Lecture 1 ExPhys

Skeletal Muscle

Agenda

  • Overview of skeletal muscle anatomy and related functions.

  • Excitation- contraction coupling mechanics.

  • Types of muscle fibers and their recruitment during contraction.

Objectives

  1. Identify the anatomical components of skeletal muscles.

  2. Explain the relationship between the nervous system and muscular system in muscle contraction.

  3. Describe excitation-contraction coupling and the sliding filament theory relative to muscle contraction and relaxation.

  4. Differentiate types of muscle fibers and their recruitment.

  5. Explain the mechanisms of muscle contraction.

Muscle Anatomy

Types of Muscle Tissue

  • Skeletal Muscle: Voluntary, striated, multi-nucleated.

  • Smooth Muscle: Involuntary, non-striated.

  • Cardiac Muscle: Involuntary, striated, mono-nucleated.

Connective Tissue

  • Tendon: Connective tissue that attaches muscle to bone.

Structure of a Skeletal Muscle

  • Epimysium: Outer layer that surrounds the entire muscle.

  • Perimysium: Connective tissue that surrounds each fascicle (a bundle of muscle fibers).

  • Endomysium: Connective tissue that surrounds each individual muscle fiber.

  • Muscle fiber: The individual muscle cell that contracts.

  • Blood vessels: Supply oxygen and nutrients to muscle fibers.

Muscle Fiber Anatomy Terms

  • Plasmalemma: Plasma membrane surrounding each muscle fiber.

  • Sarcoplasm: Gel-like substance filling spaces between myofibrils, analogous to cytoplasm in muscle cells.

  • Transverse Tubules: Extensions of the plasmalemma that carry nerve impulses deep into the muscle fiber.

  • Sarcoplasmic Reticulum: Calcium storage site crucial for muscle contraction.

  • Myofibrils: Long, thread-like structures made of muscle protein, connected to sarcomeres.

  • Sarcomere: The basic functional unit of a myofibril/muscle composed of actin and myosin filaments.

The Sarcomere

  • Thick Filaments: Primarily composed of myosin with heads that create cross-bridges.

  • Thin Filaments: Composed of actin, tropomyosin, and troponin.   

  • - Actin: Main structural component of thin filaments.   

  • - Tropomyosin: A protein that blocks the binding sites on actin.   

  • - Troponin: A protein that binds calcium ions and causes tropomyosin to shift, exposing binding sites on actin.

  • Titin: A structural protein that acts like an internal spring, allowing muscle to stretch and produce force when stretched.

Muscle Fiber Contraction

Role of the CNS and Neurons

  • Motor Unit: A single alpha motor neuron and all the muscle fibers it innervates.

  • Communication between the nervous system and muscular system causes muscle contraction.

  • Membrane potential varies from resting state (-70 mV) to threshold (-55 mV) and overshoot (+30 mV) during action potential.

Excitation-Contraction Coupling

  • A complex sequence of events begins with excitation of the motor nerve and culminates in muscle contraction.

Sliding Filament Theory

  • Explains how muscle shortening occurs during contraction requiring ATP for the interaction between actin and myosin.

Muscle Relaxation

  • Calcium ions are pumped back into the sarcoplasmic reticulum when contraction ceases, requiring ATP.

  • Troponin and tropomyosin return to their resting states blocking binding sites on actin.

Proprioceptors: Golgi Tendon Organs vs Muscle Spindles

  • Golgi Tendon Organs: Signal muscles to relax when excessive tension is detected.

  • Muscle Spindles: Signal muscles to contract when excessive stretch is detected.

Muscle Fiber Types

Classification of Muscle Fiber Types

  • Type I Fibers (Slow-Twitch): Takes approximately 110 ms to reach peak tension; high oxidative capacity, low glycolytic capacity.

  • Type II Fibers (Fast-Twitch): Takes about 50 ms to reach peak tension; divided into:   - Type IIa: Fast oxidative/glycolytic.   - Type IIx: Fast glycolytic.   - Characteristics include:     - High fatigue resistance for Type I.     - Lower fatigue resistance for Type II.

Individual Variability in Muscle Fiber Composition

  • Muscle fiber type distribution can vary per individual, with similarities in arm and leg muscles.

  • The soleus muscle typically has a higher amount of Type I fibers.

Effects of Exercise on Muscle Fibers

  • Type I Fibers: High aerobic endurance, important for low-intensity activities.

  • Type II Fibers: Better for anaerobic activities and quick energy demands, less endurance.

Distribution of Muscle Fiber Types

  • Muscle fiber proportions vary between individuals and muscles; genetics influence distribution, training can induce minor shifts in composition.

Muscle Fiber Recruitment and Contraction

Recruitment Mechanism

  • More muscle fibers stimulated = greater force produced.

  • Recruitment depends on required force: fewer motor units activated for small tasks, more for greater force demands.

  • Size Principle: Smaller motor units (Type I) are recruited before larger ones (Type II).

Exercise Influence on Recruitment

  • Type and intensity of exercise dictate which type of muscle fibers are primarily recruited.

Muscle Contraction Types

  • Concentric: Muscle shortens while contracting.   - Example: Elbow flexion using biceps brachii.

  • Isometric: Muscle generates force without changing length.   - Example: Lifting an object heavier than force my muscles can produce.

  • Eccentric: Muscle exerts force while lengthening.   - Example: Slowly lowering a heavy weight using biceps brachii.

Factors Influencing Force Generation

  1. Motor Units and Muscle Size: More motor units = more force. Type II fibers contribute more to force production.

  2. Frequency of Stimulation: Determined by the frequency of action potentials leading to increased force output: (i) Twitch, (ii) Summation, (iii) Tetanus.

  3. Muscle Fiber Length: Optimal sarcomere length for maximum force production depends on the degree of overlap between thick and thin filaments.

  4. Speed of Contraction: The relationship between contraction speed and force generation influences an exercise’s execution direction.

Force-Velocity Relation

  • In concentric contractions, increasing load causes a decrease in speed to maintain force.

  • In isometric contractions, applied load equals maximal isometric force, leading to zero contraction velocity.

Muscle Memory

  • Muscle fibers adapt to training, increasing in size and number of nuclei. The rebound speed in trained individuals following a period of inactivity can be attributed to muscle memory derived from neural adaptations.