LO 6

Muscle Tissue Types

  • Skeletal Muscle: Voluntary; attached to bones or skin.

  • Cardiac Muscle: Involuntary; found in heart wall (myocardium).

  • Smooth Muscle: Involuntary; located in walls of internal organs (e.g., intestines, blood vessels).

Functions of Skeletal Muscles

  • Body movement.

  • Maintenance of posture.

  • Protection and support.

  • Regulating material elimination.

  • Heat production.

Connective Tissues Surrounding Muscles

  • Epimysium: Dense irregular tissue wrapping the entire muscle.

  • Perimysium: Dense irregular tissue wrapping fascicles.

  • Endomysium: Areolar tissue wrapping individual muscle fibers.

Microstructure of Muscle Fiber

  • Nuclei: Multiple, as fibers are multinucleated.

  • Sarcolemma: Plasma membrane of muscle fiber.

  • Sarcoplasm: Cytoplasm of muscle fiber.

  • Myofibril: Rod-like elements composed of sarcomeres.

  • T-tubules: Extensions of the sarcolemma.

  • Sarcoplasmic Reticulum: Stores and releases Ca²⁺ for contraction.

  • Thin Filaments: Composed of actin.

  • Thick Filaments: Bundled myosin molecules.

Muscle Contraction Mechanism

  • Cross-Bridge Formation: Myosin heads attach to actin.

  • Sliding Filament Theory: Myosin heads pull actin filaments, resulting in sarcomere shortening.

Neuromuscular Junction

  • The point where a motor neuron innervates a muscle fiber.

  • Sequence: ACh release → ends plate potential → muscle fiber action potential.

  • Termination of ACh: By acetylcholinesterase.

Muscle Fiber States

  • Rigor Mortis: State of muscle stiffness post-mortem due to lack of ATP.

Motor Unit

  • Composed of a motor neuron and muscle fibers it controls.

  • Size varies: Small (few fibers) to large (thousands of fibers).

Muscle Contraction Dynamics

  • Frequency of Action Potentials: Increased rate enhances muscle contraction force.

Muscle Contractions Types

  • Isotonic: Muscle tension overcomes resistance; tension is same

  • Isometric: Muscle tension increases without changing length.

  • Concentric: Muscle shortens while contracting.

  • Eccentric: Muscle lengthens while contracting.

ATP Generation

  • Aerobic Metabolism: Requires oxygen; occurs in mitochondria.

  • Glycolysis: Short-term ATP supply that does not require oxygen; occurs in cytoplasm.

Oxygen Debt

  • The extra oxygen required to restore body to resting state post-exercise. 

Creatine Kinase

  • An enzyme that transfers phosphate from creatine phosphate to ADP, aiding energy production.

Muscle Fatigue Factors

  • Caused primarily by decreased glycogen stores; also involves altered ion concentrations and impaired neurotransmitter release.

  • Other possible causes of fatigue:

    • Insufficient Ca2+ to enter synaptic knob 

    • Decreased number of synpatic vesicles 

    • Alteried ion concentration impair AP conduction and Ca2+ release from sacroplasmic reticulum 

    • Excessive P slows release of P from myosin head 

    • Less Ca2+ avail for troponin (some Ca2+ is bound to P) 

Fiber Type Comparison

  • Slow Oxidative: Fatigue-resistant, small diameter, high myoglobin.

  • Fast Glycolytic: Fast fatigue, large diameter, low myoglobin.

  • Fast Oxidative: Moderate fatigue resistance, intermediate characteristics.

Muscle Mass Changes

  • Atrophy: Muscle mass loss.

  • Hypertrophy: Increase in muscle mass.

Muscle Naming

  • Based on action, attachments, location, or fiber orientation.