Anatomy & Physiology: Muscle Tissue

Chapter 10: Muscle Tissue Overview

Introduction to Muscle Tissue

  • Muscle tissue is a primary tissue type divided into:
    • Skeletal muscle tissue
    • Cardiac muscle tissue
    • Smooth muscle tissue

Functions of Skeletal Muscle

  • Movement of the body
    • Moves bones, facilitates facial expressions, enables speaking, breathing, and swallowing.
  • Maintenance of posture
    • Stabilizes joints and maintains body position.
  • Protection and support
    • Packages internal organs and retains their positions.
  • Regulation of elimination of materials
    • Circular sphincters control the passage of materials at orifices.
  • Production of heat
    • Helps maintain body temperature.

Characteristics of Skeletal Muscle Tissue

  • Excitability
    • Ability to respond to stimuli by changing electrical membrane potential.
  • Conductivity
    • Involves sending an electrical change along the length of the cell membrane.
  • Contractility
    • Exhibited when filaments slide past each other; enables muscle movement.
  • Extensibility
    • Ability to be stretched.
  • Elasticity
    • Ability to return to original length after being lengthened or shortened.

Anatomy of Skeletal Muscle

Gross Anatomy of Skeletal Muscle

  • Hierarchy of muscle structure
    • Muscle → Fascicle → Muscle Fiber (cell) → Myofibril → Thick and Thin Filaments.
  • Each skeletal muscle is classified as an organ, consisting of:
    • Skeletal muscle fibers
    • Connective tissue
    • Blood vessels
    • Nerves
  • Muscle fibers are bundled within fascicles:
    • A whole muscle contains many fascicles.
    • A fascicle consists of numerous muscle fibers.
    • A muscle fiber is a muscle cell.

Connective Tissue Components

  • Muscles have three layers of connective tissue:
    • Epimysium
    • Dense irregular connective tissue wrapping the entire muscle.
    • Perimysium
    • Dense irregular connective tissue surrounding a fascicle; houses many blood vessels and nerves.
    • Endomysium
    • Areolar connective tissue wrapping individual muscle fibers; provides electrical insulation and supports capillaries.

Muscle Attachments

  • At the ends of muscles, epimysium, perimysium, and endomysium merge to form:
    • Tendons (bundle) or aponeurosis (sheet) to attach skeletal muscle to bone matrix.

Vascularization and Innervation

  • Skeletal muscle tissue contains extensive blood vessels, providing:
    • Oxygen and nutrients and removing waste.
  • The tissue is innervated by somatic motor neurons:
    • Axons of neurons branch and terminate at neuromuscular junctions;
    • It is classified as voluntary muscle because contractions are under voluntary control.

Microscopic Anatomy of Skeletal Muscle

Structure of Muscle Cells (Fibers)

  • Sarcoplasm
    • Cytoplasm containing organelles, contractile proteins, and several nuclei (muscle cells are multinucleated due to fusion of myoblasts).
  • Sarcolemma
    • Plasma membrane with voltage-gated ion channels and T-tubules that extend into the cell, containing voltage-sensitive calcium channels.
  • Myofibrils
    • Bundles of myofilaments enclosed in sarcoplasmic reticulum.
  • Sarcoplasmic Reticulum
    • Internal membrane complex similar to smooth ER that contains terminal cisternae, which serve as calcium reservoirs.

Myofilaments

  • Thick filaments
    • Comprised of many myosin protein molecules with myosin heads pointing towards the ends of filament.
  • Thin filaments
    • Twisted strands of actin, with G-actin monomers having myosin binding sites.
    • Contains regulatory proteins: troponin and tropomyosin.

Organization of Sarcomeres

  • Sarcomeres are repeating units of myofilaments
    • Composed of overlapping thick and thin filaments, giving rise to:
    • I bands (light regions containing only thin filaments, bisected by Z disc).
    • A bands (dark regions containing thick filaments and overlapping thin filaments, including the H zone and M line).

Other Structural and Functional Proteins

  • Connectin
    • Extends from Z disc to M line, stabilizes thick filaments.
  • Dystrophin
    • Anchors some myofibrils to sarcolemma; abnormalities lead to muscular dystrophy

Clinical View: Muscular Dystrophy

  • A hereditary disease causing skeletal muscle degeneration, with Duchenne muscular dystrophy (DMD) being the most common form.
  • In DMD, defects in dystrophin lead to:
    • Damage to the sarcolemma during contraction, resulting in an influx of calcium, causing damage.
    • Symptoms appear in early childhood, including muscle atrophy, walking difficulties, and postural issues; it's incurable, with patients rarely living beyond age 30.

Energy Production and Muscle Fibers

  • Muscle fibers contain abundant mitochondria for aerobic ATP production.
  • Myoglobin allows storage of oxygen for ATP production; Glycogen provides quick energy.
  • Creatine phosphate replenishes ATP supply quickly.

Innervation of Skeletal Muscle Fibers

  • Motor Unit:
    • A motor neuron and all muscle fibers it controls. The number of fibers controlled varies:
    • Small motor units (<5 fibers) allow for precise control.
    • Large motor units (thousands of fibers) generate significant force but offer minimal precision.

Neuromuscular Junction (NMJ)

  • Contains synaptic knob, synaptic cleft, and motor end plate:
    • Synaptic knob: Houses synaptic vesicles with ACh; has calcium gradient maintenance through pumps.
    • Motor end plate: Specialized sarcolemma region with numerous ACh receptors allowing sodium entry and potassium exit.
    • Synaptic cleft: Fluid-filled gap containing acetylcholinesterase to break down ACh.

Muscle Fiber Actions and Contractions

Muscle Fibers at Rest

  • Exhibit resting membrane potential (RMP) around -90 mV, established by leak channels and Na+/K+ pumps.

Muscle Contraction Mechanism Overview

  • Triggered by excitatory action potential leading to muscle excitation.
  • Excitation-Contraction Coupling describes the processes linking the stimulation at the NMJ to the contraction mechanics within sarcomeres.

Crossbridge Cycling in Sarcomeres

  • Occurs via:
    1. Myosin head attaching to actin's exposed binding site.
    2. Power stroke that pulls thin filaments toward the center of the sarcomere.
    3. Binding of ATP to the myosin head for release.
    4. Hydrolysis of ATP to regain cocked position.
    • Continues as long as calcium and ATP are available.

Muscle Relaxation

  • Events leading to muscle relaxation include:
    • Termination of nerve signal and Ach release.
    • Hydrolysis of ACh stopping further action potentials.
    • Closure of calcium channels returning calcium to the sarcoplasmic reticulum.
    • Restoring troponin and tropomyosin to block actin's binding site.

Muscle Fiber Energy Sources

Methods for Generating ATP

  1. Creatine Phosphate: Transfers phosphate to ADP for ATP generation for 10-15 seconds.
  2. Glycolysis: Converts glucose to pyruvate without oxygen, yielding 2 ATP per glucose.
  3. Aerobic Cellular Respiration: Requires oxygen, produces a net of 30 ATP.

Oxygen Debt

  • Refers to the additional oxygen needed post-exercise to restore conditions, covering replenishment of ATP, glycogen, and converting lactic acid back to glucose.

Classification of Skeletal Muscle Fiber Types

  • Based on contraction types and ATP supply methods:
    • Fast-twitch fibers: Powerful and quick, but brief contractions.
    • Slow-twitch fibers: Endurance-oriented and fatigue-resistant, advantageous for continuous activities.

Muscle Tension and Contraction Types

  • Muscle Twitch: Brief contraction response to a single stimulus.
    • Comprises:
    • Latent period: No tension change.
    • Contraction period: Tension increases as power strokes happen.
    • Relaxation period: Tension decreases with crossbridge releases.

Length-Tension Relationship

  • The tension produced by muscle is affected by its length; optimal overlap of myofilaments yields maximum force.
  • Shortened or extended lengths reduce force production capabilities.

Muscle Fatigue

  • Defined as reduced tension capability, primarily due to decreased glycogen during prolonged exercise, but affected by neuromuscular junction signaling, ion concentrations, and crossbridge cycling efficiency.

Effects of Exercise on Muscle

  • Regular exercise induces muscle adaptations:
    • Hypertrophy from resistance training increases muscle protein synthesis and glycogen reserves.
    • Atrophy from disuse reduces muscle size and is sometimes permanent.

Effects of Aging on Muscle

  • Declines in muscle mass start in mid-30s, affecting size, power, and endurance, alongside increases in dense connective tissue.

Clinical View: Anabolic Steroids

  • Anabolic steroids mimic testosterone, enhancing muscle protein synthesis but with numerous side effects such as increased cardiovascular disease risk and hormonal changes.