Muscle Tissue Overview and Classification

Muscle Tissue Overview

• Muscle tissue functions in contraction, enabling body movement.
• Muscle cells are elongated, often called muscle fibers.
• Specialized structures within muscle cells:
  • Sarcolemma: the plasma membrane.
  • Sarcoplasm: the cytoplasm.
  • Sarcoplasmic reticulum: smooth endoplasmic reticulum.
  • Sarcosomes: mitochondria.

Muscle Classification

• Muscles are categorized by the arrangement of contractile proteins:
  • Striated muscle: exhibits alternating dark and light bands (striations) due to organized contractile proteins.
    • Skeletal striated muscle: found in the locomotor system, responsible for voluntary movements; cells are large, multinucleated, and have nuclei at the periphery; cells are parallel; surrounded by satellite cells for regeneration.
    • Cardiac striated muscle: found only in the heart; cells are branched, have a single nucleus, and are connected via intercalated discs for coordinated contraction; has a rich capillary network, providing energy through aerobic respiration. Has a limited capacity for regeneration in contrast to other cells.
  • Smooth muscle: lacks striations, found in the walls of hollow organs and blood vessels; spindle-shaped cells (small); cells are interconnected; maintains mitotic capacity.

Skeletal Muscle Structure

• Skeletal muscle has connective tissues:
  • Endomysium: surrounds individual muscle cells (fibers).
  • Perimysium: surrounds bundles of muscle fibers (fascicles).
  • Epimysium: surrounds the entire muscle.
• Myofibrils are cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.
• Myofilaments (actin and myosin) are arranged within sarcomeres, creating the striated pattern.
• Sarcomere: repeating structural unit within myofibril; subdivided into bands (I band, A band, H zone, M line, and Z-disk).
• T tubules: tubular extensions of the sarcolemma that penetrate into the muscle fiber, allowing for rapid spread of action potentials.
• Sarcoplasmic Reticulum: network of specialized smooth endoplasmic reticulum around each myofibril; terminal cisterns alongside the T-tubules form triads.

Myofilaments

• Thin filaments: primarily composed of actin proteins, regulated by tropomyosin and troponin.
• Thick filaments: primarily composed of myosin proteins, with heads capable of binding to actin.

Sarcomere Structure

• I band: only thin filaments, light band.
• A band: thick and thin filaments, dark zone (includes H-zone).
• H zone: only thick filaments.
• M line: middle of the sarcomere.
• Z line: marks the boundaries of sarcomeres.

Muscle Contraction

• When a nerve impulse reaches a muscle, it triggers the release of calcium from the sarcoplasmic reticulum.
• Calcium binds to troponin, causing tropomyosin to move, exposing myosin-binding sites on actin.
• Myosin heads bind to actin, forming cross-bridges.
• Myosin heads move, pulling the thin filaments past the thick filaments, shortening the sarcomere (contraction).
• Muscle contraction ceases when calcium levels drop, causing tropomyosin to block the myosin-binding sites and the myosin heads to detach.

Innervation

• Motor units: a neuron and the muscle fibers it innervates; axon branches and loses myelin.
• Motor plate: where motor nerve endings and muscle cells meet.

Cardiac Muscle

• Involuntary, striated muscle found only in the heart.
• Cells are branched, have one nucleus, and are connected by intercalated discs for synchronized contraction.
• A rich capillary network provides energy through aerobic respiration.
• Has a limited capacity for regeneration.

Smooth Muscle

• Involuntary, non-striated muscle.
• Cells are spindle-shaped, with a single nucleus.
• Cells are interconnected, forming bundles or layers.
• Retains its mitotic capacity, enabling regeneration.
• Contraction mechanism differs from skeletal and cardiac muscle, with thin and thick filaments anchored to dense bodies.