Muscle Physiology

Introduction to Muscle Types

  • Types of Muscle: There are three types of muscle tissue:
    • Skeletal: Striated and under voluntary control.
    • Cardiac: Striated, involuntary muscle found only in the heart.
    • Smooth: Non-striated and involuntary, found in walls of hollow organs.

Function of Skeletal Muscle

  • Locomotion: Enables voluntary movement of the body.
  • Heat Production: Plays a role in thermoregulation by producing heat.
  • Protein Reserve: Serves as a reserve of protein for gluconeogenesis during fasting periods.
  • Metabolic Rate: Major contributor to the body's overall metabolic rate.

Structure of Skeletal Muscle

Components
  • Muscle Fiber: Basic cellular unit, long and multinucleated.
  • Muscle Fasciculus: Bundle of muscle fibers.
  • Myofibrils: Made up of myofilaments that facilitate contraction.
  • Filaments: Include actin (thin) and myosin (thick) filaments.
Sarcomere: Functional Unit
  • Defined as the section between two Z discs.
  • Composed of:
    • A Band: Contains both actin and myosin.
    • I Band: Contains only actin.
    • H Band: Contains only myosin.
    • Z Disc: Separates adjacent sarcomeres, anchors actin filaments.
Accessory Proteins
  • Titin: Connects myosin filaments to Z discs, providing elasticity.
  • Nebulin: Extends from Z discs along actin filaments, stabilizing them.

Myosin and Actin Structure

Myosin
  • Composed of two heavy chains and four light chains.
  • Forms a double helix tail and globular heads responsible for contraction via cross-bridge cycling.
Actin Filaments
  • Composed of F-actin, tropomyosin, and troponin complexes.
  • Tropomyosin: Covers binding sites on actin, preventing interaction with myosin.
  • Troponin: Complex with three subunits; binds to tropomyosin, actin, and calcium.

Muscle Contraction Mechanism

  1. Calcium Regulation: Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from binding sites.
  2. Cross-Bridge Cycle:
    • Myosin heads attach to exposed actin binding sites forming a cross-bridge.
    • ADP and inorganic phosphate are released, resulting in a power stroke that slides the actin filaments.
    • ATP binds to myosin, causing it to detach from actin.
    • ATP is hydrolyzed to ADP + Pi, re-cocking the myosin head for another cycle.

Excitation-Contraction Coupling

  • Sarcoplasmic Reticulum (SR): Stores calcium ions; releases them upon muscle stimulation.
  • T-tubules: Invaginations that conduct action potentials deep into the muscle fiber.
  • Triad Formation: The junction of T-tubule with two terminal cisternae of SR, essential for simultaneous calcium release during muscle contraction.

Neuromuscular Junction (NMJ)

  • Structure: Specialized synapse between motoneuron and muscle fiber.
  • Process:
    • Action potential from the motoneuron leads to the release of acetylcholine (ACh).
    • ACh binds to nicotinic receptors on the muscle membrane, opening sodium channels, leading to muscle action potentials.
    • Termination: ACh is broken down by acetylcholinesterase, stopping muscle contraction.

Pathophysiology

  • Myasthenia Gravis: Autoimmune disorder affecting nicotinic ACh receptors, leading to muscle weakness.
  • Curariform Drugs: Block ACh receptors, leading to paralysis.
  • Botulinum Toxin: Prevents ACh release, diminishing muscle contraction.

Summary of Muscle Contraction

  1. Action potential travels to NMJ.
  2. ACh release initiates muscle action potential.
  3. Calcium is released from SR, enabling contraction.
  4. Myosin binds to actin, sliding occurs.
  5. Calcium is pumped back into the SR, and muscle relaxes.

Types of Smooth Muscle

  • Multi-unit Smooth Muscle: Composed of discrete fibers, reacts to individual nerve signals. Examples include eye iris and vas deferens.
  • Unitary Smooth Muscle: Forms sheets connected by gap junctions, acting as one unit; seen in GI and respiratory tracts.

Control of Smooth Muscle Contraction

  • Governed by local nerve signals, hormones, and stretch.
  • Unique features include the absence of troponin, Ca2+ driven contraction, and long-lasting contractions.

Conclusion

  • Skeletal Muscle: Voluntary control, quick contractions.
  • Cardiac Muscle: Involuntary, striated, with intercalated discs, allowing coordinated contractions.
  • Smooth Muscle: Non-striated, involuntary, slower contractions, adaptable to various stimuli.