Ch 10: Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue: Structure, Function, and Physiology

Introduction to Muscle Tissue

Muscle tissue is one of the four primary tissue types in the human body. It is specialized for contraction, which enables movement and various physiological functions. There are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. This chapter focuses primarily on skeletal muscle, with comparisons to cardiac and smooth muscle tissues.

Common Properties and Functions of Muscle Tissue

Properties of Muscle Tissue

• Excitability (Responsiveness/Irritability): Ability to receive and respond to stimuli, typically from nerve impulses, resulting in a change in membrane potential.

• Contractility: Ability of muscle cells to shorten when stimulated.

• Extensibility: Ability to be stretched without being damaged.

• Elasticity: Ability to return to original length after contraction or extension.

Functions of Skeletal Muscle

• Producing Movement: Skeletal muscle contractions pull on tendons to move bones, enabling activities from walking to playing instruments.

• Maintaining Posture and Body Position: Continuous muscle tension maintains posture and stabilizes joints.

• Supporting Soft Tissues: Muscles of the abdominal wall and pelvic floor support internal organs.

• Guarding Body Entrances and Exits: Sphincter muscles control voluntary opening and closing of digestive and urinary tracts.

• Maintaining Body Temperature: Muscle contractions generate heat, helping regulate body temperature.

• Storing Nutrients: Muscle proteins can be broken down for energy during malnutrition.

Organization of Skeletal Muscle Tissue

Muscle Structure at the Tissue Level

• Muscle (Organ): Composed of muscle tissue, connective tissue, blood vessels, and nerves.

• Fascicles: Bundles of muscle fibers within the muscle.

• Muscle Fibers: Individual muscle cells, multinucleate and elongated.

Connective Tissue Layers

• Epimysium: Dense collagen layer surrounding the entire muscle; separates muscle from surrounding tissues.

• Perimysium: Surrounds fascicles; contains blood vessels and nerves.

• Endomysium: Surrounds individual muscle fibers; contains capillaries, myosatellite cells (muscle stem cells), and nerve fibers.

At the ends of muscles, connective tissue layers merge to form tendons (cord-like) or aponeuroses (sheet-like), which attach muscles to bones.

Microscopic Structure of Skeletal Muscle Fibers

Characteristics of Skeletal Muscle Fibers

• Large, multinucleate cells formed by fusion of myoblasts during development.

• Striated appearance due to arrangement of myofilaments.

• Contain many mitochondria and glycogen granules for energy.

Key Components

• Sarcolemma: Plasma membrane of muscle fiber; conducts action potentials.

• Transverse (T) Tubules: Invaginations of sarcolemma that transmit action potentials into the cell interior.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions; forms triads with T tubules and terminal cisternae.

• Myofibrils: Cylindrical structures containing myofilaments (actin, myosin, titin); responsible for contraction.

• Sarcomeres: Repeating contractile units within myofibrils; composed of thick and thin filaments.

Sarcomere Structure

• A Band: Dark band; contains thick (myosin) filaments and overlapping thin (actin) filaments.

• M Line: Center of A band; stabilizes thick filaments.

• H Band: Lighter region on either side of M line; thick filaments only.

• Zone of Overlap: Area where thick and thin filaments overlap.

• I Band: Light band; contains thin filaments only.

• Z Line (Z Disc): Boundary between sarcomeres; anchors thin filaments via actinins and titin.

Myofilament Structure

• Thin Filaments: Composed of F-actin (two twisted rows of G-actin), nebulin, tropomyosin, and troponin. Tropomyosin covers active sites on actin; troponin binds calcium to initiate contraction.

• Thick Filaments: Composed of myosin molecules (each with a tail and two heads) and a core of titin. Myosin heads form cross-bridges with actin during contraction.

Sliding Filament Theory of Muscle Contraction

Mechanism of Contraction

• During contraction, thin filaments slide toward the center of the sarcomere, alongside thick filaments.

• H bands and I bands narrow; zones of overlap widen; Z lines move closer together; A band width remains constant.

• Shortening of all sarcomeres in a myofibril leads to muscle fiber contraction.

Example: When a muscle contracts to lift a weight, the insertion (movable end) moves toward the origin (fixed end).

Muscle Fiber Tension: Length-Tension Relationship and Frequency of Stimulation

Length-Tension Relationship

• Tension depends on the number of cross-bridges formed, which is determined by the overlap between thick and thin filaments.

• Maximum tension is produced at optimal sarcomere length; too much or too little overlap reduces tension.

Frequency of Stimulation

• Twitch: Single, brief contraction-relaxation cycle in a muscle fiber.

• Treppe: Gradual increase in tension with repeated stimulation after relaxation phase; rare in skeletal muscle, common in cardiac muscle.

• Wave Summation: Increased tension due to repeated stimulation before relaxation is complete.

• Incomplete Tetanus: Rapid cycles of contraction and relaxation; tension increases but some relaxation occurs.

• Complete Tetanus: No relaxation between stimuli; maximum, sustained tension.

Whole Muscle Contraction: Motor Units and Recruitment

Motor Units

• A motor unit consists of a motor neuron and all the muscle fibers it controls.

• Small motor units (few fibers) allow fine control (e.g., eye muscles); large motor units (many fibers) generate powerful contractions (e.g., thigh muscles).

• Fasciculations are involuntary contractions of a motor unit.

Recruitment and Muscle Tone

• Recruitment: Increasing the number of active motor units to increase muscle tension.

• Asynchronous Motor Unit Summation: Motor units alternate activity to sustain contraction and delay fatigue.

• Muscle Tone: Resting tension in a muscle; stabilizes bones and joints, maintains posture, and increases resting metabolic rate.

Types of Muscle Contractions

Isotonic Contractions

• Isotonic Concentric: Muscle tension exceeds load; muscle shortens (e.g., lifting a weight).

• Isotonic Eccentric: Peak tension is less than load; muscle lengthens (e.g., lowering a weight).

Isometric Contractions

• Muscle tension does not exceed load; muscle length remains constant (e.g., holding a heavy object steady).

Load and Speed of Contraction

• Speed of contraction is inversely related to load; heavier loads slow contraction.

Muscle Relaxation

• Muscles return to resting length via elastic forces, contraction of opposing muscles, and gravity.

Muscle Metabolism: Energy for Contraction and Recovery

ATP Generation in Muscle Fibers

• ATP is the direct energy source for muscle contraction; stores are limited and must be regenerated rapidly.

• Creatine Phosphate (CP): Stores high-energy phosphate groups; transfers phosphate to ADP to form ATP.

• Glycolysis: Anaerobic breakdown of glucose to pyruvate; yields 2 ATP per glucose.

• Aerobic Metabolism: Mitochondrial process using oxygen; yields up to 15 ATP per pyruvate (much more efficient).

Energy Use at Different Activity Levels

• At rest: Fatty acids are primary fuel; ATP produced by aerobic metabolism.

• Moderate activity: Glucose from glycogen and blood is used; aerobic metabolism predominates.

• Peak activity: Glycolysis becomes primary ATP source; lactate accumulates, leading to lactic acidosis.

Recovery Period and Oxygen Debt

• After exercise, muscle fibers restore ATP, CP, and glycogen reserves; lactate is converted back to pyruvate and glucose (Cori cycle).

• Oxygen Debt (EPOC): Extra oxygen required to restore pre-exertion conditions.

• Muscle activity generates heat; most is lost to maintain body temperature.

Hormonal Regulation

• Growth hormone and testosterone promote muscle growth.

• Thyroid hormones increase metabolic rate.

• Epinephrine increases muscle metabolism and contraction force during stress.

Cardiac Muscle Tissue

Structural Characteristics

• Found only in the heart; cells are small, branched, and usually have a single central nucleus.

• Striated due to organized sarcomeres; T tubules are short and broad, encircle sarcomeres at Z lines.

• Sarcoplasmic reticulum lacks terminal cisternae; stores less calcium than skeletal muscle.

• Cells are connected by intercalated discs containing gap junctions and desmosomes, allowing electrical and mechanical coupling.

• Rely almost exclusively on aerobic metabolism; abundant mitochondria and myoglobin.

Functional Characteristics

• Contracts without neural stimulation (automaticity); pacemaker cells set rhythm.

• Nervous system can modulate rate and force of contraction.

• Contractions last about 10 times longer than skeletal muscle; longer refractory period prevents tetanus.

• Action potentials increase sarcolemma permeability to extracellular calcium; both intracellular and extracellular Ca2+ are required for contraction.

• Cardiac muscle acts as a functional syncytium due to electrical connectivity.

Comparison Table: Skeletal vs. Cardiac Muscle

Feature	Skeletal Muscle	Cardiac Muscle
Location	Attached to bones	Heart
Cell Shape	Long, cylindrical, multinucleate	Short, branched, usually single nucleus
Striations	Present	Present
Control	Voluntary	Involuntary (automaticity)
Intercalated Discs	Absent	Present
Contraction Duration	Short	Long (10x longer)
Refractory Period	Short	Long
Energy Source	Aerobic & anaerobic	Primarily aerobic

Additional info: Smooth muscle tissue, not detailed here, is found in walls of hollow organs and blood vessels, is non-striated, involuntary, and contracts via different mechanisms. For more on smooth muscle, see subsequent chapters.