32. Cross striated skeletal muscle tissue

Cross-Striated Skeletal Muscle Tissue and Myofibrils

Skeletal muscle consists of bundles of very long, cylindrical multinucleated cells (muscle fibers) with cross striations. These muscle fibers exhibit quick, forceful, and voluntary contractions due to the interaction between actin and myosin (the sliding filament theory). Key features include:

1. Muscle Fiber Structure:
   • Muscle fibers are highly eosinophilic (stain pink) due to their high collagen content.
   • Each muscle fiber is organized into the following layers:
     • Epimysium: The external sheath of dense connective tissue surrounding all the muscle fiber masses.
     • Perimysium: Thinner dense connective tissue that surrounds fascicles (bundles of muscle fibers).
     • Endomysium: Surrounds each muscle fiber individually, containing reticular fibers.

Myofibrils and Sarcomeres:

1. • Muscle fibers contain myofibrils, long cylindrical filament bundles.
   • The functional unit of a myofibril is the sarcomere (approximately 2.5 μm).
   • Within a sarcomere:
     • A Band: Dark band where myosin and actin overlap.
     • I Band: Lighter band bisected by the dark Z line.
     • Z Line: Border between two adjacent sarcomeres.
     • H Zone: Lighter zone within the A band, consisting of myosin filaments only.
     • M Line: Connection between two myosin myofilaments in the middle of the H zone.

1. Actin and Myosin:
   • Actin: Composed of F-actin associated with tropomyosin and troponin.
     • F-actin consists of long filamentous polymers made up of two strands of G-actin.
     • Alpha-actinin anchors actin to the Z line.

1. • Tropomyosin: Forms a 40 nm polypeptide chain coil in the groove between two actin strands.
   • Troponin: A complex of three subunits:
     • TnT (attaches to tropomyosin).
     • TnC (binds to calcium).
     • TnI (regulates actin-myosin interaction).
2. Myosin:
   • Consists of two identical heavy chains and two pairs of light chains.
   • Each myosin head has ATP and actin binding sites.

1. Sarcoplasmic Reticulum and T Tubules:
   • Sarcoplasmic Reticulum: Membranous smooth ER containing pumps for calcium sequestration.
   • T Tubules: Finger-like invaginations of the cell membrane that penetrate deeply into the sarcoplasm, encircling each myofibril near the A and I bands.
   • Terminal Cisternae: Adjacent to T tubules, forming a triad (2 T cisternae + 1 T tubule) for depolarization.

1. Muscle Contraction Process:

1. Calcium ions bind to troponin, causing a shape change that moves tropomyosin on actin, exposing active sites on thin filaments.
2. Myosin heads of thick filaments attach to the exposed active sites, forming cross-bridges.

1. Myosin heads pivot, moving thin filaments toward the sarcomere center. ATP bound to myosin heads is broken down to ADP+Pi.
2. Repeating cycles slide thick and thin filaments, shortening the sarcomere and causing muscle contraction:
   • Z lines move closer.
   • I band decreases in size as filaments penetrate the A band.
   • H band diminishes.

Types of Skeletal Muscle Fibers:

• Slow Oxidative (Type I):
  • Adapted for slow contraction over long periods without fatigue.
  • Rich in mitochondria, capillaries, and myoglobin (appearing dark or red).
• Fast Glycolytic (Type IIb):
  • Specialized for rapid short-term contractions.
  • Few mitochondria, relying on anaerobic glucose metabolism (appear white due to stored glycogen).
• Fast Oxidative-Glycolytic (Type IIa):
  • Intermediate features between slow oxidative and fast glycolytic fibers.

Myofibrillar Proteins:

• Composed of myosin, actin, and regulatory proteins (tropomyosin, troponin, and actinin).