Excitation-Contraction Coupling

Excitation-Contraction Coupling

• Definition: Excitation-contraction coupling is the physiological process by which muscle fibers contract.

• Key Components:

  • Sarcolemma: This is the plasma membrane of the muscle cell, around which the electrical signal (action potential) travels.
  • T Tubules: These are invaginations of the sarcolemma that allow the action potential to quickly penetrate into the muscle fiber.
  • Voltage-sensitive Proteins: Located in the T tubules, these proteins detect the action potential.
  • Sarcoplasmic Reticulum (SR): A specialized endoplasmic reticulum in muscle cells that stores calcium ions.

• Calcium Release:

  • The action potential traveling down the T tubules triggers the voltage-sensitive proteins to open calcium channels in the sarcoplasmic reticulum.
  • Calcium ions are then released into the cytosol of the muscle fiber.

• Binding to Troponin:

  • Released calcium binds to troponin, causing a conformational change.
  • This change displaces tropomyosin, which normally blocks the active sites on actin, allowing myosin to interact with actin.

• Latent Period:

  • There is a brief period known as the latent period where no tension is observed.
  • This phase is essentially the time taken for the action potential to result in muscle contraction, highlighting that the process, while quick, is not instantaneous.
  • The latent period includes all necessary steps from the neuromuscular junction relay to the onset of muscle contraction.

• Tension Measurement:

  • On a graph, muscle tension is plotted on the Y-axis and shows phases of muscle activity.
  • During the latent period, the graph shows a flat line, indicating no tension is being produced yet despite the impulse being sent.
  • The tension is measured in millisecond intervals, indicating this process occurs very quickly, typically within a few milliseconds.

• Contraction Phase:

  • After the initial latent period, the tension begins to increase, indicating the muscle is entering the contraction phase.
  • During this phase, myosin heads form cross-bridges with actin filaments, pulling them together, which generates tension within the muscle fiber.
  • This active contractile process is crucial for muscle movement and function.