Microscopic Anatomy of Muscle Fibers

Overview

• Focus on microscopic anatomy of muscle fiber.
• Detailed look at the myofibril and muscle fiber adaptations.

Muscle Fiber Characteristics

• Muscle fiber/cell can be very long; e.g., length of a bicep is a single muscle cell.
• Diameter varies between 10-100 micrometers.
• Adaptations allow for coordinated contraction as a unit.

Key Structures

• Sarcolemma: Muscle-specific plasma membrane.
• Transverse Tubules (T-Tubules): Invaginations of sarcolemma for deep electrical signal penetration, aiding in contraction.
• Sarcoplasmic Reticulum (SR): Muscle-specific endoplasmic reticulum for calcium storage, heavily branched.
  • Terminal Cisterna: Part of SR, coincides with T-Tubules, forms a triad.
  • Calcium Storage: Crucial for muscle contraction.

Special Proteins

• Myoglobin: Cousin of hemoglobin, allows temporary oxygen storage, more prevalent in darker muscle tissues.
• Glycogen: Stored as vesicular structures, source of glucose for metabolism.

Sarcomere Structure

• Consists of thin filaments (actin) and thick filaments (myosin).
• Actin: Globular protein, part of thin filament, binds to myosin.
• Myosin: Motor protein, thick filament component, has ATPase activity.
  • Tail and Head Structure: Head interacts with actin; ATP hydrolysis for movement.

Sarcomere Components

• Titan: Large protein, acts as a shock absorber, anchors thick filaments to Z disk.
• Z Disk: Anchors actin filaments and connects sarcomeres.
• M Line: Central point, anchors myosin, site of protein interactions.

Regulatory Proteins

• Tropomyosin: Blocks actin-myosin interaction at rest.
• Troponin: Calcium-binding protein that shifts tropomyosin to allow interaction.

Sliding Filament Model

• Describes histological differences between relaxed and contracted muscles.
• Striations: Light and dark bands due to overlap of thin and thick filaments.
• Muscle Contraction: Myosin heads move, causing actin filaments to slide toward M line, forming cross-bridges.

Cross Bridges

• Interaction between actin and myosin; more cross-bridges result in greater muscle tension.

Conclusion

• Overview of muscle fiber anatomy and the sliding filament model.
• Next focus: Overall muscle physiology.