Chapter 9: Muscles and Muscle Tissue - Detailed Notes

Types of Muscle

  • Three Main Types:
    • Skeletal Muscle: Voluntary control, striated appearance.
    • Cardiac Muscle: Involuntary control, striated, found in the heart.
    • Smooth Muscle: Involuntary control, non-striated, found in walls of hollow organs.

Characteristics of Muscle Tissue

  • All muscles share four main characteristics:
    • Excitability (responsiveness): Ability to receive and respond to stimuli.
    • Contractility: Ability to shorten forcibly when stimulated.
    • Extensibility: Ability to be stretched.
    • Elasticity: Ability to recoil to resting length.

Muscle Functions

  • Four Important Functions:
    1. Movement: Responsible for locomotion and manipulation.
    2. Maintain Posture: Helps keep body positions stable.
    3. Stabilize Joints: Provides support to joints through muscle tension.
    4. Generate Heat: Muscles produce heat as they contract (thermogenesis).

Skeletal Muscle Anatomy

  • Hierarchy of Muscle Structure:
    • A whole muscle contains numerous fascicles.
    • A fascicle consists of many muscle fibers (muscle cells).
  • Connective Tissue Sheaths:
    • Epimysium: Dense irregular connective tissue surrounding entire muscle.
    • Perimysium: Surrounds fascicles (groups of muscle fibers).
    • Endomysium: Areolar connective tissue surrounding each muscle fiber.

Attachments

  • Muscles attach to bones at least two places:
    • Insertion: Attachment to the movable bone.
    • Origin: Attachment to an immovable or less movable bone.
  • Attachments can be direct (fleshy) or indirect (via tendons).

Microscopic Anatomy of Skeletal Muscle

  • Key Features:
    • Long, multi-nucleated fibers.
    • Sarcoplasm: Muscle fiber's cytoplasm containing glycosomes for glycogen storage and myoglobin for oxygen storage.
    • Myofibrils: Contractile units that make up muscle fibers.

Myofibrils and Myofilaments

  • Myofibrils:
    • Densely packed rodlike elements accounting for ~80% of muscle cell volume.
    • Create striations and consist of sarcomeres, which are repeating contractile units made of myofilaments (actin and myosin).
  • Myofilaments:
    • Actin (thin filament): Extends across I band and partway into A band; anchored to Z discs.
    • Myosin (thick filament): Extends length of A band; connected at M line.

Cross Bridge Cycling

  • Process of contraction involving the cyclical interactions between actin and myosin:
    • Myosin must bind to actin for muscle contraction to occur.
    • ATP is crucial for both the formation and breaking of the cross bridges:
    • Cross bridge formation occurs when myosin heads attach to actin.
    • Power stroke occurs when ADP and P are released from myosin heads, pulling actin fibers toward the center of the sarcomere.
    • Myosin heads detach from actin when a new ATP molecule binds to myosin, breaking the cross bridge.
  • Calcium (Ca²+) plays a key role in the binding of myosin and actin by unfurling tropomyosin via its attachment to troponin.

Excitation-Contraction Coupling

  • Events Triggering Muscle Contraction:
    1. Action Potential (AP) arrives at the axon terminal of the motor neuron, releasing acetylcholine (ACh).
    2. ACh binds to receptors on the sarcolemma, generating an end plate potential.
    3. Local depolarization leads to AP generation.
    4. AP propagates along sarcolemma into T tubules.
    5. This triggers release of Ca²+ from the sarcoplasmic reticulum, initiating contraction via cross bridge cycling.

Muscle Fiber Types

  • Three Types of Skeletal Muscle Fibers:
    • Type I (Slow Oxidative): Endurance activities, slow contraction speed, high fatigue resistance.
    • Type IIa (Fast Oxidative): Moderate contraction speed, intermediate fatigue resistance.
    • Type IIb (Fast Glycolytic): Powerful, rapid contractions, low fatigue resistance.

Muscle Contraction and Relaxation

  • Isometric Contraction: No shortening occurs; muscle tension increases but does not exceed load.
  • Isotonic Contraction: Muscle shortens because muscle tension exceeds load.
  • Muscle Fatigue: Can result from ionic imbalances, decreased glycogen, or increased inorganic phosphate in muscle cells leading to compromised function.

Physiological Considerations

  • Recovery and Oxygen Debt: Post-exercise, oxygen reserves are replenished and lactic acid is converted back into usable energy forms.
  • Effects of Exercise: Regular activities lead to increased muscle capillaries, mitochondria, and myoglobin synthesis, enhancing endurance and strength.
  • Homeostasis and Muscle Health: Disuse atrophy can occur without regular exercise, necessitating regular motor neuron stimulation to maintain muscle health.