Lecture Notes: Muscle Tissue

Introduction

• Learning Outcomes:
  • Describe the development of muscle tissue.
  • Describe the structure, location, and function of skeletal, cardiac, and smooth muscle tissue.
  • Compare and contrast the microscopic characteristics of skeletal, cardiac, and smooth muscle.

Development of Muscle Tissue

• Muscle tissue develops from myoblasts (muscle-forming stem cells) from embryonic mesoderm.
• Myoblasts develop into three types of muscle tissue:
  • Smooth Muscle
  • Cardiac Muscle
  • Skeletal Muscle
• Cell Nuclei:
  • Uninucleate cells (cardiac and smooth) originate from myoblasts without fusion.
  • Multinucleated skeletal muscle fibers form from fused myoblasts.

Types of Muscle Tissue

Smooth Muscle Tissue

• Structure: Fusiform, short, nonstriated, one central nucleus.
• Function: Involuntary movements, moves materials through organs.
• Location: Walls of hollow organs, vessels, airways, stomach, bladder, uterus.

Cardiac Muscle Tissue

• Structure: Short, bifurcated, striated, one or two central nuclei, intercalated discs.
• Function: Involuntary contractions, pumps blood in the heart.
• Location: Heart wall (myocardium).

Skeletal Muscle Tissue

• Structure: Long, cylindrical, parallel, unbranched, multinucleated.
• Function: Voluntary movement, locomotion, heat production.
• Location: Attached to bones and skin, facial muscles, voluntary sphincters.

Microscopic Characteristics

1. Skeletal Muscle: Striated appearance due to myofilaments.
2. Cardiac Muscle: Intercalated disks hold cells together during contractions.
3. Smooth Muscle: Uninucleated, nonstriated, smooth appearance, tapered ends.

Functions of Muscles

1. Producing body movements.
2. Stabilizing body positions.
3. Storing and moving substances within the body.
4. Generating heat.

Skeletal Muscle Structure

• Fascia: Superficial connective tissue anchoring muscles.
• Fascicles: Bundles of muscle fibers covered by perimysium.
• Muscle Fiber: Covered by endomysium; striated appearance with sarcolemma and sarcoplasm.

Connective Tissue Layers

• Epimysium: Outermost layer, allows powerful muscle movement.
• Perimysium: Separates fascicles.
• Endomysium: Surrounds muscle fibers, supplies nutrients.

Anatomy of Skeletal Muscle

• Cell Parts:
  • Sarcolemma: Plasma membrane.
  • T Tubules: Extensions into muscle fiber.
  • Sarcoplasmic Reticulum: Smooth ER, stores Ca+.
  • Myofibrils: Contractile organelles with proteins.

Proteins

• Contractile: Myosin and actin.
• Regulatory: Troponin and tropomyosin.
• Structural: Align filaments and attach myofibrils to the sarcolemma.

Sarcomere

• Structure: Functional unit from one Z-line to the next.
• Bands and Zones: A Band, I Band, Z Discs, H Zone, M Line.

Muscle Fiber Contraction & Relaxation

• Resting Membrane Potential: Difference in ion concentration across membrane.
• Action Potentials: Arise at neuromuscular junction, result in calcium release.

Muscle Contraction: Sliding Filament Mechanism

• Contraction Cycle:
  1. ATP hydrolysis.
  2. Cross-Bridge formation.
  3. Power stroke.
  4. Detachment from actin.
• Relaxation: Ca++ ions pumped back to SR, tropomyosin blocks binding sites.

Muscle Metabolism and Energy

• ATP Production: Creatine phosphate, anaerobic glycolysis, aerobic respiration.
• Muscle Metabolism: ATP storage and regeneration, effects of exercise.

Control of Muscle Tension

• Twitch Contraction: Brief contraction in response to action potential.
• Isotonic & Isometric Contractions: Muscle length changes or remains constant.
• Motor Unit: Composed of motor neuron and muscle fibers it innervates.

Muscle Fiber Types

• Slow Oxidative Fibers (SO): Endurance, aerobic, high fatigue resistance.
• Fast Glycolytic (FG): Rapid, powerful contractions, fatigue quickly.
• Fast Oxidative (FO): Intermediate, aerobic, more fatigue-resistant than FG.

Muscle Size and Regeneration

• Hypertrophy: Increase in fiber size, more filaments.
• Atrophy: Decrease in muscle mass, loss of structural proteins.

Exercise & Muscle Fatigue

• Aging: Sarcopenia, decrease in muscle strength and motor unit size.

These notes cover the key aspects of muscle tissue development, structure, function, and mechanics, providing a foundational understanding for further study of muscular systems.