Chapter 10: Muscle Tissue – Structure, Function, and Physiology

Muscle Tissue: Types and Properties

Types of Muscle Tissue

Muscle tissue is specialized for contraction and movement. There are three main types:

• Skeletal muscle: Voluntary, striated, attached to bones.

• Cardiac muscle: Involuntary, striated, found only in the heart.

• Smooth muscle: Involuntary, non-striated, found in internal organs.

Common Properties of Muscle Tissue

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten forcibly.

• Extensibility: Ability to stretch without damage.

• Elasticity: Ability to return to original shape after stretching.

Skeletal Muscle Structure and Function

Organization of Skeletal Muscle

Skeletal muscles attach to bones and perform essential functions:

• Produce movement

• Maintain posture and body position

• Support soft tissues

• Guard entrances and exits

• Maintain body temperature

• Store nutrient reserves

Muscle structure is organized in layers:

• Epimysium: Surrounds entire muscle

• Perimysium: Surrounds bundles (fascicles) of muscle fibers

• Endomysium: Surrounds individual muscle fibers

• Tendons/Aponeuroses: Attach muscle to bone

The perimysium and endomysium contain blood vessels and nerves.

Skeletal Muscle Fiber Anatomy

• Sarcolemma: Plasma membrane of muscle fiber

• Sarcoplasm: Cytoplasm of muscle fiber

• Sarcoplasmic Reticulum (SR): Stores calcium ions

• Transverse (T) tubules: Conduct action potentials into the fiber

• Myofibrils: Bundles of myofilaments, responsible for contraction

Myofibrils are composed of repeating units called sarcomeres, the functional unit of muscle contraction.

Myofilaments: Thin and Thick Filaments

• Thin filaments: Composed of F-actin, nebulin, tropomyosin, and troponin

• Thick filaments: Composed of myosin molecules around a titin core

Tropomyosin covers active sites on actin; troponin binds actin and tropomyosin, holding tropomyosin in place. Myosin heads form cross-bridges with actin during contraction.

Muscle Contraction: Mechanisms and Control

Sliding Filament Theory

Muscle contraction occurs as thick and thin filaments slide past each other, shortening the sarcomere.

• One end of the muscle (origin) is fixed; the other (insertion) moves during contraction.

Excitable Membranes and Neuromuscular Junction

• Both neurons and muscle fibers have excitable membranes capable of propagating action potentials.

• A neuromuscular junction (NMJ) is where a motor neuron communicates with a muscle fiber.

• When an action potential reaches the axon terminal, acetylcholine (ACh) is released into the synaptic cleft.

• ACh binds to receptors on the motor end plate, generating an action potential in the sarcolemma.

Excitation–Contraction Coupling

• Action potential travels along T tubules, triggering Ca2+ release from SR.

• Ca2+ binds to troponin, moving tropomyosin and exposing actin sites.

• Myosin heads bind to actin, pivot, detach, and reactivate in a cycle.

• Acetylcholinesterase (AChE) breaks down ACh, ending stimulation.

Muscle Tension and Contraction Types

Factors Affecting Muscle Tension

• Tension depends on the number of cross-bridges formed.

• Optimal contraction occurs within a narrow range of sarcomere lengths.

Types of Muscle Contractions

• Twitch: Single contraction-relaxation cycle from one stimulus

• Treppe: Progressive increase in twitch tension with repeated slow stimulation

• Wave summation: Addition of one twitch to another before relaxation ends

• Incomplete tetanus: Tension peaks and falls at intermediate stimulus rates

• Complete tetanus: Rapid stimuli eliminate relaxation phase

Motor Units and Muscle Tone

• Number and size of motor units affect precision of movement.

• Resting muscle tone stabilizes bones and joints.

Isotonic and Isometric Contractions

• Isotonic: Tension rises, muscle length changes (concentric: shortens; eccentric: lengthens)

• Isometric: Tension rises, muscle length remains unchanged

Load and speed of contraction are inversely related.

Muscle Metabolism and Energy

Energy Sources for Muscle Contraction

• Muscle contractions require large amounts of ATP.

• Creatine phosphate (CP) stores energy to convert ADP to ATP.

• Aerobic metabolism (citric acid cycle, electron transport chain) provides most ATP at rest/moderate activity.

• Anaerobic metabolism (glycolysis) predominates at peak activity when oxygen is limited.

Muscle Fatigue and Recovery

• Fatigue occurs due to pH drop, energy depletion, or other factors.

• Recovery period restores muscle to pre-exertion state.

• Oxygen debt (EPOC): Extra oxygen needed to restore muscle after exercise.

• Hormones can alter muscle metabolism.

Muscle Fiber Types and Performance

Types of Skeletal Muscle Fibers

• Fast fibers: Large diameter, rapid/powerful contractions, fatigue quickly, few mitochondria, pale (white muscles)

• Slow fibers: Smaller diameter, contract slowly, resist fatigue, many mitochondria, high myoglobin, red (red muscles)

• Intermediate fibers: Similar to fast fibers, but more resistant to fatigue

Muscle Conditioning and Aging

• Anaerobic endurance: Duration of contraction supported by glycolysis and ATP/CP reserves

• Aerobic endurance: Duration of contraction supported by mitochondrial activity

• Training increases muscle size (hypertrophy)

• Aging leads to fibrosis, decreased exercise tolerance, slower repair

Cardiac Muscle Tissue

Structure and Function

• Found only in the heart

• Small cells, single nucleus, short/broad T tubules

• Dependent on aerobic metabolism

• Intercalated discs bind neighboring cells

Contraction Characteristics

• Automaticity: Contracts without neural stimulation

• Longer contractions than skeletal muscle

• No wave summation or tetanic contractions

Smooth Muscle Tissue

Structure and Function

• Non-striated, involuntary

• Lacks sarcomeres; thin filaments anchored to dense bodies

Contraction Mechanism

• Ca2+ interacts with calmodulin, activating myosin light chain kinase

• Functions over wide range of lengths (plasticity)

Types of Smooth Muscle

• Multiunit: Each cell acts independently

• Visceral: Cells not always innervated; neurons not under voluntary control

Summary Table: Muscle Tissue Types

Type	Striations	Control	Location	Special Features
Skeletal	Yes	Voluntary	Attached to bones	Multiple nuclei, rapid contraction
Cardiac	Yes	Involuntary	Heart	Intercalated discs, automaticity
Smooth	No	Involuntary	Internal organs	Plasticity, calmodulin-mediated contraction

Key Equations

• ATP regeneration from creatine phosphate: $\mathrm{ADP} + \mathrm{CP} \rightarrow \mathrm{ATP} + \mathrm{Creatine}$

• Aerobic metabolism (simplified): $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \mathrm{ATP}$

• Anaerobic glycolysis (simplified): $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 \rightarrow 2\mathrm{C}_3\mathrm{H}_6\mathrm{O}_3 + \mathrm{ATP}$

Example: Muscle Contraction Cycle

• Action potential arrives at NMJ

• ACh released, binds to motor end plate

• Action potential spreads along sarcolemma and T tubules

• Ca2+ released from SR

• Ca2+ binds troponin, moves tropomyosin

• Myosin binds actin, contraction occurs

• AChE breaks down ACh, contraction ends

Additional info: Academic context was added to expand brief points into full explanations, clarify muscle fiber types, contraction mechanisms, and metabolic pathways, and to provide a summary table and key equations for exam preparation.