Muscle Tissue

Types of Muscle Tissue

Muscle tissue is a primary tissue in the human body, specialized for contraction and movement. There are three main types of muscle tissue:

• Skeletal muscle: Responsible for voluntary movements by pulling on bones.

• Cardiac muscle: Found only in the heart, controls rhythmic contractions.

• Smooth muscle: Controls involuntary movements within internal organs.

Functions of Muscle Tissue

Muscle tissue performs several essential functions:

• Producing movement: Skeletal muscles move the body.

• Maintaining posture: Stabilizes body position.

• Supporting soft tissues: Protects internal organs.

• Guarding entrances and exits: Controls passage through body openings.

• Maintaining body temperature: Generates heat during contraction.

• Storing nutrients: Stores glycogen and other nutrients.

Organization of Skeletal Muscle

Structural Hierarchy

Skeletal muscle is organized into several structural levels, each surrounded by connective tissue:

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers called fascicles.

• Endomysium: Surrounds individual muscle fibers.

Connective Tissue and Attachments

• Collagen fibers from all three layers merge to form tendons (bundles) or aponeuroses (sheets), which attach muscles to bones.

Skeletal Muscle Fibers

Development and Structure

Skeletal muscle fibers are large, multinucleate cells formed by the fusion of embryonic cells called myoblasts.

• Each fiber contains hundreds of nuclei.

• Striated appearance due to organized arrangement of contractile proteins.

Key Components

• Sarcolemma: Plasma membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber.

• Transverse tubules (T tubules): Invaginations of the sarcolemma that transmit action potentials.

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

• Myofibrils: Bundles of protein filaments responsible for contraction.

Sarcomere Structure

Functional Unit of Muscle

The sarcomere is the smallest functional unit of a muscle fiber. Its arrangement of thick and thin filaments produces the striated pattern and enables contraction.

• A band: Dark region containing thick filaments.

• I band: Light region containing thin filaments.

• H band: Area with only thick filaments.

• M line: Center of the A band, stabilizes thick filaments.

• Z line: Boundary between sarcomeres, anchors thin filaments.

• Titin: Elastic protein maintaining alignment and restoring length.

Levels of Functional Organization

Muscle structure is organized from the whole muscle down to the sarcomere:

• Skeletal muscle → Muscle fascicle → Muscle fiber → Myofibril → Sarcomere

Myofilaments: Thin and Thick Filaments

Thin Filaments

• Composed of F-actin (filamentous actin), nebulin, tropomyosin, and troponin.

• Active sites on G-actin bind to myosin heads.

• Tropomyosin covers active sites, preventing interaction.

• Troponin binds tropomyosin, G-actin, and Ca2+.

Thick Filaments

• Composed of myosin molecules (each with a tail and two heads).

• Myosin heads project toward thin filaments and form cross-bridges during contraction.

• Core of titin provides elasticity.

Sliding-Filament Theory

Mechanism of Contraction

Muscle contraction occurs as thin filaments slide toward the center of the sarcomere, causing:

• H and I bands to narrow

• Zones of overlap to widen

• Z lines to move closer together

• A band width remains constant

Neuromuscular Junction and Excitation-Contraction Coupling

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber. The process involves:

• Release of acetylcholine (ACh) from the neuron

• ACh binds to receptors on the muscle fiber, opening Na+ channels

• Depolarization generates an action potential

Excitation-Contraction Coupling

• Action potential travels along T tubules to the triads

• Ca2+ released from SR binds to troponin

• Troponin-tropomyosin complex exposes active sites

• Contraction cycle begins

Contraction Cycle Steps

1. Contraction cycle begins (Ca2+ arrives)

2. Active-site exposure (troponin changes shape)

3. Cross-bridge formation (myosin binds actin)

4. Myosin head pivoting (power stroke)

5. Cross-bridge detachment (ATP binds myosin)

6. Myosin reactivation (ATP hydrolysis)

Muscle Tension and Contraction Types

Tension Production

• Amount of tension depends on number of power strokes, fiber length, and stimulation frequency.

• Maximum tension occurs when the zone of overlap is optimal.

Types of Contractions

• Isotonic contraction: Muscle changes length (concentric = shortens, eccentric = lengthens).

• Isometric contraction: Muscle develops tension without changing length.

Energy for Muscle Contraction

ATP Production

• ATP is the direct energy source for contraction.

• Produced by:

  • Direct phosphorylation (creatine phosphate)

  • Anaerobic metabolism (glycolysis)

  • Aerobic metabolism (citric acid cycle, electron transport chain)

Muscle Metabolism

• At rest: Fatty acids metabolized, glycogen and CP stored.

• Moderate activity: Aerobic breakdown of glucose.

• Peak activity: Glycolysis produces lactate, leading to fatigue.

Muscle Performance and Fiber Types

Types of Skeletal Muscle Fibers

• Fast fibers: Contract quickly, fatigue rapidly, large diameter, few mitochondria.

• Slow fibers: Contract slowly, fatigue-resistant, small diameter, many mitochondria, high myoglobin.

• Intermediate fibers: Mid-sized, moderate endurance.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle Tissue

• Found only in the heart.

• Striated, single nucleus, branched cells.

• Connected by intercalated discs (gap junctions and desmosomes).

• Automaticity: Contracts without neural stimulation.

Smooth Muscle Tissue

• Found in walls of internal organs.

• Spindle-shaped, single nucleus, non-striated.

• Contractions regulated by calmodulin and myosin light chain kinase.

• Plasticity: Can function over a wide range of lengths.

Summary Table: Muscle Tissue Types

Key Equations

• ATP hydrolysis:

• Creatine phosphate reaction:

• Glycolysis (anaerobic):

Additional info:

• Muscle tone is essential for posture and balance.

• Muscle hypertrophy results from training; atrophy from inactivity.

• Hormones such as growth hormone, testosterone, thyroid hormones, and epinephrine influence muscle metabolism.