Lecture Notes: Muscle Tissue and Contraction

Overview of Muscle Tissue

• Muscles are essential parts of the muscular system.
• Focus on muscle tissue types and muscle contraction, particularly actin-myosin cycling.

Types of Muscle Tissue

1. Cardiac Muscle

   • Found in the heart.
   • Characteristics:
     • Branched, striated fibers with one nucleus.
     • Contains intercalated discs aiding in synchronized contraction.
   • Involuntary control.

2. Smooth Muscle

   • Appears smooth without striations.
   • Characteristics:
     • Spindle-shaped fibers with one nucleus.
     • Found in digestive system, arteries, veins, bladder, eyes.
   • Involuntary control.

3. Skeletal Muscle

   • Associated with voluntary movements.
   • Characteristics:
     • Striated, cylindrical fibers with multiple nuclei.
     • Controls movements like picking up objects.

Characteristics of Muscle Tissue

• Extensibility: Ability to stretch.
• Elasticity: Ability to retract to original length.
• Excitability: Ability to be stimulated, enabling electrical changes and action potentials.
• Contractility: Ability to contract.

Skeletal Muscle

Naming and Structure

• Often named by location or shape; Latin/Greek roots.
• Examples:
  • Rectus femoris (thigh)
  • Rectus abdominis (abdomen)
  • Deltoids (triangular-shaped)

Attachment and Function

• Insertion: Part attached to the moving bone.
• Origin: Part attached to the fixed bone.
• Agonist: Primary muscle in movement.
• Antagonist: Muscle that opposes the movement.

Muscle Fiber Anatomy

• Muscle fibers consist of myofibrils, composed of repeating sarcomeres.
• Sarcomeres: Fundamental units contributing to striated appearance.
  • Contain actin (thin filaments) and myosin (thick filaments).

Muscle Contraction

Sliding-Filament Model

• Sarcomeres shorten without filament shortening - thick and thin filaments slide past each other.
• Process:
  • Myosin heads hydrolyze ATP to ADP + phosphate.
  • Myosin heads form cross bridges with actin.
  • Power stroke occurs as ADP and phosphate are released.
  • New ATP binds myosin heads to detach them for the next cycle.
  • Continuous cross bridge cycling during contraction.

Regulation of Contraction

• Tropomyosin: Blocks myosin binding sites on actin.
• Troponin Complex: Regulatory proteins involved in contraction regulation.
• Neuronal stimulation releases calcium ions which bind to troponin.
• Conformational change in troponin moves tropomyosin, allowing myosin binding.

Conclusion

• Muscle contraction involves complex and fascinating processes, vital to voluntary and involuntary movements.
• Encouragement to reflect on these processes during everyday activities.