Lecture on Muscles and Muscle Contraction

Introduction to Muscular System

• Muscles are more than just parts like biceps or triceps visible under the skin.
• Focus is on muscle tissue and the contraction process: actin-myosin cycling.

Types of Muscle Tissue

1. Cardiac Muscle Tissue

   • Located in the heart.
   • Features branched, striated fibers, sometimes with two nuclei.
   • Contains intercalated discs for organized contraction.
   • Involuntary control.

2. Smooth Muscle Tissue

   • Lacks striations.
   • Spindle-shaped fibers with one nucleus.
   • Found in digestive system, vessels, bladder, eyes.
   • Involuntary control.

3. Skeletal Muscle Tissue

   • Attaches to bone or skin.
   • Striated, cylindrical fibers with multiple nuclei.
   • Under voluntary control.
   • Known for extensibility, elasticity, excitability, and contractility.

Skeletal Muscle Details

• Muscles named by location and shape, often with Greek or Latin origins, e.g., rectus femoris (thigh), deltoid (triangle-shaped).
• Muscle parts: insertion (attaches to moving bone) and origin (attaches to fixed bone).
• Prime movers known as agonists and opposing muscles as antagonists.

Muscle Contraction Process

• Muscle fibers contain myofibrils which have repeating units called sarcomeres.
• Sarcomeres provide muscle's striated appearance.
• Actin: forms thin filaments.
• Myosin: forms thick filaments.
• Sliding-filament model: Sarcomeres shorten without filaments shortening; filaments slide past each other.
• Z lines: define ends of sarcomere and where thin filaments attach.
• M line: midsection where thick filaments are anchored.

Actin-Myosin Interaction

• Myosin heads bind ATP and hydrolyze it (ATP → ADP + phosphate).
• Cross bridges form between actin and myosin.
• Power stroke: myosin head bends, sliding actin filament towards sarcomere center.
• New ATP binds to detach myosin head.
• Lack of ATP causes rigor mortis.

Regulation of Muscle Contraction

• Tropomyosin: blocks myosin binding sites on actin.
• Troponin complex: regulatory proteins that prevent contraction by blocking myosin binding.
• Neuronal stimulation releases calcium ions (Ca2+) which bind to troponin, changing its shape and moving tropomyosin to expose binding sites.
• Enables muscle contraction.

Conclusion

• Reflect on the complex processes occurring as you use your muscles.
• Encouragement to stay curious about biological processes.