • ADP is released from myosin, and the globular heads tilt toward the center of the sarcomere, producing tension
    • At this stage, the rigor complex is formed between actin and myosin.
    • ATP then binds to myosin, and the myosin-ATP complex breaks the cross- bridge and the globular heads snap back to the upright position.
    • The cycle is ready to start again in the presence of Ca2+.
  • Skeletal muscle enters a state of prolonged stiffness termed rigor mortis at death.
  • Rigor mortis occurs because, with death, muscle cells are no longer able to

synthesize ATP. (MCQ)

  • In the absence of ATP, the crossbridges between myosin and actin are unable

to dissociate.

 

  • After 15–25 hours, proteolytic enzymes released from lysosomes begin to

 

break down actin and myosin.

    • Relationship between muscle length and tension.
      • muscle shortens while exerting a constant force. (MCQ)
      • Isometric contraction
        • muscle length is held constant during the development of force.
        • An example would be an individual pushing against an immovable object such as the wall of a house.
      • In an isotonic contraction

 

  • An example would be an individual lifting a Weight with his biceps
  • Active tension
  • The tension that a stimulated muscle develops when it contracts isometrically (MCQ)

 

        • (total tension) and the passive tension exerted by the unstimulated muscle vary with the length of the muscle fiber
        • The difference between the two values is the tension produced by the contractile process, the active tension
        • The amount of active tension developed with a contraction decreases from its maximum as the muscle is either shortened or lengthened prior to the contractile stimulus.
        • Active tension developed is proportional to the number of crossbridges formed. (MCQ)
        • Tension is reduced when the sarcomere is shortened to a point where thin filaments overlap and prevent one another from forming crossbridges with myosin.

 

  • Thus, isometric tension produced depends on the degree of overlap of the thick and thin filaments, which dictates the number of crossbridges that can be formed. (MCQ)

 

      • Force-velocity relationship
        • refers to the relationship between the load (or weight) placed on a muscle and the velocity at which that muscle contracts while lifting the load.
        • Velocity is the distance an object moves per unit time.

 

  • Load
  • A load can be thought of as a weight that the muscle is attempting to move via an isotonic contraction, for example, when a weightlifter tries to lift a series of progressively heavier weights.

 

        • A muscle can contract most rapidly with no load (MCQ)

 

  • As loads increase, however, the velocity at which the muscle lifts the weight decreases.

 

      • When the weight equals the maximum amount of force that the muscle can generate, the velocity becomes zero. In this case the contraction becomes isometric (eg, the muscle contracts but does not shorten). (MCQ)
    • The functional unit of a muscle is called a motor unit.
    • Increased tension development in skeletal muscle is attained by wave summation (eg, increasing stimulus frequency of a single motor

      neuron).

    • Summation, or recruitment, of motor units.
      • Besides increasing tension development, recruitment allows a movement to be continuous and smooth because different motor units fire asynchronously; that is, while one motor unit is contracting, another might be at rest.
      • A contraction can be a single, brief contraction or a maintained contraction due to continuous excitation of muscle fibers.
      • A single contractile event (eg, twitch) is initiated by a single action potential from a motor neuron reaching the neuromuscular junction. (MCQ)
      • If a second stimulus is applied before the muscle fibers in the motor unit have relaxed, the second contractile event builds on the first.
      • It can be said that the two contractions summate.
      • This summation of contractions occurs when stimulation frequencies reach about 10 per second.
      • As the frequency of stimulation is increased, the developed force continues to sum until a maximum developed force is reached.
      • At this point, the individual contraction-relaxation cycles fuse to produce a single smooth curve called tetanus
      • Tetanus occurs in skeletal muscle because the refractory period  is short relative to the contraction time. (MCQ)