Fundamentals of Anatomy and Physiology - Muscle Tissue

Types of Muscle Tissue

• Skeletal Muscle

  • Striated muscle (striped appearance)
  • Long muscle fibers (cells)
  • Attached to bones, cartilage, or fascia
  • Voluntary: Subject to conscious control

• Cardiac Muscle

  • Striated muscle
  • Found exclusively in the heart
  • Short muscle cells with intercalated discs (gap junctions)
  • Involuntary: Contracts without neural or hormonal stimulation

• Smooth Muscle

  • Non-striated muscle
  • Found in walls of blood vessels, respiratory passages, hollow visceral organs (stomach, intestines, urinary bladder, uterus)
  • Involuntary

Functions of Skeletal Muscle

1. Produce movement
2. Maintain posture & body position
3. Support and protection of soft tissue (e.g., abdominal organs)
4. Guard body entrances and exits
   • Sphincters regulate material entrance and exit
5. Generate heat (e.g., shivering)
6. Store nutrients
   • Glycogen
   • Proteins can be broken down into amino acids for glucose production

Characteristics of Skeletal Muscle Fibers

1. Size
   • Can be up to 100 μm in width and 12 inches in length
2. Multinucleated
   • Several hundred nuclei formed by fusion of embryonic stem cells (myoblasts)
   • Unfused myoblasts become myosatellite cells, aiding in repair of damaged muscle tissue by fusing with damaged fibers
3. Parallel fiber arrangement
4. Striated appearance
5. Abundant mitochondria for energy production
6. Plasma membrane:
   • Known as the sarcolemma
7. Cytoplasm:
   • Known as the sarcoplasm
8. Endoplasmic Reticulum:
   • Known as the sarcoplasmic reticulum

Organization of Skeletal Muscle

• Composed of:
  • Skeletal muscle tissue
  • Connective tissues
  • Blood vessels
  • Nerves
• Naming:
  • 1 skeletal muscle cell (myocyte) = 1 skeletal muscle fiber (myofiber)

Connective Tissue Sheaths

1. Epimysium
   • Outermost sheath, bundles numerous fascicles, separates muscle from surrounding tissues and organs
2. Perimysium
   • Bundles numerous muscle fibers into fascicles
3. Endomysium
   • Delicate sheath surrounding each individual muscle fiber, contains capillaries and nerves

• Continuity: All connective tissues are continuous with each other and the tendons.

Structure of Skeletal Muscle Fibers

• Muscle fiber: Elongated multinucleate cell with banded (striated) appearance
• Contains myofibrils
  • Complex organelles composed of bundles of myofilaments
  • Thick filaments: Myosin
  • Thin filaments: Actin
  • Organized into sarcomeres

Sarcomere Structure

• Actin (Thin Filaments)
  • Consists of two twisted F-actin strands, anchored to Z-disc
  • G-actin proteins have binding sites for myosin cross-bridges
• Myosin (Thick Filaments)
  • Consists of about 200 myosin molecules, with each head (cross-bridge) having an actin-binding site and ATPase activity to provide energy
• Controlling Proteins
  • Tropomyosin: Covers myosin-binding sites in resting muscles; moved away by troponin-bound Ca2+
  • Titin: Spring-like protein that helps maintain the structure of the sarcomere during contraction/relaxation

Contraction Mechanism: Sliding Filament Model

• Involved the interaction between thick (myosin) and thin (actin) filaments, where muscle fibers shorten during contraction
  • Relaxed State:
    • The distance between Z-discs increases, allowing movement of the H-zone and I-band
  • Contracted State:
    • Distance between Z-discs decreases, and I-band shortens while A-band stays the same

T-tubules and the Sarcoplasmic Reticulum

• Sarcoplasmic Reticulum: Network of membranes around sarcomeres, stores Ca2+ for muscle contraction
• T-tubules: Invaginations of plasma membrane that facilitate action potential propagation and Ca2+ release from the sarcoplasmic reticulum.

Summary of Muscle Tissue Organization

• Muscle belly composed of:
  • Myofiber (actin & myosin)
  • Fascicle: Bundle of muscle fibers
  • Myofibril: Composed of myofilaments
  • Surrounded by endomysium, perimysium, and epimysium