10+muscle
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.