Muscle Tissue Types Overview

Smooth Muscle Tissue

• Shape and Structure: Small, elongated, spindle-shaped cells with tapered ends; lack striations due to absence of sarcomeres. Actin and myosin filaments are arranged less organized. Dense bodies anchor actin filaments.

• Function: Specialized for slow, prolonged contractions.

• Stimulation: Contraction triggered by mechanical (stretching), electrical (nerve impulses), and chemical (hormones) stimuli.

• Location and Function Examples:

  • Vascular smooth muscle regulates blood vessel diameter, impacting blood pressure.
  • Digestive tract smooth muscle facilitates peristalsis, moving food through the tract.
  • Urinary tract smooth muscle facilitates urination.

• Regeneration: Possesses high capacity for regeneration; cells can divide and increase in number. Pericytes can differentiate into smooth muscle cells. Cells can also hypertrophy.

Skeletal Muscle Tissue

• Shape and Structure: Largest cells in the body; single, multinucleated fibers; nuclei located at periphery. Approximately one nucleus every 3μm along the fiber length.

• Formation: Formed by the fusion of multiple myoblasts during development and growth.

• Subunit: Myofibril, the structural and functional subunit, composed of precisely aligned myofilaments (myosin-thick and actin-thin filaments).

• Striations: Exhibit striated appearance due to repeating sarcomeres (contractile units) in longitudinal sections.

• Sarcomere Structure:

  • ~2.5 μm in length in skeletal muscle.
  • A fiber (30 cm long) contains a staggering 120,000 sarcomeres.
  • I band - light staining, primarily thin filaments.
  • A band - dark staining, primarily thick filaments.
  • Z line - anchors thin filaments.

• Contraction: Contraction achieved through the actomyosin cross-bridge cycle.

• Differentiation and Repair: Muscle fibers (myofibers) are terminally differentiated and don't undergo mitosis. Satellite cells, skeletal muscle stem cells, repair damaged fibers.

Cardiac Muscle Tissue

• Shape and Structure: Striated, short, cylindrical cells; centrally located single nucleus; connected by intercalated discs. Intercalated discs are specialized cell-to-cell junctions.

• Size: Cardiomyocytes are smaller than skeletal muscle fibers (~80-100 μm long and ~15 μm in diameter).

• Sarcomere Length: Resting sarcomere length (about 2.2 μm) is slightly shorter than in skeletal muscle.

• Function: Specialized cardiac conducting muscle cells rhythmically generate and transmit action potentials.

• Contraction: Cells can hypertrophy or hypotrophy but cannot divide.

• Repair: Limited ability to regenerate; heart attack damage results in scar tissue formation.

Muscle Tissue Regeneration Capacity

• Smooth Muscle: High regeneration capacity.
• Skeletal Muscle: Limited regeneration capacity; satellite cells facilitate repair.
• Cardiac Muscle: Minimal regeneration capacity; damage results in scar tissue.

Connective Tissue Layers in Muscle

• Skeletal Muscle:

  • Endomysium: surrounds individual fibers.
  • Perimysium: surrounds groups of fibers (fascicles).
  • Epimysium: surrounds the entire muscle.

• Tendon:

  • Epitendineum: surrounds the entire tendon.
  • Peritendineum: divides fascicles within the tendon.
  • Endotendineum: surrounds individual fibers within a fascicle.

Red and White Muscle Fibers

• Red Muscle Fibers (Type I):

  • Rich in capillaries, mitochondria, and myoglobin.
  • Adapted for aerobic metabolism, slow-twitch with prolonged contractions and low force.
  • Dark colored.

• White Muscle Fibers (Type II):

  • Less dense in capillaries, mitochondria, and myoglobin (pale colored).
  • Fast-twitch, high force, but fatigue quickly.
  • Adapted for short bursts of anaerobic activity.

Fascia

• Dense regular connective tissue with wavy collagen fibers oriented parallel to the direction of pull.
• Surrounds muscles, muscle groups, blood vessels, and nerves.
• Flexible but highly resistant to unidirectional tension.

Tendon Sheath

• Dense regular connective tissue;
• Parallel collagen fibers;
• Fewer blood vessels;
• Tendinocytes (fibroblasts) positioned between collagen bundles.

Synovial Bursa

• Sac-like cavity lined with synovial membrane.
• Secretes lubricating synovial fluid.
• Interposed between tendons and bony prominences, or other friction points.