Lecture Notes: Muscular System and Muscle Contraction

Introduction to Muscles

• Muscles are part of the muscular system, familiar examples include biceps and triceps.
• Focus on muscle tissue and muscle contraction through actin-myosin cycling.

Types of Muscle Tissue

• Cardiac Muscle Tissue
  • Located in the heart, striated and branched fibers.
  • Contains intercalated discs for organized, wave-like contraction.
  • Involuntary control (unconscious action).
• Smooth Muscle Tissue
  • Smooth appearance without striations.
  • Spindle-shaped fibers with a single nucleus.
  • Found in the digestive system, blood vessels, bladder, and eyes.
  • Involuntary control.
• Skeletal Muscle Tissue
  • Associated with voluntary control (conscious action).
  • Striated fibers, long cylinders, multinucleated.

Characteristics of Muscle Tissue

• Extensibility: Ability to stretch or extend.
• Elasticity: Ability to retract to original length.
• Excitability: Ability to be stimulated and produce electrical changes.
• Contractility: Ability to contract.

Focus on Skeletal Muscle

• Naming and Arrangement
  • Names often stem from location or shape (e.g., rectus femoris, deltoids).
  • Skeletal muscle attachment: origin (fixed point) and insertion (movable point).
  • Involves agonists (prime mover) and antagonists (muscles for opposite actions).

Muscle Contraction at the Cellular Level

• Structure of Skeletal Muscle Fiber
  • Comprised of myofibrils, which contain sarcomeres.
  • Sarcomeres are responsible for the striated look of skeletal muscle.
• Key Proteins in Sarcomere
  • Actin: Makes up thin filaments.
  • Myosin: Makes up thick filaments.

Sliding-Filament Model

• Sarcomere shortens during contraction; thin and thick filaments slide past each other but do not shorten.
• Mechanics of Muscle Contraction
  • Myosin heads bind to actin, forming cross bridges.
  • ATP is hydrolyzed, myosin heads perform a power stroke.
  • New ATP binds to detach myosin, preventing rigor mortis.
  • Hundreds of cross bridges form and break during a contraction cycle.

Regulation of Muscle Contraction

• Role of Regulatory Proteins
  • Tropomyosin: Blocks myosin binding sites on actin.
  • Troponin complex: Works with tropomyosin to regulate binding.
• Calcium's Role
  • Neuronal stimulation triggers calcium release.
  • Calcium binds to troponin, causing tropomyosin to move and allowing myosin binding.

Conclusion

• Reflect on the complexity and coordination involved in muscle contraction.
• Encouragement to remain curious about muscle biology.