Muscle Tissue and Contraction

Introduction to Muscles

• Muscles are often recognized by those under skin, such as biceps or triceps.
• The focus here is not on the nomenclature of muscles but on muscle tissue and contraction, specifically actin-myosin cycling.

Types of Muscle Tissue

• Cardiac Muscle Tissue:
  • Located in the heart; fibers are branched and striated.
  • Each fiber has one or sometimes two nuclei.
  • Contains intercalated discs for organized contraction.
  • Involuntary control.
• Smooth Muscle Tissue:
  • Lacks striations; spindle-shaped fibers with one nucleus.
  • Found in digestive system, blood vessels, bladder, and eyes.
  • Involuntary control.
• Skeletal Muscle Tissue:
  • Attached to bones or skin; voluntary control.
  • Striated appearance; long, cylindrical, multinucleated fibers.

Characteristics of Muscle Tissue

• Extensibility: Ability to stretch.
• Elasticity: Ability to return to original length.
• Excitability: Ability to respond to stimuli.
• Contractility: Ability to contract.

Skeletal Muscle Details

• Named by location or shape with Latin/Greek origins.
  • Example: Rectus femoris (thigh) and rectus abdominis (abdomen).
  • Deltoids: triangular muscle, named after the Greek letter delta.
• Muscles attach to bones at the insertion (movable) and origin (fixed).
• Agonist: Main muscle doing work.
• Antagonist: Muscle that performs the opposite action.

Muscle Contraction

Structure of Skeletal Muscle

• Composed of muscle fibers (cells) with multiple myofibrils.
• Myofibrils contain repeating sections called sarcomeres.
• Sarcomeres give the striated appearance.

Sarcomere Composition

• Thin Filaments: Composed of actin.
• Thick Filaments: Composed of myosin.
• Actin: Remember "thin" is almost in "actin".

Sliding-Filament Model

• Sarcomere shortens during contraction; filaments slide past each other.
• Z Lines are pulled closer as thin filaments overlap with thick filaments.

Detailed Contraction Process

• Myosin Heads: Bind to actin to form cross bridges.
• ATP hydrolysis allows myosin heads to bind to actin, causing power strokes.
• ADP and phosphate release triggers the power stroke.
• ATP binding allows myosin head detachment.

Role of ATP and Rigor Mortis

• ATP is crucial for detachment of myosin from actin.
• Lack of ATP leads to rigor mortis post-mortem.

Regulation of Contraction

• Tropomyosin: Blocks myosin binding sites on actin.
• Troponin Complex: Regulatory proteins that prevent binding until Ca2+ binds.
• Calcium Release: Triggers conformational changes, allowing myosin binding.

Conclusion

• Reflect on the complexity and regulation of muscle contraction when using muscles.