Muscle Tissue and Physiology: Chapter 10 - Structure and Function

Muscle Tissue and Physiology

Introduction to Muscle Tissue

Muscle tissue is essential for movement, posture, and various bodily functions. It transforms chemical energy (ATP) into directed mechanical energy, enabling force and motion. Nearly half of the body's mass is muscle tissue, which is classified into three main types based on structure and function.

• Muscle tissue: Specialized for contraction and movement.

• ATP: Adenosine triphosphate, the energy currency for muscle contraction.

• Ability to generate heat: Muscles contribute to thermoregulation.

Classification of Muscle Tissue

Muscle tissue is categorized into three types, each with distinct characteristics and functions.

Type	Location	Features	Control
Skeletal	Attached to bones and skin	Striated, elongated cells, rapid contraction	Voluntary (conscious control)
Cardiac	Heart	Striated, branched cells, intercalated discs	Involuntary (autonomic control)
Smooth	Walls of hollow organs (e.g., stomach, blood vessels)	Non-striated, spindle-shaped cells, slow contraction	Involuntary (autonomic control)

Overview of Muscle Tissue Functions

Main Functions

Muscle tissue performs several vital functions in the body:

• Movement: Locomotion and manipulation of the environment.

• Posture: Maintenance of body position.

• Joint stabilization: Supports and stabilizes joints.

• Heat generation: Especially by skeletal muscle during contraction.

• Additional functions: Protects organs, forms valves, controls pupil size, and causes "goosebumps".

Properties of Muscle Tissue

Key Characteristics

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched beyond resting length.

• Elasticity: Ability to recoil to resting length after stretching.

• Conductivity: Ability to conduct electrical changes across the plasma membrane.

Skeletal Muscle Structure

Organization and Attachments

Skeletal muscle is highly organized, with each muscle served by arteries, veins, and nerves. Muscle fibers are grouped into fascicles, which are surrounded by connective tissue layers.

• Epimysium: Surrounds entire muscle.

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

• Endomysium: Surrounds individual muscle fibers.

Muscle attaches to bone via tendons (dense regular connective tissue) or aponeuroses (sheet-like connective tissue).

Levels of Organization

Muscle tissue is organized from the macroscopic to the microscopic level:

• Muscle (organ)

• Fascicle (bundle of muscle fibers)

• Muscle fiber (cell)

• Myofibril (organelle)

• Myofilament (protein filaments: actin and myosin)

Structure of Muscle Cells

Muscle Fiber Anatomy

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm of muscle fiber, contains glycosomes (glycogen storage) and myoglobin (oxygen storage).

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum for calcium storage and release.

• T tubules: Invaginations of sarcolemma that conduct electrical impulses into the cell.

Myofibril Structure and Banding

Myofibril Organization

Myofibrils are composed of repeating units called sarcomeres, which are the smallest contractile units of muscle fiber. Sarcomeres align end-to-end along the length of the myofibril.

• Thick filaments: Composed of myosin.

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

• Elastic filaments: Composed of titin, which stabilizes the myofibril structure.

Sarcomere Structure

• Z disc: Boundary of each sarcomere.

• M line: Center of sarcomere, anchors thick filaments.

• A band: Dark region, contains thick filaments.

• I band: Light region, contains thin filaments.

Myofilament Arrangement

Filament Type	Protein Components	Function
Thick	Myosin	Motor protein for contraction
Thin	Actin, Troponin, Tropomyosin	Regulates contraction
Elastic	Titin	Stabilizes and allows recoil

Ultrastructure of Thick Filaments

Myosin Structure

Thick filaments are composed of myosin molecules, each with two heavy and four light polypeptide chains. Myosin heads bind to actin during contraction, forming cross-bridges.

• Myosin head: Contains ATPase activity, binds to actin.

• Cross-bridge: Connection between myosin head and actin filament during contraction.

Sarcoplasmic Reticulum and T Tubules

Sarcoplasmic Reticulum (SR)

The SR is a network of smooth endoplasmic reticulum surrounding each myofibril. It regulates intracellular calcium levels, which are essential for muscle contraction.

• Terminal cisterns: Enlarged areas of SR that store calcium.

• Calcium release: Triggers contraction by binding to troponin.

T Tubules

T tubules are extensions of the sarcolemma that penetrate into the muscle fiber, allowing electrical impulses to reach deep into the cell and coordinate contraction.

• Function: Enhance cellular communication during muscle contraction.

Summary Table: Muscle Tissue Types

Type	Striations	Control	Location	Special Features
Skeletal	Yes	Voluntary	Bones, skin	Rapid contraction, multinucleated
Cardiac	Yes	Involuntary	Heart	Intercalated discs, branched cells
Smooth	No	Involuntary	Walls of hollow organs	Spindle-shaped cells, slow contraction

Key Equations

• ATP Hydrolysis (energy for contraction):

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Force Generation (simplified):

$\text{Force} \propto \text{Number of cross-bridges formed}$

Additional info:

• Muscle contraction is regulated by the interaction of actin and myosin filaments, controlled by calcium ions and ATP.

• Muscle tissue also plays a role in metabolic processes and homeostasis.