Chapter 10: Muscle Tissue and Organization

Unique Properties of Muscle Tissue

• Excitability: Ability to respond to stimuli, often neurotransmitters.
  • Ion movements across muscle cell membranes trigger contractions.
• Contractility: Muscle cells contract by sliding filament theory; filaments slide past each other.
• Elasticity: Ability to return to original shape after contraction.
• Extensibility: Ability to extend or lengthen while another muscle contracts.

Structure of Skeletal Muscle

• Skeletal muscle is an organ containing four tissue types: muscle, epithelial, connective, and nervous.
• Functions:
  • Body movement
  • Posture maintenance
  • Temperature regulation
  • Storage and movement of materials
  • Support for body structure

Organization of Muscle Tissue

• Muscles are structured as tubes within tubes:
  • Organ Level: Named muscles (e.g., biceps brachii)
  • Tissue Level: Fascicles
  • Cellular Level: Muscle fibers
  • Myofibrils/Myofilaments: Composed of thin and thick filaments.

Connective Tissue Layers

• Endomysium: Surrounds muscle fibers (areolar connective tissue).
• Perimysium: Surrounds fascicles (dense irregular connective tissue).
• Epimysium: Surrounds entire muscle (dense irregular connective tissue).
• Deep/Superficial Fascia: Separate individual muscles.

Muscle Fiber Structure

• Sarcolemma: Plasma membrane of muscle fibers.
• Sarcoplasm: Cytoplasm of muscle fibers.
• Sarcoplasmic Reticulum: Specialized smooth ER storing calcium.
  • T-tubules: Invaginations of the sarcolemma aiding calcium release.
  • Triad: Consists of T-tubule and two terminal cisternae.

Myofilaments

• Thin Filaments: Actin, tropomyosin, troponin (regulatory proteins).
• Thick Filaments: Myosin.
• Sarcomere: Functional unit of muscle contraction.
  • Regions: M line, A band (thick), I band (thin only), H zone (thick only).

Muscle Contraction: Sliding Filament Theory

• Thick and thin filaments slide past each other without shortening.
• Changes in Contraction:
  • A band remains constant.
  • H zone disappears.
  • I band shortens.

Motor Units

• Motor Unit: Single motor neuron and all muscle fibers it innervates.
• All-or-none principle: All fibers in a motor unit contract if the neuron activates.
• Fine Control: Fewer muscle fibers per motor unit (e.g., hands).
• Force: More motor units recruited.

Types of Skeletal Muscle Fibers

• Slow Oxidative (Type I): Fatigue-resistant, less force (e.g., back, calf).
• Fast Oxidative (Type IIa): Intermediate force, more fatigue-resistant.
• Fast Glycolytic (Type IIb): Quick energy, fatigues rapidly (e.g., eyes, hands).

Muscle Adaptation

• Atrophy: Reduction in muscle size and strength due to lack of use.
• Hypertrophy: Increase in muscle size due to increased myofibrils.
• Hyperplasia: Increase in muscle fiber number, often disease-related.

Tendons and Aponeuroses

• Tendons: Attach muscles to bones, cord-like.
• Aponeuroses: Flat tendons, provide broad attachment.

Muscle Levers

• First Class Lever: Fulcrum between effort and resistance (e.g., neck).
• Second Class Lever: Resistance between fulcrum and effort (e.g., heel lift).
• Third Class Lever: Effort between fulcrum and resistance (e.g., elbow flexion).

Muscle Movements

• Agonist: Prime mover of muscle action.
• Synergist: Assists the agonist.
• Antagonist: Opposes the movement of the agonist.