Muscle Tissues and Skeletal Muscle Anatomy – Chapter 9 Study Notes

Muscle Tissues: Overview

General Properties of Muscle Tissue

Muscle tissue is a fundamental component of the human body, accounting for nearly half of its mass. Its primary function is to transform chemical energy, mainly from ATP, into mechanical energy, enabling movement and force generation.

• Muscle fibers: Elongated cells of skeletal and smooth muscle (not cardiac muscle).

• Myo, mys, and sacro: Prefixes commonly used for muscle-related terms (e.g., sarcoplasm = muscle cell cytoplasm).

Review: Types of Muscle Tissue

Classification and Functions

Muscle tissue is one of four major tissue types in the body, specialized for movement. There are three main types:

• Skeletal muscle: Attaches to bones, responsible for voluntary movements.

• Cardiac muscle: Found only in the heart, responsible for pumping blood.

• Smooth muscle: Located in walls of hollow organs (e.g., intestines, blood vessels), controls involuntary movements.

Comparison Table: Skeletal, Cardiac, and Smooth Muscle

Characteristic	Skeletal	Cardiac	Smooth
Body Location	Attached to bones or facial muscles	Walls of the heart	Walls of hollow organs (except heart)
Cell Shape & Appearance	Single, very long, cylindrical, multinucleate cells with striations	Branching chains of cells; uni- or binucleate; striations	Single, spindle-shaped, uninucleate; no striations

Skeletal Muscle Anatomy

Structural Organization

Skeletal muscles span joints and attach to bones. Each skeletal muscle is an organ composed of muscle tissue, nerve and blood supply, connective tissue sheaths, and attachments.

• Muscle attachments: Muscles attach to bones via origin (immovable or less movable bone) and insertion (movable bone).

• Attachments can be direct (epimysium fused to bone/cartilage) or indirect (connective tissue forms tendons or aponeuroses).

Nerve & Blood Supply

Each muscle receives a nerve, artery, and veins. Every muscle fiber is controlled by nerves, and contracting muscles require abundant oxygen and nutrients, necessitating a rich blood supply.

Connective Tissue Sheaths

Muscles are supported by connective tissue sheaths, organized from external to internal:

• Epimysium: Surrounds entire muscle; dense irregular connective tissue.

• Perimysium: Surrounds fascicles (groups of muscle fibers); fibrous connective tissue.

• Endomysium: Surrounds each muscle fiber; fine areolar connective tissue.

Table: Structure and Organization Levels of Skeletal Muscle

Level	Description	Connective Tissue Wrapping
Muscle (organ)	Thousands of muscle fibers, blood vessels, and nerve fibers	Epimysium
Fascicle	Bundle of muscle fibers	Perimysium
Muscle fiber (cell)	Elongated multinucleate cell	Endomysium

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

Muscle fibers are the basic cellular units of skeletal muscle. They contain specialized organelles and structures for contraction:

• Sarcolemma: Muscle fiber plasma membrane.

• Sarcoplasm: Muscle fiber cytoplasm, rich in glycosomes (glycogen storage) and myoglobin (oxygen storage).

• Mitochondria: Numerous, providing energy for contraction.

• Myofibrils: Densely packed, rodlike elements; account for ~80% of muscle cell volume.

Myofibril Structure

• Striations: Due to alternating dark A bands and light I bands.

• Sarcomeres: Contractile units aligned end to end along myofibril, separated by Z discs.

• Myofilaments: Sarcomeres are composed of myofilaments, which include actin (thin) and myosin (thick) filaments.

Summary Table: Muscle Organization

Level	Made of
Muscle	Fascicles
Fascicle	Muscle fibers
Muscle fiber	Myofibrils
Myofibril	Sarcomeres
Sarcomere	Myofilaments (actin & myosin)

Myofilaments: Types and Arrangement

Types of Myofilaments

• Actin (thin) filaments: Extend across I band and partway into A band; anchored at Z discs.

• Myosin (thick) filaments: Extend length of A band; connected at M line.

Dark A bands contain myosin; light I bands lack myosin.

Arrangement in Sarcomere

• Orderly arrangement of actin and myosin within sarcomere.

• Hexagonal cross-section: Each thick filament is surrounded by six thin filaments.

Molecular Composition

• Thick filaments: Made of myosin, which has 6 polypeptide chains (2 heavy chains form tail, 4 light chains form globular head).

• During contraction, myosin heads link thick and thin filaments.

• Thin filaments: Made of actin (F-actin strands twisted together); G-actin subunits have active sites for myosin attachment.

• Tropomyosin and troponin: Regulatory proteins bound to actin.

Sarcoplasmic Reticulum & T Tubules

Sarcoplasmic Reticulum (SR)

The SR is a specialized smooth endoplasmic reticulum surrounding each myofibril. Its key function is to store and release Ca2+ ions into the cytosol, which is essential for muscle contraction.

• Most SR tubules run longitudinally.

• Terminal cisterns: Perpendicular cross channels at A–I band junctions.

T Tubules

T tubules are formed by protrusions of the sarcolemma deep into the cell interior at A–I band junctions. Their lumen is continuous with the extracellular space.

• Key function: Allow electrical signals to reach deep into the muscle fiber.

• Triad: Consists of one T tubule and two terminal cisterns of neighboring sarcomeres.

Triad Function

• T tubule membrane proteins: Act as voltage sensors, changing shape in response to electrical current.

• SR cistern membrane proteins: Control opening of Ca2+ channels in SR cisterns.

• When an electrical impulse passes by, T tubule proteins change shape, causing SR proteins to release Ca2+ into the cytosol.

Key Terms and Definitions

• Epimysium: Outer connective tissue sheath of muscle.

• Perimysium: Connective tissue surrounding fascicles.

• Endomysium: Connective tissue surrounding individual muscle fibers.

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm of muscle fiber.

• Myofibril: Rodlike unit within muscle fiber, composed of sarcomeres.

• Sarcomere: Contractile unit of muscle, composed of actin and myosin filaments.

• Triad: Structure formed by a T tubule and two terminal cisterns.

Summary Diagram: Muscle Organization

• Muscle → Fascicles → Muscle fibers → Myofibrils → Sarcomeres → Myofilaments (Actin & Myosin)

Equations and Scientific Principles

• ATP Hydrolysis: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Calcium Release: $\text{SR} \xrightarrow{\text{electrical signal}} \text{Ca}^{2+} \text{release}$

Example: Muscle Contraction

During voluntary movement, an electrical signal travels along the sarcolemma and down the T tubules, triggering the release of Ca2+ from the SR. This initiates the interaction between actin and myosin filaments, resulting in muscle contraction.

Additional info: The notes provide a foundational overview of muscle tissue structure and function, focusing on skeletal muscle as covered in Chapter 9 of a standard Anatomy & Physiology textbook. For more advanced study, students should review the molecular mechanisms of contraction and the role of regulatory proteins in detail.