Muscle Physiology

Muscle Physiology Notes

Chapter 20 – Learning Objectives

  • Understand muscle fiber functional proteins (actin and myosin)

  • Explain the sliding filament theory of muscle contraction

  • Describe the molecular events during muscle contraction

  • Explain excitation-contraction coupling and its role in muscle contraction regulation

  • Differentiate between skeletal muscle fiber types

Types of Muscle

  • Skeletal Muscle: Striated; controlled by somatic nervous system.

  • Cardiac Muscle: Striated, involuntary; myogenic action potentials.

  • Smooth Muscle: Non-striated; controlled by autonomic nervous system.

Muscle Structure

  • Muscle fibers (cells) are multinucleated cylindrical cells organized into myofibrils.

  • Tendons: Connect muscles to bones.

  • Ligaments: Connect bones to other bones.

  • Sarcomere: Functional unit of muscle; contains myofilaments (actin and myosin).

Myofilaments Arrangement in Sarcomeres

  • Myofilaments:

  • Actin: Thin filaments

  • Myosin: Thick filaments

  • Z-disc: Boundary of each sarcomere.

  • M-line: Center of the sarcomere where myosin filaments are anchored.

Sarcomere Structure

  • Arrangement includes:

  • A-band: Overlap of actin and myosin.

  • I-band: Only actin, between A-bands.

  • H-zone: Area in the A-band where only myosin is present.

Sliding Filament Theory of Muscle Contraction

  • Interaction between actin and myosin leads to the shortening of sarcomeres.

Cycle of Contraction

  • Rigor: Myosin binds to actin.

  • Cross-bridge: Myosin head pivots, pulling actin.

  • Power Stroke: Myosin returns to original position, dragging actin with it repeat.

  • Rigor Mortis: Stiffness post-death due to lack of ATP.

Regulation of Muscle Contraction

  • Role of ATP:

  • Powers cross-bridge cycling.

  • Breaks bonds between actin and myosin.

  • Role of Calcium (Ca2+):

  • Released from sarcoplasmic reticulum.

  • Binds to troponin, causing tropomyosin to shift and expose actin binding sites.

Neuromuscular Junction (NMJ)

  • Connects motor neuron to muscle fiber.

  • Acetylcholine (ACh): Neurotransmitter released at NMJ, triggers muscle contraction.

  • Resting potential of the sarcolemma is around -80 to -90 mV.

Excitation-Contraction Coupling

  1. Action potential generated at NMJ.

  2. Propagated along sarcolemma and down T-tubules.

  3. Dihydropyridine receptors (DHPR) and Ryanodine receptors (RyR) respond to depolarization, facilitating Ca2+ release.

  4. Ca2+ floods into cytoplasm, initiating contraction.

Muscle Fiber Types

Types of Twitch Fibers:

  1. Tonic Fibers: Slow, sustained contractions; common in postural muscles of reptiles/amphibians.

  2. Skeletal Muscle Fiber Types:

  • Slow Oxidative (Type I): Red, aerobic.

  • Fast Oxidative Glycolytic (Type IIa): Red/pink, both aerobic & anaerobic.

  • Fast Glycolytic (Type IIb): White, anaerobic.

Characteristics of Muscle Fiber Types

  • Mitochondria Density:

  • SO: High

  • FOG: Intermediate

  • FG: Low

  • Capillary Density:

  • SO: High

  • FOG: Intermediate

  • FG: Low

  • Fibers Diameter:

  • SO: Small

  • FOG: Intermediate

  • FG: Large

  • Myoglobin Content:

  • SO: High

  • FOG: Intermediate

  • FG: Low

  • Glycolytic Enzymes:

  • SO: Intermediate

  • FOG: High

  • FG: High

  • Myosin ATPase Activity:

  • SO: Slow

  • FOG: Fast

  • FG: Fast

Summary

  • Muscle contraction is a complex interaction of proteins (actin & myosin) regulated by calcium and ATP.

  • Understanding these components helps in grasping how various muscle types function under different physiological conditions.