Chapter 10 Muscle Tissue: Structure, Function, and Physiology

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Muscle Tissue: Structure, Function, and Physiology

Introduction to Muscle Tissue

Muscle tissue is a primary tissue type responsible for contraction and movement in the human body. There are three main types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. Each type has unique structural and functional characteristics that enable specific roles in the body.

Types and Functions of Muscle Tissue

Common Properties of Muscle Tissue

Excitability (Responsiveness): Ability to receive and respond to stimuli.
Contractility: Ability of cells to shorten and generate force.
Extensibility: Ability to stretch without being damaged.
Elasticity: Ability to return to original length after stretching.

Primary Functions of Skeletal Muscle

Producing movement by pulling on bones
Maintaining posture and body position
Supporting soft tissues
Guarding entrances and exits (e.g., digestive and urinary tracts)
Maintaining body temperature through heat production
Storing nutrients as protein reserves

Organization of Skeletal Muscle

Structural Organization

Skeletal muscles are composed of muscle tissue, connective tissues, blood vessels, and nerves. They are organized into several hierarchical levels:

Epimysium: Surrounds the entire muscle and separates it from surrounding tissues.
Perimysium: Surrounds bundles of muscle fibers called fascicles; contains blood vessels and nerves.
Endomysium: Surrounds individual muscle fibers; contains capillaries, nerve fibers, and myosatellite cells (stem cells for repair).

The organization of skeletal muscles, showing epimysium, perimysium, and endomysium Muscle fascicle structure within skeletal muscle Muscle fiber structure within a fascicle

The connective tissue layers merge at the ends of muscles to form tendons (bundles) or aponeuroses (sheets), which attach muscles to bones.

Vascular and Neural Supply

Skeletal muscles have extensive blood vessels for nutrient and waste exchange.
Muscle fibers are innervated by branches of motor neurons, allowing voluntary control.

Skeletal Muscle Fibers

Development and Structure

Skeletal muscle fibers are large, multinucleate cells formed by the fusion of embryonic myoblasts. They are also called striated muscle cells due to their banded appearance.

Formation of a multinucleate skeletal muscle fiber from myoblasts Diagram and micrograph of a muscle fiber

Sarcolemma: Plasma membrane of the muscle fiber.
Sarcoplasm: Cytoplasm of the muscle fiber.
Transverse (T) Tubules: Invaginations of the sarcolemma that transmit action potentials into the cell.
Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions; forms terminal cisternae adjacent to T tubules, creating a triad.

Internal organization of a skeletal muscle fiber, showing myofibrils, SR, and T tubules Myofibril and sarcolemma structure

Myofibrils and Sarcomeres

Myofibrils are bundles of contractile proteins (myofilaments) responsible for muscle contraction. The basic functional unit of a myofibril is the sarcomere.

Thin filaments: Composed mainly of actin.
Thick filaments: Composed mainly of myosin.

Sarcomere Structure

A bands: Dark bands containing thick filaments.
I bands: Light bands containing only thin filaments.
M line: Center of the A band; stabilizes thick filaments.
H band: Region with only thick filaments.
Z lines: Boundaries of each sarcomere; anchor thin filaments.
Titin: Elastic protein that helps restore sarcomere length.

Sarcomere structure, longitudinal view Micrograph of a sarcomere in a muscle fiber Superficial and cross-sectional views of a sarcomere

Thin and Thick Filaments

Thin filaments: Made of F-actin (two rows of G-actin), nebulin, tropomyosin (blocks active sites), and troponin (binds calcium).
Thick filaments: Made of myosin molecules with heads that bind actin; contain a core of titin.

Structure of thin filaments, showing actin, tropomyosin, and troponin Structure of thick filaments and myosin molecule

Sliding Filament Theory

During contraction, thin filaments slide toward the M line alongside thick filaments, shortening the sarcomere. The A band remains constant, while the I and H bands narrow.

Changes in sarcomere appearance during contraction Shortening of myofibrils during contraction

Neuromuscular Junction and Muscle Contraction

Excitable Membranes and Action Potentials

Muscle fibers maintain a negative resting membrane potential (~ -85 mV).
Action potentials are electrical impulses that trigger contraction.

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber. Key components include:

Axon terminal: End of the motor neuron.
Motor end plate: Folded region of the muscle fiber's sarcolemma.
Synaptic cleft: Space between axon terminal and motor end plate.
Neurotransmitter (ACh): Acetylcholine is released, binds to receptors, and initiates an action potential in the muscle fiber.

Vesicles of acetylcholine in the axon terminal at the NMJ Arrival of action potential at the axon terminal Exocytosis of ACh into the synaptic cleft ACh binding to receptor channels, sodium influx Action potential generation and ACh breakdown

Excitation-Contraction Coupling

This process links the action potential in the sarcolemma to the start of muscle contraction. The action potential travels down T tubules, triggering calcium release from the SR. Calcium binds to troponin, moving tropomyosin and exposing actin active sites for contraction.

Excitation-contraction coupling, neural control, and calcium release

The Contraction Cycle

Calcium binds to troponin, exposing active sites on actin.
Myosin heads bind to actin, forming cross-bridges.
Power stroke: Myosin head pivots, pulling actin toward the M line (uses ATP).
ATP binds to myosin, causing detachment from actin.
Myosin reactivation: ATP is hydrolyzed, recocking the myosin head.

Contraction cycle begins: calcium ions arrive Active-site exposure: calcium binds to troponin Cross-bridge formation: myosin binds actin Myosin head pivoting: power stroke Cross-bridge detachment: ATP binds myosin Myosin reactivation: ATP hydrolysis Resting sarcomere: myosin heads energized Contracted sarcomere: cycle repeats as long as calcium and ATP are present

Relaxation and Rigor Mortis

Relaxation occurs when stimulation ends, ACh is broken down, calcium is reabsorbed, and active sites are covered by tropomyosin.
Rigor mortis is the post-mortem stiffening of muscles due to lack of ATP, preventing cross-bridge detachment.

Steps involved in muscle contraction and relaxation (initiation)

Muscle Tension and Contraction Types

Mechanisms of Tension Production

The amount of tension depends on the number of power strokes, sarcomere length (length-tension relationship), and frequency of stimulation.
Maximum tension is produced at optimal sarcomere length.

Types of Muscle Contractions

Isotonic contractions: Muscle changes length (concentric: shortens; eccentric: lengthens).
Isometric contractions: Muscle develops tension but does not change length.

Energy for Muscle Contraction

ATP Generation

ATP is the direct energy source for contraction.
Creatine phosphate (CP) stores energy for rapid ATP regeneration.
ATP is also generated by glycolysis (anaerobic) and aerobic metabolism (mitochondria).

Muscle Fiber Types and Performance

Types of Skeletal Muscle Fibers

Fast fibers: Large, powerful, fatigue quickly (white muscles).
Slow fibers: Smaller, more endurance, rich in myoglobin (red muscles).
Intermediate fibers: Characteristics between fast and slow fibers.

Muscle Hypertrophy and Atrophy

Hypertrophy: Increase in muscle size due to training.
Atrophy: Decrease in muscle size due to inactivity.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle Tissue

Found only in the heart; cells are small, branched, and striated with a single nucleus.
Connected by intercalated discs (gap junctions and desmosomes) for synchronized contraction.
Automaticity: Can contract without neural stimulation (pacemaker cells).

Smooth Muscle Tissue

Found in walls of hollow organs and blood vessels.
Cells are spindle-shaped, non-striated, with a single nucleus.
Contraction is regulated by calcium binding to calmodulin, not troponin.
Can maintain tone and contract over a wide range of lengths (plasticity).