Muscle Tissue: Structure and Function (Anatomy & Physiology, Chapter 9)

Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is a specialized tissue found throughout the body, responsible for producing movement, maintaining posture, and generating heat. There are three primary types of muscle tissue, each with distinct structure and function.

• Skeletal muscle – Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac muscle – Involuntary, striated muscle found only in the heart; responsible for pumping blood.

• Smooth muscle – Involuntary, non-striated muscle found in walls of hollow organs (e.g., intestines, blood vessels).

Muscle Terminology

Key Terms

• Myo- and Mys-: Prefixes meaning "muscle" (e.g., myofibril, myocyte).

• Sarco-: Prefix meaning "flesh" or "muscle" (e.g., sarcolemma, sarcoplasm).

• Muscle fiber: Another term for a muscle cell or myofiber.

Universal Characteristics of Muscle

Functional Properties

• Responsiveness (Excitability): Ability to respond to stimuli by producing electrical changes across the plasma membrane.

• Contractility: Unique to muscle tissue; ability to shorten forcibly when stimulated. Muscles always pull (never push).

• Extensibility: Ability to be stretched beyond resting length without damage. Other cell types would rupture under similar conditions.

• Elasticity: Ability to recoil and return to original length after being stretched.

Skeletal Muscle

Structure and Control

• Voluntary striated muscle – under conscious control.

• Usually attached to one or more bones (exceptions: facial muscles).

• Muscle fibers (myofibers) are exceptionally long (average 3 cm, up to 30 cm).

• Striations – due to overlapping arrangement of internal contractile proteins (actin and myosin).

Connective Tissue Components

• Endomysium: Areolar connective tissue surrounding each muscle fiber.

• Perimysium: Fibrous connective tissue surrounding each fascicle (bundle of muscle fibers).

• Epimysium: Dense irregular connective tissue surrounding the whole muscle.

Attachments

Muscle-Bone Connections

• Insertion vs. Origin: Insertion is the movable attachment; origin is the fixed attachment.

• Direct (fleshy) attachment: Epimysium of muscle fused to periosteum of bone (e.g., intercostal muscles to ribs).

• Indirect attachment: Connective tissue extends beyond muscle as a tendon (rope-like) or aponeurosis (sheet-like).

Muscle Fibers

Cellular Structure

• Multiple flattened nuclei per muscle fiber.

• Sarcolemma: Plasma membrane of muscle cell.

• Sarcoplasm: Cytoplasm of muscle cell, containing:

  • Myofibrils (bundles of parallel protein myofilaments – actin & myosin)

  • Granules of stored glycogen (energy reserve)

  • Myoglobin (oxygen-binding pigment)

Specialized Structures

• Transverse (T) tubules: Tunnel-like extensions of sarcolemma that penetrate deep into the cell, carrying action potentials to the cell interior.

• Sarcoplasmic reticulum (SR): Smooth endoplasmic reticulum forming a network around myofibrils; stores and releases calcium ions (Ca2+).

• Terminal cisternae: Dilated end sacs of SR at T-tubules; store Ca2+ and release it upon stimulation.

• Triad: Combination of a T-tubule and two terminal cisternae.

Myofibrils and Myofilaments

Organization and Types

• Each muscle fiber contains many parallel myofibrils.

• Myofibrils are divided into sarcomeres, the functional units of contraction.

• Myofibrils contain three types of myofilaments:

  • Thick filaments: Composed of myosin protein.

  • Thin filaments: Composed of actin protein.

  • Elastic filaments: Composed of titin (connectin), a large, springy protein that anchors thick filaments and provides elasticity.

Summary Table: Connective Tissue Components

Component	Tissue Type	Location/Function
Endomysium	Areolar connective tissue	Surrounds each muscle fiber
Perimysium	Fibrous connective tissue	Surrounds each fascicle
Epimysium	Dense irregular connective tissue	Surrounds whole muscle

Example: Skeletal Muscle Fiber Structure

• Sarcolemma surrounds the muscle fiber and conducts action potentials.

• Sarcoplasm contains myofibrils, glycogen, and myoglobin.

• Myofibrils are composed of repeating sarcomeres, which contain thick, thin, and elastic filaments.

Additional info: Later sections of the chapter (not shown in these images) would likely cover the molecular mechanism of contraction, neuromuscular junctions, and differences between muscle tissue types in greater detail.