Skeletal, Cardiac, and Smooth Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue

Skeletal Muscle Functions

Skeletal muscle tissue is essential for movement and maintaining homeostasis in the human body. Its functions include:

• Movement of bones: Skeletal muscles contract to move the skeleton.

• Maintain posture and balance: Continuous muscle contractions stabilize joints and maintain posture.

• Support visceral organs: Muscles form walls and floors of body cavities.

• Guard entrances and exits: Sphincter muscles control passage of substances.

• Thermoregulation: Muscle contractions generate heat.

Skeletal Muscle Connective Tissues

Connective tissue layers organize and protect muscle fibers:

• Epimysium: Outer layer of collagen fibers surrounding the entire muscle (continuous with tendons).

• Perimysium: Divides muscle into compartments called fascicles; contains blood vessels and nerves.

• Endomysium: Surrounds individual muscle fibers; contains capillaries and nerve fibers.

Additional info: These layers merge at the ends of muscles to form tendons or aponeuroses.

Skeletal Muscle Cells (Fibers)

Skeletal muscle fibers are unique, multinucleated cells specialized for contraction.

• Size: Up to 100μm wide and up to a foot in length.

• Multinucleated: Multiple nuclei per cell due to fusion of myoblasts.

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm of muscle fiber, rich in glycogen and myoglobin.

• Myofibrils: Contractile organelles composed of sarcomeres.

Sarcoplasmic Reticulum (SR)

The SR is a specialized endoplasmic reticulum that regulates calcium ion concentration for muscle contraction.

• SR Tubules: Network surrounding each myofibril.

• Terminal Cisternae: Enlarged regions of SR adjacent to T-tubules; store calcium ions.

• T-tubules: Invaginations of sarcolemma that transmit action potentials into the fiber.

Sarcomere Anatomy

The sarcomere is the basic contractile unit of skeletal muscle, defined by the region between two Z-lines.

• A band: Dark region containing thick (myosin) filaments.

• I band: Light region containing thin (actin) filaments.

• H zone: Center of A band with only thick filaments.

• M line: Center of sarcomere, anchoring thick filaments.

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

Contractile Filaments

Thin Filaments

• F-actin: Filamentous actin, a twisted strand of G-actin molecules.

• G-actin: Globular actin, each with an active site for myosin binding.

• Tropomyosin: Covers active sites on actin, preventing myosin binding under resting conditions.

• Troponin: Binds calcium ions, causing tropomyosin to uncover active sites during contraction.

Thick Filaments

• Myosin: Each molecule has a tail and two heads; heads bind actin and ATP.

• Titin: Elastic protein stabilizing thick filaments and returning sarcomere to resting length.

Sliding Filament Theory

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

• Cross-bridge formation: Myosin heads bind to actin active sites.

• Power stroke: Myosin heads pivot, pulling actin filaments inward.

• ATP binding: Releases myosin head from actin, resetting for another cycle.

Skeletal Muscle Contraction

Excitation-Contraction Coupling

• Action potential arrives at neuromuscular junction.

• Acetylcholine (ACh) released, binds to receptors on sarcolemma.

• Action potential travels along sarcolemma and T-tubules.

• SR releases calcium ions, initiating contraction.

Recovery

• ACh is degraded by acetylcholinesterase.

• Calcium is pumped back into SR.

• Troponin and tropomyosin return to resting positions.

• ATP is hydrolyzed to reset myosin heads.

Length-Tension Relationship

The force a muscle can generate depends on the degree of overlap between thick and thin filaments.

• Optimal overlap produces maximal tension.

• Too much or too little overlap reduces tension.

• Number of fibers recruited also affects total tension.

Frequency of Stimulation

• Twitch: Single contraction-relaxation cycle.

• Latent period: Time between stimulus and contraction.

• Contraction period: Tension rises to peak.

• Relaxation period: Tension falls.

• Wave summation: Repeated stimulation increases tension.

• Incomplete tetanus: Partial relaxation between stimuli.

• Complete tetanus: No relaxation; maximal tension.

• Treppe: Gradual increase in tension with repeated stimulation.

Tension Production

Internal vs. External Tension

• Internal tension: Generated within muscle fibers.

• External tension: Transmitted to tendons.

Motor Units

• Consist of a motor neuron and all muscle fibers it innervates.

• Smaller units allow fine control; larger units produce more force.

• Recruitment increases force by activating more motor units.

Types of Muscle Contractions

Isotonic Contraction

• Concentric: Muscle shortens as it contracts (e.g., lifting a weight).

• Eccentric: Muscle lengthens while contracting (e.g., lowering a weight).

Isometric Contraction

• Muscle tension increases but length does not change (e.g., holding a weight steady).

Energetics of Contraction

ATP Usage

• Each thick filament uses about 2500 ATP/sec during contraction.

• ATP is required for cross-bridge cycling and calcium pumping.

Energy Sources

• Creatine phosphate: Stores high-energy phosphate to regenerate ATP.

• Aerobic metabolism: Uses oxygen to produce ATP from glucose, fatty acids.

• Anaerobic glycolysis: Produces ATP without oxygen, generating lactic acid.

Equations:

• $\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

• $\text{Glucose} + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36\text{ATP}$

• $\text{Glucose} \rightarrow 2\text{Pyruvate} + 2\text{ATP}$ (glycolysis)

Levels of Energy Use

• Rest: Glucose stored as glycogen; fatty acids metabolized aerobically.

• Moderate activity: ATP and CP used; aerobic metabolism predominates.

• Intense activity: Anaerobic glycolysis increases; lactic acid produced.

Fatigue

Muscle fatigue results from depletion of energy stores, accumulation of lactic acid, and impaired calcium handling.

• Energy supply

• Circulatory supply

• Neuromuscular function

Oxygen Debt

Oxygen debt is the extra oxygen required after exercise to restore energy reserves and remove lactic acid.

• Cori cycle: Lactic acid transported to liver and converted to glucose.

Skeletal Muscle Fiber Types

Type	Characteristics
Fast fibers	Large diameter, rapid contraction, few mitochondria, fatigue quickly
Slow fibers	Small diameter, slow contraction, many mitochondria, resistant to fatigue
Intermediate fibers	Characteristics between fast and slow

Changes in Muscle Size

• Hypertrophy: Increase in muscle fiber size due to training.

• Atrophy: Decrease in muscle fiber size due to inactivity.

Anaerobic and Aerobic Endurance

Anaerobic Endurance

• Depends on ATP, CP reserves, glycolytic capacity, and lactic acid tolerance.

Aerobic Endurance

• Depends on oxygen supply and mitochondrial activity.

• Substrates: carbohydrates, lipids, amino acids.

Cardiac Muscle

Structural Differences

• Cells are short, branched, and interconnected by intercalated discs.

• Single nucleus per cell.

• Automaticity: pacemaker cells depolarize spontaneously.

Functional Differences

• Longer contraction duration, no tetanus.

• Contractions initiated by pacemaker cells.

Smooth Muscle

Structural Differences

• Cells are small, spindle-shaped, single nucleus.

• No T-tubules, no striations.

• Filaments are more scattered.

Functional Differences

• Excitation-contraction coupling: Calcium binds calmodulin, activating myosin light chain kinase.

• Length-tension relationship: Can contract over a wide range of lengths.

• Control: Multi-unit (independent cells) and visceral (syncytial) types; regulated by autonomic nervous system, hormones, and local factors.

Additional info: Smooth muscle is found in walls of blood vessels, digestive tract, respiratory tract, and urinary tract.