L16_Muscle_2

1. Muscle Contraction Mechanism

1.1 Neural Activation

  • Motor Neuron Action Potential: Initiates contraction process.

  • Acetylcholine Release:

    • Released from the axon terminal due to Ca2+ influx through voltage-gated channels.

    • Diffuses to the motor end plate on muscle fiber.

  • Acetylcholine Binding:

    • Binds to nicotinic receptors on motor end plate, increasing permeability to Na+ and K+.

    • Greater Na+ influx leads to depolarization and produces an end-plate potential (EPP).

1.2 Excitation-Contraction Coupling

  • Action Potential Propagation:

    • EPP causes local currents that spread to adjacent muscle membrane, generating an action potential.

    • Action potential travels along the muscle fiber and into T-tubules.

  • Calcium Release from Sarcoplasmic Reticulum (SR):

    • Action potential activates dihydropyridine (DHP) receptors, opening ryanodine receptors.

    • Ca2+ released from terminal cisternae bathes myofibrils.

  • Troponin and Tropomyosin Interaction:

    • Ca2+ binds to troponin, causing tropomyosin to shift and expose cross-bridge binding sites on actin.

2. Cross-Bridge Cycling

2.1 Steps of Cross-Bridge Formation

  • Energized Cross-Bridge Attachment:

    • Cross-bridges bind to actin, forming A · M · ADP · Pi.

  • Power Stroke:

    • Cross-bridge movement occurs, releasing ADP and Pi.

    • Produces force, causing thin filament to slide.

  • Detachment:

    • ATP binds to myosin, causing dissociation from actin: A · M + ATP → A + M · ATP.

  • Energizing Cycle:

    • Hydrolysis of ATP energizes myosin for next cycle: A + M · ATP → A + M · ADP · Pi.

    • This cycle continues as long as calcium is bound to troponin.

2.2 Role of Calcium in Muscle Relaxation

  • Calcium Removal:

    • Ca2+ pumped back into SR via Ca2+-ATPase, reducing cytosolic Ca2+ concentration.

  • Restoration of Tropomyosin Blocking Action:

    • Removal of Ca2+ allows tropomyosin to block actin sites, halting contraction.

3. Muscle Properties and Reflexes

3.1 Proprioceptors

  • Muscle Spindle:

    • Senses stretch and the speed of stretch (1a afferent fibers).

    • Sends signals to prevent overstretching.

  • Golgi Tendon Organ:

    • Senses tension (1b afferent fibers).

    • Induces relaxation if muscle tension is excessive, protecting muscles from injury.

4. Role of ATP in Muscle Contraction

  • Maintaining Ion Gradients:

    • Na+/K+-ATPase keeps membrane potentials stable for action potential propagation.

  • Active Transport of Calcium:

    • Ca2+-ATPase lowers cytosolic Ca2+ after contraction ends.

  • Energizing Cross-Bridges:

    • ATP hydrolysis energizes myosin cross-bridges, enabling force generation and cyclic activity.