Skeletal Muscle Notes

Chapter 10: Muscular Tissue

Overview of Skeletal Muscle

  • Skeletal Muscle Fibers:
    • Very long cells, among the longest in the body after neurons.
    • Example: Sartorius muscle contains single fibers that are at least 30 cm long.

Formation of Skeletal Muscle Fibers

  • Myoblasts:
    • Immature muscle fiber cells that fuse to form skeletal muscle fibers.
  • Satellite Cells:
    • Involved in muscle fiber repair and regeneration.

Structure of Skeletal Muscle Fibers

  • Skeletal Muscle Fiber Components:
    • Sarcolemma:
    • Plasma membrane of an individual skeletal muscle fiber.
    • Sarcoplasm:
    • Cytoplasm of skeletal muscle fibers, rich in contractile proteins arranged in myofibrils.
  • Internal Structures to Learn:
    • Myofibrils
    • T-tubules
    • Triad (includes terminal cisterns)
    • Sarcoplasmic Reticulum
    • Sarcomere
    • Endomysium: Surrounds the muscle fiber.

Myofibril Structure

  • Filaments:
    • Myofibrils consist of:
    • Thick Filaments: Composed primarily of myosin.
    • Thin Filaments: Composed mostly of actin with other proteins.
  • Scanning Electron Micrograph of a Sarcomere:
    • The basic functional unit of skeletal muscle fibers, arranged between two Z discs.

Sarcomere and Muscle Contraction

  • Z Disc:
    • Marks the boundary of sarcomeres; contraction occurs within these structures.
  • Muscle contraction is facilitated at the level of the sarcomere.

Muscle Proteins

  • Contractile Proteins:
    • Generate force during muscle contraction.
    • Main components: Actin (thin filaments) and Myosin (thick filaments).
  • Regulatory Proteins:
    • Help switch the contraction process on and off.
    • Troponin: Binds calcium and allows for muscle contraction by exposing binding sites on actin.
    • Tropomyosin: Covers myosin binding sites in a relaxed muscle.
  • Structural Proteins:
    • Maintain alignment, stability, elasticity, and extensibility of myofibrils.
    • Examples include:
    • Myomesin
    • Nebulin
    • Titan:
      • The third most abundant protein in muscle, providing elasticity to myofibrils and extending from the Z disc.
    • Calsequestrin: Involved in calcium storage.
    • Dystrophin: Related to muscular dystrophy.

Detailed Structure of Filaments

  • Thin Filaments (Actin):
    • Comprised of actin proteins, resembling a string of pearls.
  • Thick Filaments (Myosin):
    • Comprised mainly of myosin proteins, resembling bound golf clubs.

Interaction of Proteins During Contraction

  • Myosin Binding Sites:
    • Initially covered by the troponin-tropomyosin complex.
  • Calcium Ions (Ca^2+):
    • Bind to troponin, causing a conformational change that uncovers myosin binding sites on actin, facilitating muscle contraction.
  • Dynamic Nature:
    • The troponin-tropomyosin complex can slide to expose or cover the myosin binding sites depending on the presence of Ca^2+ ions.

Summary of Muscle Fiber Properties

  • Muscle fibers are complex structures crafted from a variety of proteins, each with essential roles in muscle function and contraction. The interaction between these proteins is crucial for muscle movement and overall function, highlighting the sophisticated nature of muscular tissue.