Muscle Tissue: Structure, Function, and Physiology (Chapter 10 Study Notes)

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Muscle Tissue Overview

Introduction

Muscle tissue is a specialized tissue found throughout the human body, responsible for producing movement, maintaining posture, and generating heat. There are three main types of muscle tissue: skeletal, cardiac, and smooth, each with distinct structural and functional characteristics.

Types of Muscular Tissue

Skeletal Muscle Tissue

Striated: Exhibits alternating light and dark protein bands (striations).
Location: Attached to bones of the skeleton.
Function: Moves bones and enables voluntary body movements.
Control: Voluntary via the nervous system; some subconscious control (e.g., diaphragm).
Appearance: Multi-nucleated and striated.

Cardiac Muscle Tissue

Striated: Contains striations similar to skeletal muscle.
Location: Found only in the heart.
Function: Responsible for contraction and relaxation of the heart, pumping blood.
Control: Involuntary; regulated by the autonomic nervous system and hormones.
Special Feature: Autorhythmicity—natural pacemaker initiates contractions.
Appearance: One nucleus per cell, striated, with intercalated discs.

Smooth Muscle Tissue

Non-striated: Lacks striations; appears smooth under the microscope.
Location: Walls of hollow internal structures (blood vessels, airways, digestive tract, most organs).
Function: Propels substances (e.g., food, blood) through internal passageways.
Control: Involuntary; regulated by hormones and neurotransmitters.
Special Feature: Some smooth muscle exhibits autorhythmicity (e.g., digestive canal).
Appearance: One nucleus per cell, no striations.

Type	Location	Function	Appearance	Control
Skeletal	Skeletal	Move bones	Multi-nucleated, striated	Voluntary
Cardiac	Heart	Pump blood	One nucleus, striated, intercalated discs	Involuntary
Visceral (Smooth)	Various organs (e.g., GI tract)	Various functions (e.g., peristalsis)	One nucleus, no striations	Involuntary

Functions of Muscular Tissue

Major Functions

Producing body movements: Muscles contract to produce voluntary and involuntary movements.
Stabilizing body positions: Continual contractions maintain posture (e.g., neck muscles holding the head upright).
Storing and mobilizing substances within the body:
- Sphincters: Bands of smooth muscle that regulate outflow of contents in organs.
- Cardiac muscle: Pumps blood through blood vessels.
- Smooth muscle: Moves food and substances through digestive canals.
Generating heat:
- Thermogenesis: Heat production through muscle contractions helps maintain body temperature.
- Shivering: Involuntary contractions of skeletal muscles to increase heat production.

Properties of Muscular Tissue

Key Properties

Electrical excitability: Ability to respond to stimuli by producing electrical signals called action potentials (stimulated by electrical or chemical signals).
Contractility: Ability to contract forcefully when stimulated, resulting in movement.
Conductivity: Ability to propagate electrical changes along the plasma membrane.
Extensibility: Ability to stretch without being damaged (e.g., smooth muscle stretching when filled with food).
Elasticity: Ability to return to original length and shape after contraction or extension.

Connective Tissue Components

Muscle Protection and Organization

Connective tissue surrounds and protects muscles.
Subcutaneous tissue: Separates muscle from skin, provides pathways for nerves, blood vessels, and lymphatics; includes adipose tissue to reduce heat loss.
Fascia: Dense sheet of irregular connective tissue that supports and surrounds muscles and organs, holds muscles with similar functions together, and allows free movement.

Layers of Connective Tissue

Epimysium: Outer layer, dense irregular connective tissue encircling the entire muscle.
Perimysium: Dense irregular connective tissue surrounding groups of 10 to 100 muscle fibers, forming bundles called muscle fascicles.
Endomysium: Penetrates the interior of each muscle fascicle, separating individual muscle fibers.

Connective tissue can extend beyond muscle fibers to form tendons that attach muscle to bone.

Skeletal Muscle Tissue Structure

Nerve and Blood Supply

Somatic motor neurons stimulate skeletal muscles to contract; axons branch onto muscle fibers.
Blood capillaries supply muscles with oxygen and nutrients, and remove waste and heat.

Skeletal Muscle Fiber Anatomy

Cellular Structure

Each muscle fiber has hundred or more nuclei due to fusion of myoblasts during development.
Sarcolemma: Plasma membrane of muscle fiber.
T-tubules: Tube-shaped tunnels extending from the membrane, filled with interstitial fluid, allowing rapid transmission of muscle action potentials.

Sarcoplasmic Reticulum

Membranous sacs that store calcium ions critical for muscle contractions.
Calcium ions are released from terminal cisterns (enlarged ends).
Triad: Terminal cisterns sandwich a T-tubule from both sides.

Sarcoplasm and Myofibrils

Sarcoplasm: Cytoplasm of muscle fiber, contains glycogen (for ATP synthesis), myoglobin (binds oxygen), and mitochondria.
Myofibrils: Contractile organelles extending the entire length of the muscle fiber, giving striated appearance.

Filaments and Sarcomere

Thin filaments: Composed of actin protein.
Thick filaments: Composed of myosin protein.
Sarcomere: Basic functional unit of myofibril, contains both thin and thick filaments (2:1 ratio), separated by Z-disc proteins.

Bands and Lines of Sarcomeres

A band: Entire length of thick filaments (dark part).
I band: Region with only thin filaments (light part).
H band: Region with only thick filaments.
M line: Middle of the sarcomere, contains proteins that hold thick filaments together.

Muscle Proteins

Contractile Proteins

Myosin: Main component of thick filament; motor protein that converts chemical energy in ATP to mechanical energy of motion.
Actin: Main component of thin filament; contains myosin-binding sites for cross-bridge formation.

Regulatory Proteins

Tropomyosin: Blocks myosin from binding to actin when muscle is relaxed.
Troponin: Binds to calcium ions, causing a conformational change that moves tropomyosin away from myosin-binding sites, allowing contraction.

Structural Proteins

Titin (Connectin): Connects Z disc to M line, stabilizes thick filament, provides elasticity and extensibility.
Dystrophin: Links thin filaments to integral membrane proteins of the sarcolemma, reinforcing the membrane and transmitting tension to tendons.

Duchenne Muscular Dystrophy

Genetic Muscle Disease

Inheritance: Sex-linked recessive.
Pathology: Lack of normal dystrophin leads to weak sarcolemma, muscle fiber damage, and replacement with fat tissue.
Symptoms: Progressive muscle weakness, especially in proximal limb muscles; onset between ages 2 and 12; wheelchair-bound by age 12; death by age 20 due to respiratory or cardiac failure.

The Sliding Filament Mechanism

Muscle Contraction Process

Myosin heads attach to thin filaments and pull them toward the M line, increasing overlap.
H-band and I-band disappear during contraction; filament length remains unchanged.

The Contraction Cycle

Steps of the Cycle

ATP Hydrolysis: at myosin heads, energizing them and reorienting to a "cocked" position.
Attachment of Myosin to Actin: Myosin head binds to actin, forming a cross-bridge; requires Ca2+ to move tropomyosin.
Power Stroke: Myosin head swivels, pulling thin filament; releases phosphate, changes angle, and draws Z disc closer.
Detachment: Another ATP binds to myosin head, causing detachment from actin; cycle repeats as long as ATP and Ca2+ are present.

Summary Equation:

Movement: Cross-bridge cycling shortens sarcomeres, whole muscle shortens, and tendons pull on bones to create movement.