8

Muscle Physiology

Overview of Muscle Types

  • Three Main Types of Muscle:

    • Skeletal Muscle

    • Smooth Muscle

    • Cardiac Muscle

Types of Skeletal Muscle Fibers

  • Classification Based on ATP Hydrolysis and Synthesis:

    • Slow-Oxidative (Type I) Fibers

    • Fast-Oxidative (Type IIa) Fibers

    • Fast-Glycolytic (Type IIx) Fibers

Slow-Twitch Oxidative Muscle Fibers

  • Characteristics:

    • Large amounts of red myoglobin

    • Numerous mitochondria

    • Extensive capillary blood supply

    • Smaller diameter

    • Darker color due to myoglobin

    • Fatigue-resistant

Fast-Twitch Glycolytic Muscle Fibers

  • Characteristics:

    • Larger diameter

    • Pale color

    • Easily fatigued

Comparative Table of Muscle Fiber Types

  • | Characteristics | Slow-Oxidative (Type I) | Fast-Oxidative (Type IIa) | Fast-Glycolytic (Type IIx) |
    -|--------------------------------|-------------------------|---------------------------|-----------------------------|
    -| Myosin-ATPase activity | Low | High | High |
    -| Speed of contraction | Slow | Fast | Fast |
    -| Resistance to fatigue | High | Intermediate | Low |
    -| Oxidative phosphorylation capacity| High | High | Low |
    -| Enzymes for anaerobic glycolysis| Low | Intermediate | High |
    -| Mitochondria | Many | Many | Few |
    -| Capillaries | Many | Many | Few |
    -| Myoglobin content | High | High | Low |
    -| Color of fiber | Red | Red | White |
    -| Glycogen content | Low | Intermediate | High |

Muscle Fiber Recruitment and Fatigue

  • Types of Fatigue:

    • Muscle Fatigue:

    • Occurs when an exercising muscle can no longer respond to stimulation with the same degree of contractile activity.

    • Central Fatigue:

    • Occurs when the central nervous system (CNS) no longer adequately activates motor neurons.

  • Post-exercise Oxygen Consumption:

    • Increased oxygen consumption is necessary to recover from exercise, leading to excess post-exercise oxygen consumption (EPOC).

Muscle Fiber Adaptations

  • Adaptations of Muscle Fibers:

    • Improvement in oxidative capacity

    • Muscle hypertrophy

    • Influence of testosterone

    • Interconversion between fast muscle types

    • Muscle atrophy

    • Limited repair of muscle fibers

Effects of Exercise Type on Muscle Fibers

  • Aerobic Exercise:

    • Leads to adaptations like increased energy demand and oxidative stress, promoting various cellular mechanisms such as angiogenesis and enhanced mitochondrial dynamics.

  • Resistance Exercise:

    • Triggers myogenic program leading to muscle protein synthesis through pathways involving:

    • PI3K

    • AKT

    • mTOR

    • p-70S6K

    • 4E-BP1

  • Key Factors in Muscle Fiber Adaptation:

    • VEGF (Vascular Endothelial Growth Factor)

    • IL-8 (Interleukin-8)

    • NRF (Nuclear Respiratory Factor)

    • ERR (Estrogen Related Receptor)

    • PGC-1α (Peroxisome proliferator-activated receptor gamma coactivator 1-alpha)

    • Mitochondrial fusion proteins: MFN1, MFN2, OPA1

    • Mitochondrial fission proteins: DRP1, FIS1

    • Myogenic factors: MGF (Mechano Growth Factor), HGF (Hepatocyte Growth Factor), Myogenin, Myf5 & Myf6

    • Satellite cell activation

  • Outcomes of Resistance Training:

    • Improved oxygen extraction

    • Submaximal energy efficiency

    • Myofiber hypertrophy

    • Myonuclear addition

Smooth Muscle Physiology

  • Characteristics:

    • Unstriated (lacks sarcomeres)

    • Autonomic (involuntary)

    • Requires calcium but lacks troponin

    • Phasic vs. Tonic contraction:

    • Phasic: Contracts in bursts

    • Tonic: Maintains tonus

    • Capable of contracting over a range of lengths

    • Exhibits slow, graded contractions

    • Dual innervation from sympathetic and parasympathetic

    • Contains pacemaker cells; electrical impulses spread through gap junctions

    • Responds to neurotransmitters and hormones

Smooth Muscle Contraction Mechanism

  • Structure of Smooth Muscle Cells:

    • Composed of dense bodies, thick and thin filaments

  • Contractile Unit Variations:

    • Relaxed: One contractile unit extending side to side

    • Contracted: Unit shows increased density due to filament interaction

Unique Properties of Smooth Muscle

  • Stress Relaxation Response:

    • Smooth muscle can relax when stretched, which is beneficial for maintaining function

  • Latch Phenomenon:

    • Allows smooth muscle to maintain tension efficiently with low energy expenditure

Cardiac Muscle Physiology

  • Characteristics:

    • Combines features of both skeletal and smooth muscle

  • Location:

    • Found exclusively in the heart

  • Structure:

    • Highly organized, with striated and branched fibers

    • Interconnected by gap junctions found in intercalated discs, facilitating communication between cells

  • Function:

    • Pumps blood out of the heart

    • Controlled by the autonomic nervous system