Cross-striated muscle tissue, common features structure and function

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Cross-striated muscle

Definition

Cross-striated or striated muscle tissue is a type of muscle tissue characterized by a regular transverse striation pattern visible under light microscopy. The striation is caused by the ordered arrangement of actin and myosin filaments into sarcomeres. This category includes:

• Skeletal muscle

• Cardiac muscle

Common structural features

Muscle Fiber Layers

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.

Sarcomere

Myofibrils and Sarcomeres

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.

Actin and Troponin

Actin and Myosin

Thin Filament - Actin: Double helix of fibrous 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.
• Tropomyosin: Forms a 40 nm polypeptide chain coil in the groove between two actin strands.
• Troponin: A complex of three subunits:
  • TnT - connecting troponin with tropomyosin.
  • TnC - binds to calcium.
  • TnI - changing the position of tropomyosin and thus revealing the binding sites for myosin.

Myosin

Thick Filament - Myosin: Each filament is formed by two myosin molecules that twist around each other (tail) and expand at the end (head). The part between the head and tail has the ability to bend (neck). The head has ATPase activity and binds to the active sites of actin. The thick filament is made up of many myosin molecules. The tails form the axis of the filament, and the heads protrude into space.

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.

Molecular mechanism of muscle contraction

Function

Excitation and Contraction have several successive phases:

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.
3. Myosin heads pivot, moving thin filaments toward the sarcomere center. ATP bound to myosin heads is broken down to ADP+Pi.
4. 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.

References

• TROJAN, Stanislav, et al. Lékařská fyziologie. 4. dopl. vyd. Praha : Grada Publishing, 2003. 772 s. ISBN 80-247-0512-5.
• GANONG, William F. Přehled lékařské fyziologie. 20. vyd. Praha : Galén, 2005. 890 s. ISBN 80-7262-311-7./>