10.2 Skeletal Muscle - Anatomy and Physiology 2e

Key Features of Skeletal Muscle

• Contraction and Movement

  • Skeletal muscle is known for its ability to contract and produce movement.
  • It also stops movement (e.g., resistance to gravity for posture).
  • Maintains skeletal stability, prevents damage or deformation.
  • Located throughout the body to control opening of internal tracts, allowing voluntary control of swallowing, urination, and defecation.
  • Protects internal organs and supports their weight.

• Homeostasis and Heat Generation

  • Contraction requires energy, releasing heat noticeable during exercise or shivering in cold conditions.

Structural Organization

• Components

  • Consists of skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue.
  • Three layers of connective tissue (mysia):
    • Epimysium: Dense irregular connective tissue surrounding each muscle, allows contraction while maintaining structure.
    • Perimysium: Middle layer organizing muscle fibers into bundles (fascicles).
    • Endomysium: Thin layer encasing muscle fibers, containing extracellular fluid and nutrients.

• Tendon and Bone Connection

  • Collagen in connective tissue layers intertwines with tendon collagen, fusing with bone periosteum, facilitating bone movement.
  • Aponeuroses and fascia may also connect muscles to bone or skin.

• Blood and Nerve Supply

  • Rich vascular supply for nourishment, oxygen, and waste removal.
  • Each muscle fiber is innervated by a somatic motor neuron.

Skeletal Muscle Fibers

• Characteristics

  • Long, cylindrical, also known as muscle fibers.
  • Large cell size.
  • Multinucleated due to fusion of embryonic myoblasts.

• Terminology

  • Sarcolemma: Plasma membrane.
  • Sarcoplasm: Cytoplasm.
  • Sarcoplasmic Reticulum (SR): Stores, releases, and retrieves calcium ions.

The Sarcomere

• Structure
  • Functional unit, striated appearance due to actin and myosin arrangement.
  • Bordered by Z-discs, actin anchored here.
  • Composed of thin (actin) and thick (myosin) filaments.

Neuromuscular Junction (NMJ)

• Function
  • Site where motor neuron meets muscle fiber.
  • The only method for skeletal muscle activation.

Excitation-Contraction Coupling

• Process

  • Muscle fibers generate electrical signals (action potentials) using membrane potentials.
  • Action potentials travel and trigger calcium release from SR, initiating contraction.

• Steps

  • Neuronal action potential travels to NMJ, releasing acetylcholine (ACh).
  • ACh binds to receptors on sarcolemma, causing depolarization.
  • Depolarization triggers sodium channels, spreading action potential.
  • ACh is degraded by acetylcholinesterase to prevent prolonged contraction.

• Triad Structure

  • T-tubules carry action potentials to SR, causing calcium release into sarcoplasm, initiating contraction.