Muscle Tissue Classification

Classification of Muscles

• Skeletal muscle:
  • Very long, multinucleated cells
  • Cross-striations
  • Voluntary, forceful
• Cardiac muscle:
  • Branched
  • Cross-striations
  • Intercalated discs
  • Involuntary, rhythmic
• Smooth muscle:
  • Fusiform cells
  • Lack striations
  • Involuntary, slow

Specialized Terms

• Sarcolemma: Cell membrane
• Sarcoplasm: Cytoplasm
• Sarcoplasmic reticulum: SER

Organization of Skeletal Muscle

• Epimysium: Dense connective tissue surrounding entire muscle
• Perimysium: Thin connective tissue surrounding bundles of muscle fibers (fascicle)
• Endomysium: Reticular fiber and fibroblast surrounding individual muscle fibers

Structure of Skeletal Muscle Fibers

• Long, cylindrical cell: 10-100µm in diameter and 1-40mm in length
• Multinucleated, with nuclei lying beneath sarcolemma
• Cross-striations: alternating dark and light bands
• Myofibrils: Thread-like structures arranged in parallel in muscle fibers

Myofibrils

• Long, cylindrical filamentous bundles
• Parallel to the long axis of muscle fibers
• 1-2µm in diameter
• Alternate A bands and I bands
• I band is bisected by a dark transverse line, Z line
• A band: light zone present, H band, bisected by M line
• Sarcomere: The unit of structure and function of muscle fibers
• Myofilaments: Thick and thin filaments

Thick Filaments

• Occupy A band
• Made up of myosin molecules
• A single myosin molecule has a rod and two globular heads
• The rods overlap; the globular heads forming cross-bridges and having ATPase activity

Thin Filaments

• One end inserted into the Z line, the other extends into the A band
• Composed of actin, tropomyosin, and troponin
• Actin: Globular monomers (G-actin) and filamentous polymers (F-actin)
• Tropomyosin: A long, thin molecule containing two polypeptide chains
• Troponin: A complex of three subunits: TnT (attaches to tropomyosin), TnI (inhibits actin-myosin interaction), and TnC (binds calcium ions)

Transverse Tubules and Sarcoplasmic Reticulum

• Transverse tubules: Sarcolemma tubular infoldings at A- and I-band boundaries, responsible for rapid conduction of impulses
• Sarcoplasmic reticulum: Stores calcium ions, located between two adjacent T-tubules, terminal cisternae

Mechanism of Contraction

• Sliding filament hypothesis
• Impulse of NF: Calcium binds to TnC of troponin, myosin combines with actin to form cross-bridges, myosin heads pivot, moving thin filaments towards the sarcomere center

Cardiac Muscle

• Found in the wall of the heart and proximal portions of the aorta and venae
• Involuntary, striated muscle
• Contracting spontaneously and displaying a rhythmic beat
• LM structure:
  • Short column and branched
  • One or two centrally placed nucleus
  • Show cross-striations and myofibrils, but less distinct
  • Intercalated disks link cells to one another
• Ultrastructure:
  • Discrete myofibril bundles not present
  • Larger T-tubules at Z-line level
  • Sarcoplasmic reticulum: Not well-developed, small terminal cisternae
  • Dyads: T-tubule and terminal cisternae on one side
  • More mitochondria, glycogen, and lipid droplets

Smooth Muscle

• Seen in blood vessels and hollow viscera (visceral muscle)
• Arranged in layers
• Enclosed by a network of type I and type III collagen fibers
• Non-striated cells
• LM structure:
  • Spindle in shape, with an oval, centrally located nucleus
  • A range of diameters
  • Without striations
• EM structure:
  • Gap junctions
  • Caveolae
  • A rudimentary sarcoplasmic reticulum
  • Myofibrillar arrays
  • Thick filaments
  • Thin filaments
  • Dense body: Works as Z line
  • Intermediate filament: Desmin, attaches to dense bodies