Bio chap. 10

Chapter 10: Muscular Tissue

Overview

• Focuses on three primary types of muscular tissue: Skeletal, Cardiac, and Smooth.

• Examines properties, structure, function, and metabolism of muscle tissues.

Properties of Muscular Tissue

• Excitability: Ability to respond to stimuli.

• Contractility: Capacity to contract forcefully when stimulated.

• Extensibility: Ability to stretch without being damaged.

• Elasticity: Ability to return to original length after stretching.

Functions of Muscular Tissue

• Producing Body Movements: Involves both voluntary (skeletal muscle) and involuntary (cardiac and smooth muscle) movements.

• Stabilizing Body Positions: Maintains posture.

• Moving Substances Within the Body: Includes blood circulation (heart) and digestion (smooth muscle in digestive tract).

• Generating Heat: Muscle contraction generates heat; shivering increases heat production.

Types of Muscular Tissue

Skeletal Muscle Tissue

• Appearance: Striated, multi-nucleated, fibers are parallel.

• Control: Voluntary; consciously controlled.

  • Example: Diaphragm operates subconsciously.

Cardiac Muscle Tissue

• Location: Found in walls of the heart.

• Appearance: Striated, one central nucleus.

• Control: Involuntary; contraction initiated by a pacemaker node.

Smooth Muscle Tissue

• Location: Found in walls of hollow organs (e.g., blood vessels, digestive tract).

• Appearance: No striations, one central nucleus.

• Control: Involuntary.

Structure of Skeletal Muscle Tissue

Connective Tissue Components

• Endomysium: Encloses individual muscle fibers.

• Perimysium: Encloses fascicles (bundles of muscle fibers).

• Epimysium: Outer layer encircling the whole muscle.

• Fascia: Holds muscles and allows movement.

• Tendon: Connects muscle to bone.

• Aponeurosis: Broad and flattened tendon.

Microscopic Anatomy and Function

• Muscle Fibers: Number is set before birth, and growth occurs via hypertrophy (enlargement of existing fibers).

• Sarcolemma: Plasma membrane of muscle cells.

• Myofibrils: Contractile organelles within skeletal muscle fibers containing thin (actin) and thick (myosin) filaments.

Sarcomere Structure

• The basic functional unit of myofibrils, extends from one Z disc to another.

• A band: Thick myosin filaments; overlaps with thin filaments at the ends.

• I band: Light area containing only thin filaments.

• H zone: Center of A band containing only thick filaments.

• M line: Supporting proteins in the center of the H zone.

Muscle Proteins

Types of Proteins in Myofibrils

• Contractile Proteins: Myosin and actin generate force.

• Regulatory Proteins: Troponin and tropomyosin, switch contraction on and off.

• Structural Proteins: Titin and dystrophin, maintain alignment.

Contraction Mechanism

The Sliding-Filament Mechanism

• Muscle contraction results from the sliding of thin filaments past thick filaments, shortening the sarcomere without changing filament lengths.

• Excitation-contraction coupling links action potentials in muscle fibers to mechanical contraction.

• Calcium ions play a crucial role in exposing binding sites on actin for myosin.

Excitation-Contraction Coupling Steps

1. Release of Calcium Ions: Action potential triggers SR to release Ca++.

2. Troponin Interaction: Ca++ binds to troponin, moving tropomyosin.

3. Cross-Bridge Formation: Myosin binds to exposed actin sites, initiating contraction.

Muscle Action Potential and Relaxation

• ACh is released at the neuromuscular junction, creating action potential.

• Ca++ reuptake into SR leads to muscle relaxation.

Muscle Metabolism

ATP Production

• ATP is necessary for muscle contraction, calcium ion storage, and cellular repair.

• Creatine phosphate, anaerobic respiration, and aerobic respiration are pathways for ATP production.

Muscle Fatigue

• Caused by inadequate calcium release, depletion of energy resourced, and buildup of metabolic byproducts (lactic acid, ADP).

Control of Muscle Tension

Motor Units

• Consist of a single motor neuron and muscle fibers it innervates.

• Strength of contraction correlates with motor unit size and activation.

Types of Contractions

1. Twitch Contraction: A brief contraction in response to a single action potential.

2. Isotonic Contraction: Muscle changes length without change in tension.

3. Isometric Contraction: Muscle remains the same length despite tension.

Types of Skeletal Muscle Fibers

1. Slow Oxidative Fibers (SOF): Endurance, high myoglobin; resist fatigue.

2. Fast Oxidative-Glycolytic Fibers (FOG): Moderate speed; used in activities like walking.

3. Fast Glycolytic Fibers (FG): Quick bursts of energy; fatigue quickly.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle

• Unique intercalated discs facilitate spread of action potentials.

• Dependent on aerobic metabolism; can use lactic acid for energy.

Smooth Muscle

• Involved in involuntary functions; operates via gap junctions.

• Can maintain prolonged contraction and respond to various stimuli (neurotransmitters, stretching).

Regeneration of Muscle Tissue

• Hyperplasia: Rare; more common in smooth muscle (e.g., uterus).

• Hypertrophy: Common in skeletal muscles; results from increased workload.

• Aging affects muscle mass, strength, reflexes, and flexibility.

Summary

• Understanding of muscular tissue, its properties, functions, contraction mechanism, and metabolism is crucial for grasping human physiology and physical performance.