Physiology of Skeletal Muscle Contraction

Overview

• Skeletal muscle contraction is a complex process that involves muscle fiber anatomy, excitation-contraction coupling, and sliding filament theory.

Muscle Fiber Structure

Components of Muscle Fiber

• Structure: Long cylindrical cells (muscle fibers) with peripheral nuclei.

• Membranes:

  • Sarcolemma: Plasma membrane of muscle fiber.

  • Sarcoplasm: Cytoplasm containing myoglobin and glycogen for energy storage.

Connective Tissue Wrappings

• Epimysium: Outer layer around the entire muscle.

• Perimysium: Surrounds fascicles (bundles of muscle fibers).

• Endomysium: Surrounds individual muscle fibers, connects to neighboring fibers, and contains blood vessels.

Skeletal Muscle Attachments

• Attachments: Skeletal muscles usually attach to bones at two points.

  • Insertion: Point of attachment that moves during contraction.

  • Origin: Stationary point of attachment.

• Types of Attachments: Direct (epimysium fuses with bone) or indirect (tendons and aponeurosis).

Microscopic Anatomy

Myofibrils

• Definition: Rod-like structures that make up muscle fibers and contain contractile units called sarcomeres.

• Sarcomeres: Made up of thick (myosin) and thin (actin) filaments, responsible for striations observed in skeletal muscle.

Sarcomere Structure

• Z Line: Ends of sarcomeres, where actin filaments attach.

• M Line: Middle of the sarcomere that anchors thick filaments.

• A Band: Length of thick filaments.

• I Band: Areas where only thin filaments are present, bisected by Z lines.

• H Zone: Area within the A band where thin filaments do not overlap.

Contractile Proteins

Myosin

• Structure: Thick filaments with heads that interact with actin.

• Function: Binds to actin during contraction, resulting in the power stroke.

Actin

• Structure: Thin filaments containing G-actin which forms binding sites for myosin.

• Function: Interacts with myosin to facilitate contraction.

Role of Tropomyosin and Troponin

• Tropomyosin: Covers myosin binding sites on G-actin, preventing contraction.

• Troponin: Binds calcium; changes conformational shape to move tropomyosin away, exposing binding sites for myosin.

Mechanism of Contraction

Sliding Filament Theory

• Process: 1) Myosin binds to actin, forming cross bridges. 2) Power stroke pulls actin towards the center of sarcomere. 3) ATP binds to myosin head causing detachment from actin. 4) Myosin head re-cocks into high-energy configuration using ATP.

Excitation-Contraction Coupling

• Mechanism: Action potentials stimulate muscle fibers via T-tubules, releasing calcium from the sarcoplasmic reticulum (SR).

• Calcium's Role: Binds to troponin, causing tropomyosin to move, thus exposing binding sites for myosin.

Muscle Contraction Types

Isometric Contractions

• Definition: Muscle generates force without changing length.

Isotonic Contractions

• Definition: Muscle changes length while loading an object or moving.

Muscle Fatigue

Causes

• Central Fatigue: Originates in the CNS, associated with perceptions of effort.

• Peripheral Fatigue: Involves neurotransmitter depletion and muscle fiber response issues (e.g., myasthenia gravis).

Muscle Fiber Types

Slow-Twitch vs. Fast-Twitch Fibers

• Slow-Twitch: Red fibers, high myoglobin, fatigue-resistant, used for endurance activities.

• Fast-Twitch: White fibers, low myoglobin, fatigue quickly, capable of explosive power.

Energy Supply for Muscle Contraction

Energy Sources

• Creatine Phosphate (CP): Quick energy, approximately 15 seconds, used during initial high-intensity activities.

• Glycolysis: Anaerobic process generating lactic acid and 2 ATP over 30-60 seconds.

• Aerobic Respiration: Longer duration (hours), yields 36 ATP per glucose molecule, reliant on oxygen.

Conclusion

• Understanding these physiological principles of skeletal muscle contraction is vital for comprehending how movements and muscle performance occur in the body.