Learning Objective

By the end of this module, learners will be able to identify the functional proteins of the thin and thick filaments in striated muscle and explain how calcium and ATP regulate cross-bridge cycling and muscle contraction.

Overview

Striated muscle (skeletal and cardiac) contraction depends on the coordinated interaction between proteins of the thin filament and thick filament within the sarcomere. These proteins regulate actin–myosin interaction and convert chemical energy (ATP) into mechanical force.

Proteins of the Thin Filament

The thin filament is primarily composed of actin, along with regulatory proteins tropomyosin and troponin.

• Actin: The structural backbone of the thin filament. It contains specific binding sites for myosin heads.
• Tropomyosin: Lies along the actin filament and blocks the myosin-binding sites under resting conditions.
• Troponin Complex (three subunits):
  • Troponin T (TnT) – Binds to tropomyosin
  • Troponin I (TnI) – Binds to actin and inhibits contraction
  • Troponin C (TnC) – Binds calcium

Regulation by Calcium

• At rest: Calcium is not bound to troponin C. Tropomyosin remains positioned over actin’s myosin-binding sites, preventing cross-bridge formation.
• During contraction: Calcium binds to troponin C → conformational change in the troponin complex → tropomyosin shifts → myosin-binding sites on actin are exposed → cross-bridge formation occurs.

Proteins of the Thick Filament

The thick filament is composed primarily of myosin, a motor protein with ATPase activity.

• Myosin ATPase Activity
  Hydrolysis of ATP places the myosin head in a “high-energy” state and increases its affinity for actin.
• Cross-Bridge Formation and Power Stroke
  When myosin binds to actin:
  • Chemical energy from ATP hydrolysis is converted into mechanical energy.
  • The myosin head pivots, pulling the actin filament toward the center of the sarcomere.
  • This movement generates active tension and is known as the power stroke.

Activity

Types of Contraction

• Isotonic Contraction
  Occurs when the force generated by cross-bridge cycling exceeds the load, causing muscle shortening.
• Isometric Contraction
  Occurs when the force generated is insufficient to overcome the load; tension increases, but muscle length does not change.

Activity