Chapter 9C Muscles and Fibers

Factors Affecting Force of Contraction

  • High Frequency of Stimulation (Temporal Summation)

    • More cross bridges attached due to increased stimulation rate.

    • Includes large muscle fibers and tetanus scenarios.

  • Number of Muscle Fibers Recruited

    • Groups of motor units activated leads to an increased force.

  • Muscle and Sarcomere Stretch

    • Optimal length for maximum tension is slightly over 100% of resting length.

  • Contractile Force

    • Increased number of cross bridges leads to a greater force of contraction.

Force of Muscle Contraction

  • General Overview

    • The force of contraction is influenced by:

    1. Frequency of Stimulation

      • Higher frequency induces greater force due to stimuli summation.

    2. Number of Muscle Fibers Stimulated (Recruitment)

      • More motor units engaged results in increased force.

    3. Fiber Size

      • Larger fibers can generate more tension.

      • Hypertrophy (muscle cells increase in size) occurs with regular exercise.

        • Increases in length through adding more sarcomeres sequentially.

        • Increases in diameter by:

        • Increasing myofibril diameter by adding actin and myosin.

        • Splitting existing myofibrils which run parallel to existing fibers.

    4. Degree of Muscle Stretch

      • Optimal sarcomere length ranges from 80-120% of normal resting length for maximum force generation.

      • Below 80%: filaments overlap too much, reducing force.

      • Above 120%: essential filaments overlap less, also leading to decreased force.

Length-Tension Relationships of Sarcomeres in Skeletal Muscles

  • Description of optimal sarcomere lengths for effective contraction and force generation.

Velocity and Duration of Contraction

  • Factors affecting contraction speed and duration:

    • Muscle Fiber Type

    • Load

    • Recruitment

Muscle Fiber Type Characteristics

  • Speed of Contraction:

    • Determined by:

    • Catalytic Rate of Myosin ATPases

    • Pattern of Electrical Activity of Motor Neurons

  • Pathways for ATP regeneration:

    • Oxidative Fibers: Utilize aerobic pathways.

    • Glycolytic Fibers: Engage anaerobic glycolysis.

Classification of Muscle Fibers

  • Three Types of Muscle Fibers:

    • Slow Oxidative (SO)

    • Characteristics:

      • Slow contraction speed.

      • High myoglobin content.

      • Resistant to fatigue.

      • Suitable for endurance-type activities (e.g., maintaining posture).

    • Fast Oxidative (FO)

    • Characteristics:

      • Intermediate contraction speed.

      • Moderate resistance to fatigue.

      • Suitable for medium-intensity activities.

    • Fast Glycolytic (FG)

    • Characteristics:

      • Fast contraction speed.

      • Low myoglobin content and high glycogen stores.

      • Fatigue quickly, suited for short, intense movements (e.g., sprinting, hitting a baseball).

Velocity vs Duration of Contraction

  • Light Load: Increases contractile velocity and duration.

  • Heavy Load: Decreases velocity and duration.

Adaptation to Exercise

  • Aerobic (Endurance) Exercise:

    • Leads to increases in:

    • Capillarization.

    • Number of mitochondria and myoglobin synthesis.

    • Enhanced endurance and fatigue resistance.

    • Functional conversion of FG fibers to FO fibers.

  • Resistance Exercise:

    • Promotes:

    • Muscle hypertrophy due to increases in fiber size, myofibrils, myofilaments.

    • Increased strength and size.

    • Functional shifts from FO to FG fibers.

Clinical Significance: Homeostatic Imbalance

  • Disuse Atrophy:

    • Muscle degeneration and loss of mass due to immobilization or lack of neural stimulation.

    • Noticeable loss within 24 hours.

    • Strength reduction of approximately 5% per day, and paralyzed muscles may reduce to a quarter of original size.

Smooth Muscle Overview

  • Location: Found in walls of most hollow organs (excluding the heart).

    • Organs include respiratory, digestive, urinary, reproductive, and circulatory systems.

  • Organization: Usually organized into sheets of densely packed fibers, often in two layers:

    • Longitudinal Layer: Fibers oriented parallel to the organ’s long axis.

    • Circular Layer: Fibers wrap around the organ's circumference.

Differences between Smooth and Skeletal Muscle Fibers

  • Morphology:

    • Smooth muscle fibers are spindle-shaped, less developed SR, only one nucleus (centered).

    • No striations or T tubules; contains varicosities instead of neuromuscular junctions for neurotransmitter release.

  • Innervation:

    • Smooth muscle fibers receive autonomic nervous system (ANS) inputs; lack outer sheaths found in skeletal muscle.

Calcium Sources for Smooth Muscle Contraction

  • Smooth muscle contraction relies on Ca²+ from both sarcoplasmic reticulum and extracellular fluids.

  • Mechanism: Ca²+ binds to calmodulin instead of troponin, activating myosin light chain kinase.

Special Features of Smooth Muscle Contraction

  • Stress-Relaxation Response: Predominant in smooth muscle adaptability, allowing it to adjust to new lengths while maintaining contractility.

Types of Smooth Muscle

  • Unitary Smooth Muscle: Commonly found in hollow organs, characterized by rhythmic contractions.

  • Multiunit Smooth Muscle: Rare gap junctions, acts more like skeletal muscle in response to neural stimuli via motor units.

Developmental Aspects of Muscle

  • Development from embryonic myoblasts; multinucleation and ACh receptor distribution are critical during muscular formation.

  • Regeneration Differences:

    • Skeletal: Limited regeneration; cardiac tissue replaced by connective tissue; smooth muscle maintains regenerative ability throughout life.

  • Muscular Development: Starts head to toe and proximal to distal, with peak neuromuscular control occurring in adolescence. Aging impacts muscle fibers and connective tissue, leading to sarcopenia; exercise counters these effects.