Lecture Notes on Muscles and Muscle Contraction

Overview of the Muscular System

• Muscles are more than just identifiable parts like biceps and triceps.
• Focus on muscle tissue and muscle contraction (actin-myosin cycling).

Types of Muscle Tissue

1. Cardiac Muscle Tissue

• Location: Heart
• Structure: Branched and striated fibers; each fiber has a nucleus (sometimes two).
• Intercalated Discs: Important for organized contraction in a wave-like pattern.
• Control: Involuntary (not consciously controlled).

2. Smooth Muscle Tissue

• Description: Smooth (no striations)
• Structure: Spindle-shaped, one nucleus.
• Location: Found in digestive system, arteries/veins, bladder, and iris of the eyes.
• Control: Involuntary.

3. Skeletal Muscle Tissue

• Description: Often associated with voluntary control (e.g., biceps, triceps).
• Structure: Striated, long cylindrical fibers, multinucleated.
• Control: Voluntary (can be consciously controlled).

Characteristics of Muscle Tissue

• Extensibility: Can stretch/extend.
• Elasticity: Can retract to starting length.
• Excitability: Can be stimulated; electrical changes can generate action potentials.
• Contractility: Ability to contract.

Naming and Arrangement of Skeletal Muscles

• Naming: Based on location or shape; often have Latin/Greek roots.
  • Example: Rectus femoris (thigh), Rectus abdominis (abdomen), Deltoids (triangular shape).
• Muscle Actions:
  • Insertion: Part that moves during contraction.
  • Origin: Fixed part of the bone.
  • Agonist: Prime mover muscle.
  • Antagonist: Opposing muscle for balance.

Muscle Contraction at the Cellular Level

• Muscle Fibers: Composed of myofibrils, with repeating sections called sarcomeres.
• Sarcomere Components:
  • Actin: Thin filament.
  • Myosin: Thick filament.
• Sliding-Filament Model:
  • Sarcomeres shorten during contraction; thick and thin filaments slide past each other.
  • Z lines move closer together.

Mechanism of Muscle Contraction

1. Myosin heads bind to ATP and hydrolyze it into ADP and phosphate, allowing binding to actin (cross-bridge formation).
2. Myosin head performs a power stroke, sliding actin towards the sarcomere center.
3. New ATP molecule binds to myosin head, allowing detachment from actin.
   • Rigor Mortis: Occurs without ATP, causing muscles to stiffen post-mortem.

Regulatory Mechanisms

• Tropomyosin: Blocks myosin bonding sites on actin.
• Troponin Complex: Regulatory proteins that work with tropomyosin.
• Calcium Ions (Ca2+): Released from neuron stimulation; bind to troponin, causing conformational change that moves tropomyosin off bonding sites, allowing myosin to bind.

Conclusion

• Reflection on the complexity and functionality of skeletal muscles during common actions (e.g., picking up a textbook).
• Encouragement to stay curious about biological processes.