Muscle Contraction Mechanisms and Types

Sep 27, 2024

Lecture Notes on Muscle Contraction

Recap of Muscle Types

  • Types of Muscles
    • Skeletal Muscle: Attached to bones.
    • Cardiac Muscle: Found around the heart.
    • Smooth Muscle: Found around organs and the viscera.
  • Primary Function
    • Generate force through contraction (muscle tension).

Functions of Muscle Types

  • Skeletal Muscles
    • Movement
    • Posture
    • Joint stabilization
    • Heat generation
  • Cardiac Muscles
    • Pumping blood through the heart
  • Smooth Muscles
    • Movement through hollow organs

Histology of Muscle Types

  • Skeletal Muscles
    • Long, straight, multinucleated cells with striations
  • Cardiac Muscles
    • Shorter, wider, branched cells with one or two nuclei
  • Smooth Muscles
    • Thin, elongated cells with a single nucleus

Properties of Muscle Cells

  • Contractility: Ability to contract
  • Excitability: Can be stimulated by electrical signals
  • Conductivity: Can conduct electrical currents
  • Extensibility: Can stretch
  • Elasticity: Can return to original shape

Muscle Cell Terminology

  • Sarcoplasm: Cytoplasm of a muscle cell
  • Sarcolemma: Plasma membrane of a muscle cell
  • Sarcoplasmic Reticulum: Specialized endoplasmic reticulum in muscle cells

Structure of Skeletal Muscle

  • Muscle Fiber Arrangement: Compartment -> Muscle -> Fascicle -> Fiber -> Myofibril -> Myofilament
  • Muscle Fibers: Thin cylinders, about 30 cm long, multinucleated

Components of Muscle Fibers

  • T-tubules: Extensions of sarcolemma
  • Terminal Cisternae: Part of sarcoplasmic reticulum; form a triad with T-tubules

Muscle Contraction: Myofilaments

  • Myosin (Thick Filament)
  • Actin (Thin Filament)
    • Troponin and Tropomyosin: Regulate binding sites

Sarcomere

  • Unit of Contraction
  • Z Disc to Z Disc: Length of a sarcomere
  • I Band: Only thin filament
  • A Band: Area of thick filament
  • H Zone: Only thick filament
  • Zones of Overlap: Both thick and thin filaments
  • Elastic Filament (Connectin): Prevents over-contraction and provides elasticity

Steps in Muscle Contraction

  1. Action Potential travels down motor neuron
  2. Calcium Channels open, allowing calcium into neuron
  3. Calcium binds to vesicles, releasing neurotransmitters (e.g., acetylcholine)
  4. Neurotransmitters cross synaptic cleft, bind to receptors
  5. Sodium enters muscle cell, changing charge
  6. Electrical current travels across muscle fiber and through T-tubules, releasing calcium from SR
  7. Calcium binds to troponin, moving tropomyosin away
  8. Myosin binds to actin, starting cross-bridge cycling
  9. Cross-Bridge Cycling: Myosin head attaches, pivots, detaches, and resets

Muscle Relaxation

  • Signal termination stops acetylcholine release
  • Acetylcholine is degraded by acetylcholinesterase
  • Calcium is returned to SR
  • Tropomyosin covers binding sites

Electrical and Chemical Aspects

  • Innervation: Nerves stimulate muscle fibers
  • Synapse: Gap between neuron and muscle fiber
  • Action Potential: Signal conversion to stimulate muscle

Muscle Twitch and Tetanus

  • Twitch: Smallest measurable contraction
  • Wave Summation: Increase in tension via repetitive stimulation
    • Unfused Tetanus: Partial relaxation between stimuli
    • Fused Tetanus: No relaxation, continuous contraction

Energy Sources for Muscle Contraction

  • Creatine Phosphate: Quick ATP regeneration
  • Glycolysis: Fast, anaerobic, 2 ATP per glucose
  • Oxidative Catabolism: Slow, aerobic, 36 ATP per glucose

Muscle Fiber Types

  • Type 1 (Slow Twitch): Endurance, more mitochondria
  • Type 2 (Fast Twitch): Burst activity
    • Fast Oxidative Glycolytic: Intermediate
    • Fast Glycolytic: Primarily anaerobic

Muscle Tone

  • Continuous, passive partial contraction of muscles

Muscle Contraction Types

  • Isotonic Concentric: Muscle shortens
  • Isotonic Eccentric: Muscle lengthens
  • Isometric: Muscle length unchanged

Myoplasticity and Muscle Adaptation

  • Endurance Training: More mitochondria and blood vessels
  • Resistance Training: More myofibrils
  • Disuse: Muscle atrophy

Smooth Muscle

  • Single Unit: Most common, linked via gap junctions
  • Multi-Unit: Less common, precise control
  • Contraction: Uses calmodulin and myosin light chain kinase

Cardiac Muscle

  • Structure: Short, branched cells with striations
  • Intercalated Discs: Allow electrical signal propagation
  • Pacemaker Cells: Enable auto-rhythmicity

This covers the major points discussed in the lecture on muscle functions, contraction mechanisms, muscle types, and their properties.